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发信人: jjk (○kernel○), 信区: Linux
标 题: FreeBSD Assembly Language Tutorial(转寄)
发信站: 荔园晨风BBS站 (Sun Mar 24 14:37:13 2002), 转信
【 以下文字转载自 jjk 的信箱 】
【 原文由 jjksam@smth.org 所发表 】
发信人: dizzy (Metal Hammer), 信区: FreeBSD
标 题: FreeBSD Assembly Language Tutorial(转寄)
发信站: BBS 水木清华站 (Thu Mar 14 20:26:37 2002)
http://www.int80h.org/
Preface
by G. Adam Stanislav
Whiz Kid Technomagic
Assembly language programing under Unix is highly undocumented. It is
generally assumed that no one would ever want to use it because
various Unix systems run on different microprocessors, so everything
should be written in C for portability.
In reality, C portability is quite a myth. Even C programs need to be
modified when ported from one Unix to another, regardless of what
processor each runs on. Typically, such a program is full of conditional
statements depending on the system it is compiled for.
Even if we believe that all of Unix software should be written in C,
or some other high-level language, we still need assembly language
programmers: Who else would write the section of C library that accesses
the kernel?
In this tutorial I will attempt to show you how you can use assembly
language writing Unix programs, specifically under FreeBSD.
This tutorial does not explain the basics of assembly language. There
are enough resources about that (for a complete online course in
assembly language, see Randall Hyde’s Art of Assembly Language; or if
you prefer a printed book, take a look at Jeff Duntemann’s Assembly
Language Step-by-Step). However, once the tutorial is finished, any
assembly language programmer will be able to write programs for
FreeBSD quickly and efficiently.
Chapter 1 – The Tools
1.1. The Assembler
The most important tool for assembly language programming is the
assembler, the software that converts assembly language code into
machine language.
Two very different assemblers are available for FreeBSD. One is as(1),
which uses the traditional Unix assembly language syntax. It comes
with the system.
The other is /usr/ports/devel/nasm. It uses the Intel syntax. Its main
advantage is that it can assemble code for many operating systems. It
needs to be installed separately, but is completely free.
This tutorial uses nasm syntax because most assembly language
programmers coming to FreeBSD from other operating systems will find
it easier to understand. And, because, quite frankly, that is what I
am used to.
1.2. The Linker
The output of the assembler, like that of any compiler, needs to be
linked to form an executable file.
The standard ld(1) linker comes with FreeBSD. It works with the code
assembled with either assembler.
Chapter 2 – System Calls
2.1. Default Calling Convention
By default, the FreeBSD kernel uses the C calling convention. Further,
although the kernel is accessed using int 80h, it is assumed the program
will call a function that issues int 80h, rather than issuing int 80h
directly.
This convention is very convenient, and quite superior to the
Microsoft convention used by MS DOS. Why? Because the Unix convention
allows any program written in any language to access the kernel.
An assembly language program can do that as well. For example, we
could open a file:
kernel:
int 80h ; Call kernel
ret
open:
push dword mode
push dword flags
push dword path
mov eax, 5
call kernel
add esp, byte 12
ret
This is a very clean and portable way of coding. If you need to port the
code to a Unix system which uses a different interrupt, or a
different way of passing parameters, all you need to change is the
kernel procedure.
But assembly language programmers like to shave off cycles. The above
example requires a call/ret combination. We can eliminate it by
pushing an extra dword:
open:
push dword mode
push dword flags
push dword path
mov eax, 5
push eax ; Or any other dword
int 80h
add esp, byte 16
The 5 that we have placed in EAX identifies the kernel function, in this
case open.
2.2. Alternate Calling Convention
FreeBSD is an extremely flexible system. It offers other ways of calling
the kernel. For it to work, however, the system must have Linux
emulation installed.
Linux is a Unix-like system. However, its kernel uses the Microsoft
system-call convention of passing parameters in registers. As with the
Unix convention, the function number is placed in EAX. The parameters,
however, are not passed on the stack but in EBX, ECX, EDX, ESI, EDI,
EBP:
open:
mov eax, 5
mov ebx, path
mov ecx, flags
mov edx, mode
int 80h
This convention has a great disadvantage over the Unix way, at least
as far as assembly language programming is concerned: Every time you
make a kernel call you must push the registers, then pop them later.
This makes your code bulkier and slower. Nevertheless, FreeBSD gives you
a choice.
If you do choose the Microsoft/Linux convention, you must let the system
know about it. After your program is assembled and linked, you need
to brand the executable:
% brandelf -f Linux filename
2.3. Which Convention Should You Use?
If you are coding specifically for FreeBSD, you should always use the
Unix convention: It is faster, you can store global variables in
registers, you do not have to brand the executable, and you do not
impose the installation of the Linux emulation package on the target
system.
If you want to create portable code that can also run on Linux, you will
probably still want to give the FreeBSD users as efficient a code as
possible. I will show you how you can accomplish that after I have
explained the basics.
2.4. Call Numbers
To tell the kernel which system service you are calling, place its
number in EAX. Of course, you need to know what the number is.
2.4.1. The syscalls File
The numbers are listed in syscalls. locate syscalls finds this file in
several different formats, all produced automatically from syscalls.
master.
You can find the master file for the default Unix calling convention
in /usr/src/sys/kern/syscalls.master. If you need to use the other
convention implemented in the Linux emulation mode, read
/usr/src/sys/i386/linux/syscalls.master.
N.B.: Not only do FreeBSD and Linux use different calling conventions,
they sometimes use different numbers for the same functions.
syscalls.master describes how the call is to be made:
0 STD NOHIDE { int nosys(void); } syscall nosys_args int
1 STD NOHIDE { void exit(int rval); } exit rexit_args void
2 STD POSIX { int fork(void); }
3 STD POSIX { ssize_t read(int fd, void *buf, size_t nbyte); }
4 STD POSIX { ssize_t write(int fd, const void *buf, size_t nbyte);
}
5 STD POSIX { int open(char *path, int flags, int mode); }
6 STD POSIX { int close(int fd); }
etc...
It is the leftmost column that tells us the number to place in EAX.
The rightmost column tells us what parameters to push. They are pushed
from right to left.
EXAMPLE 2.1: For example, to open a file, we need to push the mode
first, then flags, then the address at which the path is stored.
Chapter 3 – Return Values
A system call would not be useful most of the time if it did not
return some kind of a value: The file descriptor of an open file, the
number of bytes read to a buffer, the system time, etc.
Additionally, the system needs to inform us if an error occurs: A file
does not exist, system resources are exhausted, we passed an invalid
parameter, etc.
3.1. Man Pages
The traditional place to look for information about various system calls
under Unix systems are the man pages. FreeBSD describes its system
calls in section 2, sometimes in section 3.
For example, open(2) says:
If successful, open() returns a non-negative integer, termed a file
descriptor. It returns -1 on failure, and sets errno to indicate the
error.
The assembly language programmer new to Unix and FreeBSD will
immediately ask the puzzling question: Where is errno and how do I get
to it?
N.B.: The information presented in the man pages applies to C programs.
The assembly language programmer needs additional information.
3.2. Where Are the Return Values?
Unfortunately, it depends... For most system calls it is in EAX, but not
for all. A good rule of thumb, when working with a system call for
the first time, is to look for the return value in EAX. If it is not
there, you need further research.
N.B.: I am aware of one system call that returns the value in EDX:
SYS_fork. All others I have worked with use EAX. But I have not worked
with them all yet.
TIP: If you cannot find the answer here or anywhere else, study libc
source code and see how it interfaces with the kernel.
3.3. Where Is errno?
Actually, nowhere...
errno is part of the C language, not the Unix kernel. When accessing
kernel services directly, the error code is returned in EAX, the same
register the proper return value generally ends up in.
This makes perfect sense. If there is no error, there is no error code.
If there is an error, there is no return value. One register can
contain either.
3.4. Determining an Error Occurred
When using the standard FreeBSD calling convention, the carry flag is
cleared upon success, set upon failure.
When using the Linux emulation mode, the signed value in EAX is
non-negative upon success, and contains the return value. In case of
an error, the value is negative, i.e., -errno.
Chapter 4 – Creating Portable Code
Portability is generally not one of the strengths of assembly language.
Yet, writing assembly language programs for different platforms is
possible, especially with nasm. I have written assembly language
libraries that can be assembled for such different operating systems
as Windows and FreeBSD.
It is all the more possible when you want your code to run on two
platforms which, while different, are based on similar architectures.
For example, FreeBSD is Unix, Linux is Unix-like. I only mentioned three
differences between them (from an assembly language programmer’s
perspective): The calling convention, the function numbers, and the
way of returning values.
4.1. Dealing with Function Numbers
In many cases the function numbers are the same. However, even when they
are not, the problem is easy to deal with: Instead of using numbers
in your code, use constants which you have declared differently
depending on the target architecture:
%ifdef LINUX
%define SYS_execve 11
%else
%define SYS_execve 59
%endif
4.2. Dealing with Conventions
Both, the calling convention, and the return value (the errno problem)
can be resolved with macros:
%ifdef LINUX
%macro system 0
call kernel
%endmacro
align 4
kernel:
push ebx
push ecx
push edx
push esi
push edi
push ebp
mov ebx, [esp+32]
mov ecx, [esp+36]
mov edx, [esp+40]
mov esi, [esp+44]
mov ebp, [esp+48]
int 80h
pop ebp
pop edi
pop esi
pop edx
pop ecx
pop ebx
or eax, eax
js .errno
clc
ret
.errno:
neg eax
stc
ret
%else
%macro system 0
int 80h
%endmacro
%endif
4.3. Dealing with Other Portability Issues
The above solutions can handle most cases of writing code portable
between FreeBSD and Linux. Nevertheless, with some kernel services the
differences are deeper.
In that case, you need to write two different handlers for those
particular system calls, and use conditional assembly. Luckily, most
of your code does something other than calling the kernel, so usually
you will only need a few such conditional sections in your code.
4.4. Using a Library
You can avoid portability issues in your main code altogether by writing
a library of system calls. Create a separate library for FreeBSD, a
different one for Linux, and yet other libraries for more operating
systems.
In your library, write a separate function (or procedure, if you
prefer the traditional assembly language terminology) for each system
call. Use the C calling convention of passing parameters. But still
use EAX to pass the call number in. In that case, your FreeBSD library
can be very simple, as many seemingly different functions can be just
labels to the same code:
sys.open:
sys.close:
[etc...]
int 80h
ret
Your Linux library will require more different functions. But even
here you can group system calls using the same number of parameters:
sys.exit:
sys.close:
[etc... one-parameter functions]
push ebx
mov ebx, [esp+12]
int 80h
pop ebx
jmp sys.return
...
sys.return:
or eax, eax
js sys.err
clc
ret
sys.err:
neg eax
stc
ret
The library approach may seem inconvenient at first because it
requires you to produce a separate file your code depends on. But it has
many advantages: For one, you only need to write it once and can use it
for all your programs. You can even let other assembly language
programmers use it, or perhaps use one written by someone else. But
perhaps the greatest advantage of the library is that your code can be
ported to other systems, even by other programmers, by simply writing
a new library without any changes to your code.
If you do not like the idea of having a library, you can at least
place all your system calls in a separate assembly language file and
link it with your main program. Here, again, all porters have to do is
create a new object file to link with your main program.
4.5. Using an Include File
If you are releasing your software as (or with) source code, you can use
macros and place them in a separate file, which you include in your
code.
Porters of your software will simply write a new include file. No
library or external object file is necessary, yet your code is
portable without any need to edit the code.
N.B.: This is the approach we will use throughout this tutorial. We will
name our include file system.inc, and add to it whenever we deal with a
new system call.
We can start our system.inc by declaring the standard file descriptors:
%define stdin 0
%define stdout 1
%define stderr 2
Next, we create a symbolic name for each system call:
%define SYS_nosys 0
%define SYS_exit 1
%define SYS_fork 2
%define SYS_read 3
%define SYS_write 4
; [etc...]
We add a short, non-global procedure with a long name, so we do not
accidentally reuse the name in our code:
section .text
align 4
access.the.bsd.kernel:
int 80h
ret
We create a macro which takes one argument, the syscall number:
%macro system 1
mov eax, %1
call access.the.bsd.kernel
%endmacro
Finally, we create macros for each syscall. These macros take no
arguments.
%macro sys.exit 0
system SYS_exit
%endmacro
%macro sys.fork 0
system SYS_fork
%endmacro
%macro sys.read 0
system SYS_read
%endmacro
%macro sys.write 0
system SYS_write
%endmacro
; [etc...]
Go ahead, enter it into your editor and save it as system.inc. We will
add more to it as we discuss more syscalls.
Chapter 5 – Our First Program
We are now ready for our first program, the mandatory Hello, World!
1: %include 'system.inc'
2:
3: section .data
4: hello db 'Hello, World!', 0Ah
5: hbytes equ $-hello
6:
7: section .text
8: global _start
9: _start:
10: push dword hbytes
11: push dword hello
12: push dword stdout
13: sys.write
14:
15: push dword 0
16: sys.exit
Here is what it does: Line 1 includes the defines, the macros, and the
code from system.inc.
Lines 3-5 are the data: Line 3 starts the data section/segment. Line 4
contains the string "Hello, World!" followed by a new line (0Ah). Line 5
creates a constant that contains the length of the string from line 4
in bytes.
Lines 7-16 contain the code. Note that FreeBSD uses the elf file
format for its executables, which requires every program to start at the
point labeled _start (or, more precisely, the linker expects that).
This label has to be global.
Lines 10-13 ask the system to write hbytes bytes of the hello string
to stdout.
Lines 15-16 ask the system to end the program with the return value of
0. The SYS_exit syscall never returns, so the code ends there.
N.B.: If you have come to Unix from MS DOS assembly language background,
you may be used to writing directly to the video hardware. You will
never have to worry about this in FreeBSD, or any other flavor of Unix.
As far as you are concerned, you are writing to a file known as stdout.
This can be the video screen, or a telnet terminal, or an actual file,
or even the input of another program. Which one it is, is for the
system to figure out.
5.1. Assembling the Code
Type the code (except the line numbers) in an editor, and save it in a
file named hello.asm. You need nasm to assemble it.
5.1.1. Installing nasm
If you do not have nasm, type:
% su
Password:your root password
# cd /usr/ports/devel/nasm
# make install
# exit
%
You may type make install clean instead of just make install if you do
not want to keep nasm source code.
Either way, FreeBSD will automatically download nasm from the Internet,
compile it, and install it on your system.
N.B.: If your system is not FreeBSD, you need to get nasm from its
home page. You can still use it to assemble FreeBSD code.
Now you can assemble, link, and run the code:
% nasm -f elf hello.asm
% ld -s -o hello hello.o
% ./hello
Hello, World!
%
Chapter 6 – Writing Unix Filters
A common type of Unix application is a filter—a program that reads data
from the stdin, processes it somehow, then writes the result to
stdout.
In this chapter, we shall develop a simple filter, and learn how to read
from stdin and write to stdout. This filter will convert each byte of
its input into a hexadecimal number followed by a blank space.
%include 'system.inc'
section .data
hex db '0123456789ABCDEF'
buffer db 0, 0, ' '
section .text
global _start
_start:
; read a byte from stdin
push dword 1
push dword buffer
push dword stdin
sys.read
add esp, byte 12
or eax, eax
je .done
; convert it to hex
movzx eax, byte [buffer]
mov edx, eax
shr dl, 4
mov dl, [hex+edx]
mov [buffer], dl
and al, 0Fh
mov al, [hex+eax]
mov [buffer+1], al
; print it
push dword 3
push dword buffer
push dword stdout
sys.write
add esp, byte 12
jmp short _start
.done:
push dword 0
sys.exit
In the data section we create an array called hex. It contains the 16
hexadecimal digits in ascending order. The array is followed by a buffer
which we will use for both input and output. The first two bytes of the
buffer are initially set to 0. This is where we will write the two
hexadecimal digits (the first byte also is where we will read the
input). The third byte is a space.
The code section consists of four parts: Reading the byte, converting it
to a hexadecimal number, writing the result, and eventually exiting the
program.
To read the byte, we ask the system to read one byte from stdin, and
store it in the first byte of the buffer. The system returns the
number of bytes read in EAX. This will be 1 while data is coming, or 0,
when no more input data is available. Therefore, we check the value
of EAX. If it is 0, we jump to .done, otherwise we continue.
N.B.: For simplicity sake, we are ignoring the possibility of an error
condition at this time.
The hexadecimal conversion reads the byte from the buffer into EAX, or
actually just AL, while clearing the remaining bits of EAX to zeros.
We also copy the byte to EDX because we need to convert the upper four
bits (nibble) separately from the lower four bits. We store the result
in the first two bytes of the buffer.
Next, we ask the system to write the three bytes of the buffer, i.e.,
the two hexadecimal digits and the blank space, to stdout. We then
jump back to the beginning of the program and process the next byte.
Once there is no more input left, we ask the system to exit our program,
returning a zero, which is the traditional value meaning the program
was successful.
Go ahead, and save the code in a file named hex.asm, then type the
following (the ^D means press the control key and type D while holding
the control key down):
% nasm -f elf hex.asm
% ld -s -o hex hex.o
% ./hex
Hello, World!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A Here I come!
48 65 72 65 20 49 20 63 6F 6D 65 21 0A ^D %
N.B.: If you are migrating to Unix from MS DOS, you may be wondering why
each line ends with 0A instead of 0D 0A. This is because Unix does
not use the cr/lf convention, but a “new line” convention, which is 0A
in hexadecimal.
Can we improve this? Well, for one, it is a bit confusing because once
we have converted a line of text, our input no longer starts at the
begining of the line. We can modify it to print a new line instead of
a space after each 0A:
%include 'system.inc'
section .data
hex db '0123456789ABCDEF'
buffer db 0, 0, ' '
section .text
global _start
_start:
mov cl, ' '
.loop:
; read a byte from stdin
push dword 1
push dword buffer
push dword stdin
sys.read
add esp, byte 12
or eax, eax
je .done
; convert it to hex
movzx eax, byte [buffer]
mov [buffer+2], cl
cmp al, 0Ah
jne .hex
mov [buffer+2], al
.hex:
mov edx, eax
shr dl, 4
mov dl, [hex+edx]
mov [buffer], dl
and al, 0Fh
mov al, [hex+eax]
mov [buffer+1], al
; print it
push dword 3
push dword buffer
push dword stdout
sys.write
add esp, byte 12
jmp short .loop
.done:
push dword 0
sys.exit
We have stored the space in the CL register. We can do this safely
because, unlike Microsoft Windows, Unix system calls do not modify the
value of any register they do not use to return a value in.
That means we only need to set CL once. We have, therefore, added a
new label .loop and jump to it for the next byte instead of jumping at
_start. We have also added the .hex label so we can either have a
blank space or a new line as the third byte of the buffer.
Once you have changed hex.asm to reflect these changes, type:
% nasm -f elf hex.asm
% ld -s -o hex hex.o
% ./hex
Hello, World!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
Here I come!
48 65 72 65 20 49 20 63 6F 6D 65 21 0A
^D %
That looks better. But this code is quite inefficient! We are making a
system call for every single byte twice (once to read it, another time
to write the output).
Chapter 7 – Buffered Input and Output
We can improve the efficiency of our code by buffering our input and
output. We create an input buffer and read a whole sequence of bytes
at one time. Then we fetch them one by one from the buffer.
We also create an output buffer. We store our output in it until it is
full. At that time we ask the kernel to write the contents of the buffer
to stdout.
The program ends when there is no more input. But we still need to ask
the kernel to write the contents of our output buffer to stdout one last
time, otherwise some of our output would make it to the output buffer,
but never be sent out. Do not forget that, or you will be wondering why
some of your output is missing.
%include 'system.inc'
%define BUFSIZE 2048
section .data
hex db '0123456789ABCDEF'
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
global _start
_start:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
.loop:
; read a byte from stdin
call getchar
; convert it to hex
mov dl, al
shr al, 4
mov al, [hex+eax]
call putchar
mov al, dl
and al, 0Fh
mov al, [hex+eax]
call putchar
mov al, ' '
cmp dl, 0Ah
jne .put
mov al, dl
.put:
call putchar
jmp short .loop
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
ret
read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword stdin
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
ret
align 4
.done:
call write ; flush output buffer
push dword 0
sys.exit
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
sub edi, ecx ; start of buffer
push ecx
push edi
push dword stdout
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
ret
We now have a third section in the source code, named .bss. This section
is not included in our executable file, and, therefore, cannot be
initialized. We use resb instead of db. It simply reserves the requested
size of uninitialized memory for our use.
We take advantage of the fact that the system does not modify the
registers: We use registers for what, otherwise, would have to be global
variables stored in the .data section. This is also why the Unix
convention of passing parameters to system calls on the stack is
superior to the Microsoft convention of passing them in the registers:
We can keep the registers for our own use.
We use EDI and ESI as pointers to the next byte to be read from or
written to. We use EBX and ECX to keep count of the number of bytes in
the two buffers, so we know when to dump the output to, or read more
input from, the system.
Let us see how it works now:
% nasm -f elf hex.asm
% ld -s -o hex hex.o
% ./hex
Hello, World!
Here I come!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
48 65 72 65 20 49 20 63 6F 6D 65 21 0A
^D %
Not what you expected? The program did not print the output until we
pressed ^D. That is easy to fix by inserting three lines of code to
write the output every time we have converted a new line to 0A. I have
marked the three lines with > (do not copy the > in your hex.asm).
%include 'system.inc'
%define BUFSIZE 2048
section .data
hex db '0123456789ABCDEF'
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
global _start
_start:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
.loop:
; read a byte from stdin
call getchar
; convert it to hex
mov dl, al
shr al, 4
mov al, [hex+eax]
call putchar
mov al, dl
and al, 0Fh
mov al, [hex+eax]
call putchar
mov al, ' '
cmp dl, 0Ah
jne .put
mov al, dl
.put:
call putchar
> cmp al, 0Ah
> jne .loop
> call write
jmp short .loop
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
ret
read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword stdin
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
ret
align 4
.done:
call write ; flush output buffer
push dword 0
sys.exit
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
sub edi, ecx ; start of buffer
push ecx
push edi
push dword stdout
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
ret
Now, let us see how it works:
% nasm -f elf hex.asm
% ld -s -o hex hex.o
% ./hex
Hello, World!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
Here I come!
48 65 72 65 20 49 20 63 6F 6D 65 21 0A
^D %
Not bad for a 644-byte executable, is it!
N.B.: This approach to buffered input/output still contains a hidden
danger. I will discuss—and fix—it later, when I talk about the dark
side of buffering.
7.1. How to Unread a Character
WARNING: This may be a somewhat advanced topic, mostly of interest to
programmers familiar with the theory of compilers. If you wish, you
may skip to the next chapter, and perhaps read this later.
While our sample program does not require it, more sophisticated filters
often need to look ahead. In other words, they may need to see what the
next character is (or even several characters). If the next character
is of a certain value, it is part of the token currently being
processed. Otherwise, it is not.
For example, you may be parsing the input stream for a textual string
(e.g., when implementing a language compiler): If a character is
followed by another character, or perhaps a digit, it is part of the
token you are processing. If it is followed by white space, or some
other value, then it is not part of the current token.
This presents an interesting problem: How to return the next character
back to the input stream, so it can be read again later?
One possible solution is to store it in a character variable, then set a
flag. We can modify getchar to check the flag, and if it is set,
fetch the byte from that variable instead of the input buffer, and reset
the flag. But, of course, that slows us down.
The C language has an ungetc() function, just for that purpose. Is there
a quick way to implement it in our code? I would like you to scroll
back up and take a look at the getchar procedure and see if you can find
a nice and fast solution before reading the next paragraph. Then come
back here and see my own solution.
The key to returning a character back to the stream is in how we are
getting the characters to start with:
First we check if the buffer is empty by testing the value of EBX. If it
is zero, we call the read procedure.
If we do have a character available, we use lodsb, then decrease the
value of EBX. The lodsb instruction is effectively identical to:
mov al, [esi]
inc esi
The byte we have fetched remains in the buffer until the next time
read is called. We do not know when that happens, but we do know it will
not happen until the next call to getchar. Hence, to “return” the
last-read byte back to the stream, all we have to do is decrease the
value of ESI and increase the value of EBX:
ungetc:
dec esi
inc ebx
ret
But, be careful! We are perfectly safe doing this if our look-ahead is
at most one character at a time. If we are examining more than one
upcoming character and call ungetc several times in a row, it will
work most of the time, but not all the time (and will be tough to
debug). Why?
Because as long as getchar does not have to call read, all of the
pre-read bytes are still in the buffer, and our ungetc works without a
glitch. But the moment getchar calls read, the contents of the buffer
change.
We can always rely on ungetc working properly on the last character we
have read with getchar, but not on anything we have read before that.
If your program reads more than one byte ahead, you have at least two
choices:
If possible, modify the program so it only reads one byte ahead. This is
the simplest solution.
If that option is not available, first of all determine the maximum
number of characters your program needs to return to the input stream at
one time. Increase that number slightly, just to be sure, preferably to
a multiple of 16—so it aligns nicely. Then modify the .bss section
of your code, and create a small “spare” buffer right before your
input buffer, something like this:
section .bss
resb 16 ; or whatever the value you came up with
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
You also need to modify your ungetc to pass the value of the byte to
unget in AL:
ungetc:
dec esi
inc ebx
mov [esi], al
ret
With this modification, you can call ungetc up to 17 times in a row
safely (the first call will still be within the buffer, the remaining 16
may be either within the buffer or within the “spare”).
Chapter 8 – Command Line Arguments
Our hex program will be more useful if it can read the names of an input
and output file from its command line, i.e., if it can process the
command line arguments. But... Where are they?
Before a Unix system starts a program, it pushes some data on the stack,
then jumps at the _start label of the program. Yes, I said jumps, not
calls. That means the data can be accessed by reading [esp+offset], or
by simply popping it.
The value at the top of the stack contains the number of command line
arguments. It is traditionally called argc, for “argument count.”
Command line arguments follow next, all argc of them. These are
typically referred to as argv, for “argument value(s).” That is, we
get argv[0], argv[1], ..., argv[argc-1]. These are not the actual
arguments, but pointers to arguments, i.e., memory addresses of the
actual arguments. The arguments themselves are NUL-terminated
character strings.
The argv list is followed by a NULL pointer, which is simply a 0.
There is more, but this is enough for our purposes right now.
N.B.: If you have come from the MS DOS programming environment, the main
difference is that each argument is in a separate string. The second
difference is that there is no practical limit on how many arguments
there can be.
Armed with this knowledge, we are almost ready for the next version of
hex.asm. First, however, we need to add a few lines to system.inc:
First, we need to add two new entries to our list of system call
numbers:
%define SYS_open 5
%define SYS_close 6
Then we add two new macros at the end of the file:
%macro sys.open 0
system SYS_open
%endmacro
%macro sys.close 0
system SYS_close
%endmacro
Here, then, is our modified source code:
%include 'system.inc'
%define BUFSIZE 2048
section .data
fd.in dd stdin
fd.out dd stdout
hex db '0123456789ABCDEF'
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
align 4
err:
push dword 1 ; return failure
sys.exit
align 4
global _start
_start:
add esp, byte 8 ; discard argc and argv[0]
pop ecx
jecxz .init ; no more arguments
; ECX contains the path to input file
push dword 0 ; O_RDONLY
push ecx
sys.open
jc err ; open failed
add esp, byte 8
mov [fd.in], eax
pop ecx
jecxz .init ; no more arguments
; ECX contains the path to output file
push dword 420 ; file mode (644 octal)
push dword 0200h | 0400h | 01h
; O_CREAT | O_TRUNC | O_WRONLY
push ecx
sys.open
jc err
add esp, byte 12
mov [fd.out], eax
.init:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
.loop:
; read a byte from input file or stdin
call getchar
; convert it to hex
mov dl, al
shr al, 4
mov al, [hex+eax]
call putchar
mov al, dl
and al, 0Fh
mov al, [hex+eax]
call putchar
mov al, ' '
cmp dl, 0Ah
jne .put
mov al, dl
.put:
call putchar
cmp al, dl
jne .loop
call write
jmp short .loop
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
ret
read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword [fd.in]
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
ret
align 4
.done:
call write ; flush output buffer
; close files
push dword [fd.in]
sys.close
push dword [fd.out]
sys.close
; return success
push dword 0
sys.exit
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
sub edi, ecx ; start of buffer
push ecx
push edi
push dword [fd.out]
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
ret
In our .data section we now have two new variables, fd.in and fd.out. We
store the input and output file descriptors here.
In the .text section we have replaced the references to stdin and stdout
with [fd.in] and [fd.out].
The .text section now starts with a simple error handler, which does
nothing but exit the program with a return value of 1. The error handler
is before _start so we are within a short distance from where the
errors occur.
Naturally, the program execution still begins at _start. First, we
remove argc and argv[0] from the stack: They are of no interest to us
(in this program, that is).
We pop argv[1] to ECX. This register is particularly suited for
pointers, as we can handle NULL pointers with jecxz. If argv[1] is not
NULL, we try to open the file named in the first argument. Otherwise, we
continue the program as before: Reading from stdin, writing to stdout.
If we fail to open the input file (e.g., it does not exist), we jump to
the error handler and quit.
If all went well, we now check for the second argument. If it is there,
we open the output file. Otherwise, we send the output to stdout. If we
fail to open the output file (e.g., it exists and we do not have the
write permission), we, again, jump to the error handler.
The rest of the code is the same as before, except we close the input
and output files before exiting, and, as mentioned, we use [fd.in] and
[fd.out].
Our executable is now a whopping 768 bytes long.
Can we still improve it? Of course! Every program can be improved.
Here are a few ideas of what we could do:
Have our error handler print a message to stderr.
Add error handlers to the read and write functions.
Close stdin when we open an input file, stdout when we open an output
file.
Add command line switches, such as -i and -o, so we can list the input
and output files in any order, or perhaps read from stdin and write to a
file.
Print a usage message if command line arguments are incorrect.
I shall leave these enhancements as an exercise to the reader: You
already know everything you need to know to implement them.
Chapter 9 – Unix Environment
An important Unix concept is the environment, which is defined by
environment variables. Some are set by the system, others by you, yet
others by the shell, or any program that loads another program.
9.1. How to Find Environment Variables
I said earlier that when a program starts executing, the stack
contains argc followed by the NULL-terminated argv array, followed by
something else. The “something else” is the environment, or, to be
more precise, a NULL-terminated array of pointers to environment
variables. This is often referred to as env.
The structure of env is the same as that of argv, a list of memory
addresses followed by a NULL (0). In this case, there is no “envc”—we
figure out where the array ends by searching for the final NULL.
The variables usually come in the name=value format, but sometimes the
=value part may be missing. We need to account for that possibility.
9.2. webvars
I could just show you some code that prints the environment the same way
the Unix env command does. But I thought it would be more interesting
to write a simple assembly language CGI utility.
9.2.1. CGI: A Quick Overview
I have a detailed CGI tutorial on my web site, but here is a very
quick overview of CGI:
The web server communicates with the CGI program by setting
environment variables.
The CGI program sends its output to stdout. The web server reads it from
there.
It must start with an HTTP header followed by two blank lines.
It then prints the HTML code, or whatever other type of data it is
producing.
N.B.: While certain environment variables use standard names, others
vary, depending on the web server. That makes webvars quite a useful
diagnostic tool.
9.2.2. The Code
Our webvars program, then, must send out the HTTP header followed by
some HTML mark-up. It then must read the environment variables one by
one and send them out as part of the HTML page.
The code follows. I placed comments and explanations right inside the
code:
;;;;;;; webvars.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;
;
; Copyright (c) 2000 G. Adam Stanislav
; All rights reserved.
;
; Redistribution and use in source and binary forms, with or without
; modification, are permitted provided that the following conditions
; are met:
; 1. Redistributions of source code must retain the above copyright
; notice, this list of conditions and the following disclaimer.
; 2. Redistributions in binary form must reproduce the above copyright
; notice, this list of conditions and the following disclaimer in
the
; documentation and/or other materials provided with the
distribution.
;
; THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS''
AND
; ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
THE
; IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE
; ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE
LIABLE
; FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL
; DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
GOODS
; OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION)
; HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
STRICT
; LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
ANY WAY
; OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
OF
; SUCH DAMAGE.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;
;
; Version 1.0
;
; Started: 8-Dec-2000
; Updated: 8-Dec-2000
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;
%include 'system.inc'
section .data
http db 'Content-type: text/html', 0Ah, 0Ah
db '<?xml version="1.0" encoding="UTF-8"?>', 0Ah
db '<!DOCTYPE html PUBLIC "-//W3C/DTD XHTML Strict//EN" '
db '"DTD/xhtml1-strict.dtd">', 0Ah
db '<html xmlns="http://www.w3.org/1999/xhtml" '
db 'xml.lang="en" lang="en">', 0Ah
db '<head>', 0Ah
db '<title>Web Environment</title>', 0Ah
db '<meta name="author" content="G. Adam Stanislav" />', 0Ah
db '</head>', 0Ah, 0Ah
db '<body bgcolor="#ffffff" text="#000000" link="#0000ff" '
db 'vlink="#840084" alink="#0000ff">', 0Ah
db '<div class="webvars">', 0Ah
db '<h1>Web Environment</h1>', 0Ah
db '<p>The following <b>environment variables</b> are defined '
db 'on this web server:</p>', 0Ah, 0Ah
db '<table align="center" width="80" border="0" cellpadding="10" '
db 'cellspacing="0" class="webvars">', 0Ah
httplen equ $-http
left db '<tr>', 0Ah
db '<td class="name"><tt>'
leftlen equ $-left
middle db '</tt></td>', 0Ah
db '<td class="value"><tt><b>'
midlen equ $-middle
undef db '<i>(undefined)</i>'
undeflen equ $-undef
right db '</b></tt></td>', 0Ah
db '</tr>', 0Ah
rightlen equ $-right
wrap db '</table>', 0Ah
db '</div>', 0Ah
db '</body>', 0Ah
db '</html>', 0Ah, 0Ah
wraplen equ $-wrap
section .text
global _start
_start:
; First, send out all the http and xhtml stuff that is
; needed before we start showing the environment
push dword httplen
push dword http
push dword stdout
sys.write
; Now find how far on the stack the environment pointers
; are. We have 12 bytes we have pushed before "argc"
mov eax, [esp+12]
; We need to remove the following from the stack:
;
; The 12 bytes we pushed for sys.write
; The 4 bytes of argc
; The EAX*4 bytes of argv
; The 4 bytes of the NULL after argv
;
; Total:
; 20 + eax * 4
;
; Because stack grows down, we need to ADD that many bytes
; to ESP.
lea esp, [esp+20+eax*4]
cld ; This should already be the case, but let's be sure.
; Loop through the environment, printing it out
.loop:
pop edi
or edi, edi ; Done yet?
je near .wrap
; Print the left part of HTML
push dword leftlen
push dword left
push dword stdout
sys.write
; It may be tempting to search for the '=' in the env string next.
; But it is possible there is no '=', so we search for the
; terminating NUL first.
mov esi, edi ; Save start of string
sub ecx, ecx
not ecx ; ECX = FFFFFFFF
sub eax, eax
repne scasb
not ecx ; ECX = string length + 1
mov ebx, ecx ; Save it in EBX
; Now is the time to find '='
mov edi, esi ; Start of string
mov al, '='
repne scasb
not ecx
add ecx, ebx ; Length of name
push ecx
push esi
push dword stdout
sys.write
; Print the middle part of HTML table code
push dword midlen
push dword middle
push dword stdout
sys.write
; Find the length of the value
not ecx
lea ebx, [ebx+ecx-1]
; Print "undefined" if 0
or ebx, ebx
jne .value
mov ebx, undeflen
mov edi, undef
.value:
push ebx
push edi
push dword stdout
sys.write
; Print the right part of the table row
push dword rightlen
push dword right
push dword stdout
sys.write
; Get rid of the 60 bytes we have pushed
add esp, byte 60
; Get the next variable
jmp .loop
.wrap:
; Print the rest of HTML
push dword wraplen
push dword wrap
push dword stdout
sys.write
; Return success
push dword 0
sys.exit
This code produces a 1,396-byte executable. Most of it is data, i.e.,
the HTML mark-up we need to send out.
Assemble and link it as usual:
% nasm -f elf webvars.asm
% ld -s -o webvars webvars.o
To use it, you need to upload webvars to your web server. Depending on
how your web server is set up, you may have to store it in a special
cgi-bin directory, or perhaps rename it with a .cgi extension.
Then you need to use your browser to view its output. To see its
output on my web server, please go to http://www.int80h.org/webvars/. If
curious about the additional environment variables present in a
password protected web directory, go to http://www.int80h.org/private/,
using the name asm and password programmer.
Chapter 10 – Working with Files
We have already done some basic file work: We know how to open and close
them, how to read and write them using buffers. But Unix offers much
more functionality when it comes to files. We will examine some of it in
this section, and end up with a nice file conversion utility.
Indeed, let us start at the end, that is, with the file conversion
utility. It always makes programming easier when we know from the
start what the end product is supposed to do.
One of the first programs I wrote for Unix was tuc, a text-to-Unix
file converter. It converts a text file from other operating systems
to a Unix text file. In other words, it changes from different kind of
line endings to the newline convention of Unix. It saves the output in a
different file. Optionally, it converts a Unix text file to a DOS
text file.
I have used tuc extensively, but always only to convert from some
other OS to Unix, never the other way. I have always wished it would
just overwrite the file instead of me having to send the output to a
different file. Most of the time, I end up using it like this:
% tuc myfile tempfile
% mv tempfile myfile
It would be nice to have a ftuc, i.e., fast tuc, and use it like this:
% ftuc myfile
In this chapter, then, we will write ftuc in assembly language (the
original tuc is in C), and study various file-oriented kernel services
in the process.
At first sight, such a file conversion is very simple: All you have to
do is strip the carriage returns, right?
If you answered yes, think again: That approach will work most of the
time (at least with MS DOS text files), but will fail occasionally.
The problem is that not all non-Unix text files end their line with
the carriage return / line feed sequence. Some use carriage returns
without line feeds. Others combine several blank lines into a single
carriage return followed by several line feeds. And so on.
A text file converter, then, must be able to handle any possible line
endings:
carriage return / line feed
carriage return
line feed / carriage return
line feed
It should also handle files that use some kind of a combination of the
above (e.g., carriage return followed by several line feeds).
10.1.Finite State Machine
The problem is easily solved by the use of a technique called finite
state machine, originally developed by the designers of digital
electronic circuits. A finite state machine is a digital circuit whose
output is dependent not only on its input but on its previous input, i.
e., on its state. The microprocessor is an example of a finite state
machine: Our assembly language code is assembled to machine language
in which some assembly language code produces a single byte of machine
language, while others produce several bytes. As the microprocessor
fetches the bytes from the memory one by one, some of them simply change
its state rather than produce some output. When all the bytes of the op
code are fetched, the microprocessor produces some output, or changes
the value of a register, etc.
Because of that, all software is essentially a sequence of state
instructions for the microprocessor. Nevertheless, the concept of finite
state machine is useful in software design as well.
Our text file converter can be designed as a finite state machine with
three possible states. We could call them states 0-2, but it will make
our life easier if we give them symbolic names:
ordinary
cr
lf
Our program will start in the ordinary state. During this state, the
program action depends on its input as follows:
If the input is anything other than a carriage return or line feed,
the input is simply passed on to the output. The state remains
unchanged.
If the input is a carriage return, the state is changed to cr. The input
is then discarded, i.e., no output is made.
If the input is a line feed, the state is changed to lf. The input is
then discarded.
Whenever we are in the cr state, it is because the last input was a
carriage return, which was unprocessed. What our software does in this
state again depends on the current input:
If the input is anything other than a carriage return or line feed,
output a line feed, then output the input, then change the state to
ordinary.
If the input is a carriage return, we have received two (or more)
carriage returns in a row. We discard the input, we output a line feed,
and leave the state unchanged.
If the input is a line feed, we output the line feed and change the
state to ordinary. Note that this is not the same as the first case
above – if we tried to combine them, we would be outputting two line
feeds instead of one.
Finally, we are in the lf state after we have received a line feed
that was not preceded by a carriage return. This will happen when our
file already is in Unix format, or whenever several lines in a row are
expressed by a single carriage return followed by several line feeds, or
when line ends with a line feed / carriage return sequence. Here is how
we need to handle our input in this state:
If the input is anything other than a carriage return or line feed, we
output a line feed, then output the input, then change the state to
ordinary. This is exactly the same action as in the cr state upon
receiving the same kind of input.
If the input is a carriage return, we discard the input, we output a
line feed, then change the state to ordinary.
If the input is a line feed, we output the line feed, and leave the
state unchanged.
10.1.1. The Final State
The above finite state machine works for the entire file, but leaves the
possibility that the final line end will be ignored. That will happen
whenever the file ends with a single carriage return or a single line
feed. I did not think of it when I wrote tuc, just to discover that
occasionally it strips the last line ending.
This problem is easily fixed by checking the state after the entire file
was processed. If the state is not ordinary, we simply need to output
one last line feed.
N.B.: Now that we have expressed our algorithm as a finite state
machine, we could easily design a dedicated digital electronic circuit
(a “chip”) to do the conversion for us. Of course, doing so would be
considerably more expensive than writing an assembly language program.
10.1.2. The Output Counter
Because our file conversion program may be combining two characters into
one, we need to use an output counter. We initialize it to 0, and
increase it every time we send a character to the output. At the end
of the program, the counter will tell us what size we need to set the
file to.
10.2. Implementing FSM in Software
The hardest part of working with a finite state machine is analyzing the
problem and expressing it as a finite state machine. That accomplished,
the software almost writes itself.
In a high-level language, such as C, there are several main approaches.
One is to use a switch statement which chooses what function should
be run. For example,
switch (state) {
default:
case REGULAR:
regular(inputchar);
break;
case CR:
cr(inputchar);
break;
case LF:
lf(inputchar);
break;
}
Another approach is by using an array of function pointers, something
like this:
(output[state])(inputchar);
Yet another is to have state be a function pointer, set to point at
the appropriate function:
(*state)(inputchar);
This is the approach we will use in our program because it is very
easy to do in assembly language, and very fast, too. We will simply keep
the address of the right procedure in EBX, and then just issue:
call ebx
This is possibly faster than hardcoding the address in the code
because the microprocessor does not have to fetch the address from the
memory—it is already stored in one of its registers. I said possibly
because with the caching modern microprocessors do, either way may be
equally fast.
10.3.Memory Mapped Files
Because our program works on a single file, we cannot use the approach
that worked for us before, i.e., to read from an input file and to write
to an output file.
Unix allows us to map a file, or a section of a file, into memory. To do
that, we first need to open the file with the appropriate read/write
flags. Then we use the mmap system call to map it into the memory. One
nice thing about mmap is that it automatically works with virtual
memory: We can map more of the file into the memory than we have
physical memory available, yet still access it through regular memory op
codes, such as mov, lods, and stos. Whatever changes we make to the
memory image of the file will be written to the file by the system. We
do not even have to keep the file open: As long as it stays mapped, we
can read from it and write to it.
The 32-bit Intel microprocessors can access up to four gigabytes of
memory – physical or virtual. The FreeBSD system allows us to use up to
a half of it for file mapping.
For simplicity sake, in this tutorial we will only convert files that
can be mapped into the memory in their entirety. There are probably
not too many text files that exceed two gigabytes in size. If our
program encounters one, it will simply display a message suggesting we
use the original tuc instead.
If you examine your copy of syscalls.master, you will find two
separate syscalls named mmap. This is because of evolution of Unix:
There was the traditional BSD mmap, syscall 71. That one was
superceded by the POSIX mmap, syscall 197. The FreeBSD system supports
both because older programs were written by using the original BSD
version. But new software uses the POSIX version, which is what we
will use.
The syscalls.master file lists the POSIX version like this:
197 STD BSD { caddr_t mmap(caddr_t addr, size_t len, int prot, \
int flags, int fd, long pad, off_t pos); }
This differs slightly from what mmap(2) says. That is because mmap(2)
describes the C version.
The difference is in the long pad argument, which is not present in
the C version. However, the FreeBSD syscalls add a 32-bit pad after
pushing a 64-bit argument. In this case, off_t is a 64-bit value.
When we are finished working with a memory-mapped file, we unmap it with
the munmap syscall:
TIP: For an in-depth treatment of mmap, see W. Richard Stevens’ Unix
Network Programming, Volume 2, Chapter 12.
10.4. Determining File Size
Because we need to tell mmap how many bytes of the file to map into
the memory, and because we want to map the entire file, we need to
determine the size of the file.
We can use the fstat syscall to get all the information about an open
file that the system can give us. That includes the file size.
Again, syscalls.master lists two versions of fstat, a traditional one
(syscall 62), and a POSIX one (syscall 189). Naturally, we will use
the POSIX version:
189 STD POSIX { int fstat(int fd, struct stat *sb); }
This is a very straightforward call: We pass to it the address of a stat
structure and the descriptor of an open file. It will fill out the
contents of the stat structure.
I do, however, have to say that I tried to declare the stat structure in
the .bss section, and fstat did not like it: It set the carry flag
indicating an error. After I changed the code to allocate the
structure on the stack, everything was working fine.
10.5. Changing the File Size
Because our program may combine carriage return / line feed sequences
into straight line feeds, our output may be smaller than our input.
However, since we are placing our output into the same file we read
the input from, we may have to change the size of the file.
The ftruncate system call allows us to do just that. Despite its
somewhat misleading name, the ftruncate system call can be used to
both truncate the file (make it smaller) and to grow it.
And yes, we will find two versions of ftruncate in syscalls.master, an
older one (130), and a newer one (201). We will use the newer one:
201 STD BSD { int ftruncate(int fd, int pad, off_t length); }
Please note that this one contains a int pad again.
10.6. ftuc
We now know everything we need to write ftuc. We start by adding some
new lines in system.inc. First, we define some constants and structures,
somewhere at or near the beginning of the file:
;;;;;;; open flags
%define O_RDONLY 0
%define O_WRONLY 1
%define O_RDWR 2
;;;;;;; mmap flags
%define PROT_NONE 0
%define PROT_READ 1
%define PROT_WRITE 2
%define PROT_EXEC 4
;;
%define MAP_SHARED 0001h
%define MAP_PRIVATE 0002h
;;;;;;; stat structure
struc stat
st_dev resd 1 ; = 0
st_ino resd 1 ; = 4
st_mode resw 1 ; = 8, size is 16 bits
st_nlink resw 1 ; = 10, ditto
st_uid resd 1 ; = 12
st_gid resd 1 ; = 16
st_rdev resd 1 ; = 20
st_atime resd 1 ; = 24
st_atimensec resd 1 ; = 28
st_mtime resd 1 ; = 32
st_mtimensec resd 1 ; = 36
st_ctime resd 1 ; = 40
st_ctimensec resd 1 ; = 44
st_size resd 2 ; = 48, size is 64 bits
st_blocks resd 2 ; = 56, ditto
st_blksize resd 1 ; = 64
st_flags resd 1 ; = 68
st_gen resd 1 ; = 72
st_lspare resd 1 ; = 76
st_qspare resd 4 ; = 80
endstruc
We define the new syscalls:
%define SYS_mmap 197
%define SYS_munmap 73
%define SYS_fstat 189
%define SYS_ftruncate 201
We add the macros for their use:
%macro sys.mmap 0
system SYS_mmap
%endmacro
%macro sys.munmap 0
system SYS_munmap
%endmacro
%macro sys.ftruncate 0
system SYS_ftruncate
%endmacro
%macro sys.fstat 0
system SYS_fstat
%endmacro
And here is our code:
;;;;;;; Fast Text-to-Unix Conversion (ftuc.asm) ;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;
;;
;; Started: 21-Dec-2000
;; Updated: 22-Dec-2000
;;
;; Copyright 2000 G. Adam Stanislav.
;; All rights reserved.
;;
;;;;;;; v.1 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;
%include 'system.inc'
section .data
db 'Copyright 2000 G. Adam Stanislav.', 0Ah
db 'All rights reserved.', 0Ah
usg db 'Usage: ftuc filename', 0Ah
usglen equ $-usg
co db "ftuc: Can't open file.", 0Ah
colen equ $-co
fae db 'ftuc: File access error.', 0Ah
faelen equ $-fae
ftl db 'ftuc: File too long, use regular tuc instead.', 0Ah
ftllen equ $-ftl
mae db 'ftuc: Memory allocation error.', 0Ah
maelen equ $-mae
section .text
align 4
memerr:
push dword maelen
push dword mae
jmp short error
align 4
toolong:
push dword ftllen
push dword ftl
jmp short error
align 4
facerr:
push dword faelen
push dword fae
jmp short error
align 4
cantopen:
push dword colen
push dword co
jmp short error
align 4
usage:
push dword usglen
push dword usg
error:
push dword stderr
sys.write
push dword 1
sys.exit
align 4
global _start
_start:
pop eax ; argc
pop eax ; program name
pop ecx ; file to convert
jecxz usage
pop eax
or eax, eax ; Too many arguments?
jne usage
; Open the file
push dword O_RDWR
push ecx
sys.open
jc cantopen
mov ebp, eax ; Save fd
sub esp, byte stat_size
mov ebx, esp
; Find file size
push ebx
push ebp ; fd
sys.fstat
jc facerr
mov edx, [ebx + st_size + 4]
; File is too long if EDX != 0 ...
or edx, edx
jne near toolong
mov ecx, [ebx + st_size]
; ... or if it is above 2 GB
or ecx, ecx
js near toolong
; Do nothing if the file is 0 bytes in size
jecxz .quit
; Map the entire file in memory
push edx
push edx ; starting at offset 0
push edx ; pad
push ebp ; fd
push dword MAP_SHARED
push dword PROT_READ | PROT_WRITE
push ecx ; entire file size
push edx ; let system decide on the address
sys.mmap
jc near memerr
mov edi, eax
mov esi, eax
push ecx ; for SYS_munmap
push edi
; Use EBX for state machine
mov ebx, ordinary
mov ah, 0Ah
cld
.loop:
lodsb
call ebx
loop .loop
cmp ebx, ordinary
je .filesize
; Output final lf
mov al, ah
stosb
inc edx
.filesize:
; truncate file to new size
push dword 0 ; high dword
push edx ; low dword
push eax ; pad
push ebp
sys.ftruncate
; close it (ebp still pushed)
sys.close
add esp, byte 16
sys.munmap
.quit:
push dword 0
sys.exit
align 4
ordinary:
cmp al, 0Dh
je .cr
cmp al, ah
je .lf
stosb
inc edx
ret
align 4
.cr:
mov ebx, cr
ret
align 4
.lf:
mov ebx, lf
ret
align 4
cr:
cmp al, 0Dh
je .cr
cmp al, ah
je .lf
xchg al, ah
stosb
inc edx
xchg al, ah
; fall through
.lf:
stosb
inc edx
mov ebx, ordinary
ret
align 4
.cr:
mov al, ah
stosb
inc edx
ret
align 4
lf:
cmp al, ah
je .lf
cmp al, 0Dh
je .cr
xchg al, ah
stosb
inc edx
xchg al, ah
stosb
inc edx
mov ebx, ordinary
ret
align 4
.cr:
mov ebx, ordinary
mov al, ah
; fall through
.lf:
stosb
inc edx
ret
WARNING: Do not use this program on files stored on a disk formated by
MS DOS or Windows. There seems to be a subtle bug in the FreeBSD code
when using mmap on these drives mounted under FreeBSD: If the file is
over a certain size, mmap will just fill the memory with zeros, and then
copy them to the file overwriting its contents.
Chapter 11 – One-Pointed Mind
As a student of Zen, I like the idea of a one-pointed mind: Do one thing
at a time, and do it well.
This, indeed, is very much how Unix works as well. While a typical
Windows application is attempting to do everything imaginable (and is,
therefore, riddled with bugs), a typical Unix program does only one
thing, and it does it well.
The typical Unix user then essentially assembles his own applications by
writing a shell script which combines the various existing programs
by piping the output of one program to the input of another.
When writing your own Unix software, it is generally a good idea to
see what parts of the problem you need to solve can be handled by
existing programs, and only write your own programs for that part of the
problem that you do not have an existing solution for.
11.1. CSV
I will illustrate this principle with a specific real-life example I was
faced with recently:
I needed to extract the 11th field of each record from a database I
downloaded from a web site. The database was a CSV file, i.e., a list of
comma-separated values. That is quite a standard format for sharing
data among people who may be using different database software.
The first line of the file contains the list of various fields separated
by commas. The rest of the file contains the data listed line by line,
with values separated by commas.
I tried awk, using the comma as a separator. But because several lines
contained a quoted comma, awk was extracting the wrong field from
those lines.
Therefore, I needed to write my own software to extract the 11th field
from the CSV file. However, going with the Unix spirit, I only needed to
write a simple filter that would do the following:
Remove the first line from the file;
Change all unquoted commas to a different character;
Remove all quotation marks.
Strictly speaking, I could use sed to remove the first line from the
file, but doing so in my own program was very easy, so I decided to do
it and reduce the size of the pipeline.
At any rate, writing a program like this took me about 20 minutes.
Writing a program that extracts the 11th field from the CSV file would
take a lot longer, and I could not reuse it to extract some other
field from some other database.
N.B.: While it took me 20 minutes to write, it took me almost a day to
debug. This was because of the .code problem described in the change
log. I am just mentioning this so you do not wonder why the code
itself says it was started on one day, updated the next.
This time I decided to let it do a little more work than a typical
tutorial program would:
It parses its command line for options;
It displays proper usage if it finds wrong arguments;
It produces meaningful error messages.
Here is its usage message:
Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>]
All parameters are optional, and can appear in any order.
The -t parameter declares what to replace the commas with. The tab is
the default here. For example, -t; will replace all unquoted commas with
semicolons.
I did not need the -c option, but it may come in handy in the future. It
lets me declare that I want a character other than a comma replaced
with something else. For example, -c@ will replace all at signs
(useful if you want to split a list of email addresses to their user
names and domains).
The -p option preserves the first line, i.e., it does not delete it.
By default, we delete the first line because in a CSV file it contains
the field names rather than data.
The -i and -o options let me specify the input and the output files.
Defaults are stdin and stdout, so this is a regular Unix filter.
I made sure that both -i filename and -ifilename are accepted. I also
made sure that only one input and one output files may be specified.
To get the 11th field of each record, I can now do:
% csv '-t;' data.csv | awk '-F;' '{print $11}'
The code stores the options (except for the file descriptors) in EDX:
The comma in DH, the new separator in DL, and the flag for the -p option
in the highest bit of EDX, so a check for its sign will give us a quick
decision what to do.
Here is the code:
;;;;;;; csv.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;
;
; Convert a comma-separated file to a something-else separated file.
;
; Started: 31-May-2001
; Updated: 1-Jun-2001
;
; Copyright (c) 2001 G. Adam Stanislav
; All rights reserved.
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;
%include 'system.inc'
%define BUFSIZE 2048
section .data
fd.in dd stdin
fd.out dd stdout
usg db 'Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i
<infile>]', 0Ah
usglen equ $-usg
iemsg db "csv: Can't open input file", 0Ah
iemlen equ $-iemsg
oemsg db "csv: Can't create output file", 0Ah
oemlen equ $-oemsg
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
align 4
ierr:
push dword iemlen
push dword iemsg
push dword stderr
sys.write
push dword 1 ; return failure
sys.exit
align 4
oerr:
push dword oemlen
push dword oemsg
push dword stderr
sys.write
push dword 2
sys.exit
align 4
usage:
push dword usglen
push dword usg
push dword stderr
sys.write
push dword 3
sys.exit
align 4
global _start
_start:
add esp, byte 8 ; discard argc and argv[0]
mov edx, (',' << 8) | 9
.arg:
pop ecx
or ecx, ecx
je near .init ; no more arguments
; ECX contains the pointer to an argument
cmp byte [ecx], '-'
jne usage
inc ecx
mov ax, [ecx]
.o:
cmp al, 'o'
jne .i
; Make sure we are not asked for the output file twice
cmp dword [fd.out], stdout
jne usage
; Find the path to output file - it is either at [ECX+1],
; i.e., -ofile --
; or in the next argument,
; i.e., -o file
inc ecx
or ah, ah
jne .openoutput
pop ecx
jecxz usage
.openoutput:
push dword 420 ; file mode (644 octal)
push dword 0200h | 0400h | 01h
; O_CREAT | O_TRUNC | O_WRONLY
push ecx
sys.open
jc near oerr
add esp, byte 12
mov [fd.out], eax
jmp short .arg
.i:
cmp al, 'i'
jne .p
; Make sure we are not asked twice
cmp dword [fd.in], stdin
jne near usage
; Find the path to the input file
inc ecx
or ah, ah
jne .openinput
pop ecx
or ecx, ecx
je near usage
.openinput:
push dword 0 ; O_RDONLY
push ecx
sys.open
jc near ierr ; open failed
add esp, byte 8
mov [fd.in], eax
jmp .arg
.p:
cmp al, 'p'
jne .t
or ah, ah
jne near usage
or edx, 1 << 31
jmp .arg
.t:
cmp al, 't' ; redefine output delimiter
jne .c
or ah, ah
je near usage
mov dl, ah
jmp .arg
.c:
cmp al, 'c'
jne near usage
or ah, ah
je near usage
mov dh, ah
jmp .arg
align 4
.init:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
; See if we are to preserve the first line
or edx, edx
js .loop
.firstline:
; get rid of the first line
call getchar
cmp al, 0Ah
jne .firstline
.loop:
; read a byte from stdin
call getchar
; is it a comma (or whatever the user asked for)?
cmp al, dh
jne .quote
; Replace the comma with a tab (or whatever the user wants)
mov al, dl
.put:
call putchar
jmp short .loop
.quote:
cmp al, '"'
jne .put
; Print everything until you get another quote or EOL. If it
; is a quote, skip it. If it is EOL, print it.
.qloop:
call getchar
cmp al, '"'
je .loop
cmp al, 0Ah
je .put
call putchar
jmp short .qloop
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
ret
read:
jecxz .read
call write
.read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword [fd.in]
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
ret
align 4
.done:
call write ; flush output buffer
; close files
push dword [fd.in]
sys.close
push dword [fd.out]
sys.close
; return success
push dword 0
sys.exit
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
jecxz .ret ; nothing to write
sub edi, ecx ; start of buffer
push ecx
push edi
push dword [fd.out]
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
.ret:
ret
Much of it is taken from hex.asm above. But there is one important
difference: I no longer call write whenever I am outputing a line feed.
Yet, the code can be used interactively.
I have found a better solution for the interactive problem since I first
started writing this tutorial. I wanted to make sure each line is
printed out separately only when needed. After all, there is no need
to flush out every line when used non-interactively.
The new solution I use now is to call write every time I find the
input buffer empty. That way, when running in the interactive mode,
the program reads one line from the user’s keyboard, processes it,
and sees its input buffer is empty. It flushes its output and reads
the next line.
11.1.1. The Dark Side of Buffering
This change prevents a mysterious lockup in a very specific case. I
refer to it as the dark side of buffering, mostly because it presents
a danger that is not quite obvious.
It is unlikely to happen with a program like the csv above, so let us
consider yet another filter: In this case we expect our input to be
raw data representing color values, such as the red, green, and blue
intensities of a pixel. Our output will be the negative of our input.
Such a filter would be very simple to write. Most of it would look
just like all the other filters we have written so far, so I am only
going to show you its inner loop:
.loop:
call getchar
not al ; Create a negative
call putchar
jmp short .loop
Because this filter works with raw data, it is unlikely to be used
interactively.
But it could be called by image manipulation software. And, unless it
calls write before each call to read, chances are it will lock up.
Here is what might happen:
The image editor will load our filter using the C function popen().
It will read the first row of pixels from a bitmap or pixmap.
It will write the first row of pixels to the pipe leading to the fd.in
of our filter.
Our filter will read each pixel from its input, turn it to a negative,
and write it to its output buffer.
Our filter will call getchar to fetch the next pixel.
getchar will find an empty input buffer, so it will call read.
read will call the SYS_read system call.
The kernel will suspend our filter until the image editor sends more
data to the pipe.
The image editor will read from the other pipe, connected to the fd.
out of our filter so it can set the first row of the output image before
it sends us the second row of the input.
The kernel suspends the image editor until it receives some output
from our filter, so it can pass it on to the image editor.
At this point our filter waits for the image editor to send it more data
to process, while the image editor is waiting for our filter to send it
the result of the processing of the first row. But the result sits in
our output buffer.
The filter and the image editor will continue waiting for each other
forever (or, at least, until they are killed). Our software has just
entered a race condition.
This problem does not exist if our filter flushes its output buffer
before asking the kernel for more input data.
Chapter 12 – Using the FPU
Strangely enough, most of assembly language literature does not even
mention the existence of the FPU, or floating point unit, let alone
discuss programming it.
Yet, never does assembly language shine more than when we create
highly optimized FPU code by doing things that can be done only in
assembly language.
12.1. Organization of the FPU
The FPU consists of 8 80–bit floating–point registers. These are
organized in a stack fashion—you can push a value on TOS (top of stack)
and you can pop it.
That said, the assembly language op codes are not push and pop because
those are already taken.
You can push a value on TOS by using fld, fild, and fbld. Several
other op codes let you push many common constants—such as pi—on the
TOS.
Similarly, you can pop a value by using fst, fstp, fist, fistp, and
fbstp. Actually, only the op codes that end with a p will literally
pop the value, the rest will store it somewhere else without removing it
from the TOS.
We can transfer the data between the TOS and the computer memory
either as a 32–bit, 64–bit, or 80–bit real, a 16–bit, 32–bit, or
64–bit integer, or an 80–bit packed decimal.
The 80–bit packed decimal is a special case of binary coded decimal
which is very convenient when converting between the ASCII
representation of data and the internal data of the FPU. It allows us to
use 18 significant digits.
No matter how we represent data in the memory, the FPU always stores
it in the 80–bit real format in its registers.
Its internal precision is at least 19 decimal digits, so even if we
choose to display results as ASCII in the full 18–digit precision, we
are still showing correct results.
We can perform mathematical operations on the TOS: We can calculate
its sine, we can scale it (i.e., we can multiply or divide it by a power
of 2), we can calculate its base–2 logarithm, and many other things.
We can also multiply or divide it by, add it to, or subtract it from,
any of the FPU registers (including itself).
The official Intel op code for the TOS is st, and for the registers
st(0)–st(7). st and st(0), then, refer to the same register.
For whatever reasons, the original author of nasm has decided to use
different op codes, namely st0–st7. In other words, there are no
parentheses, and the TOS is always st0, never just st.
12.1.1. The Packed Decimal Format
The packed decimal format uses 10 bytes (80 bits) of memory to represent
18 digits. The number represented there is always an integer.
TIP: You can use it to get decimal places by multiplying the TOS by a
power of 10 first.
The highest bit of the highest byte (byte 9) is the sign bit: If it is
set, the number is negative, otherwise, it is positive. The rest of
the bits of this byte are unused/ignored.
The remaining 9 bytes store the 18 digits of the number: 2 digits per
byte.
The more significant digit is stored in the high nibble (4 bits), the
less significant digit in the low nibble.
That said, you might think that -1234567 would be stored in the memory
like this (using hexadecimal notation):
80 00 00 00 00 00 01 23 45 67
Alas it is not! As with everything else of Intel make, even the packed
decimal is little–endian.
That means our -1234567 is stored like this:
67 45 23 01 00 00 00 00 00 80
Remember that, or you will be pulling your hair out in desperation!
N.B.: The book to read—if you can find it—is Richard Startz’
8087/80287/80387 for the IBM PC & Compatibles. Though it does seem to
take the fact about the little–endian storage of the packed decimal for
granted. I kid you not about the desperation of trying to figure out
what was wrong with the filter I show below before it occurred to me I
should try the little–endian order even for this type of data.
12.2. Excursion to Pinhole Photography
To write meaningful software, we must not only understand our
programming tools, but also the field we are creating software for.
Our next filter will help us whenever we want to build a pinhole camera,
so, we need some background in pinhole photography before we can
continue.
12.2.1. The Camera
The easiest way to describe any camera ever built is as some empty space
enclosed in some lightproof material, with a small hole in the
enclosure.
The enclosure is usually sturdy (e.g., a box), though sometimes it is
flexible (the bellows). It is quite dark inside the camera. However, the
hole lets light rays in through a single point (though in some cases
there may be several). These light rays form an image, a
representation of whatever is outside the camera, in front of the hole.
If some light sensitive material (such as film) is placed inside the
camera, it can capture the image.
The hole often contains a lens, or a lens assembly, often called the
objective.
12.2.2. The Pinhole
But, strictly speaking, the lens is not necessary: The original
cameras did not use a lens but a pinhole. Even today, pinholes are used,
both as a tool to study how cameras work, and to achieve a special kind
of image.
The image produced by the pinhole is all equally sharp. Or blurred.
There is an ideal size for a pinhole: If it is either larger or smaller,
the image loses its sharpness.
12.2.3. Focal Length
This ideal pinhole diameter is a function of the square root of focal
length, which is the distance of the pinhole from the film.
D = PC * sqrt(FL)
In here, D is the ideal diameter of the pinhole, FL is the focal length,
and PC is a pinhole constant. According to Jay Bender, its value is 0.
04, while Kenneth Connors has determined it to be 0.037. Others have
proposed other values. Plus, this value is for the daylight only:
Other types of light will require a different constant, whose value
can only be determined by experimentation.
12.2.4. The F–Number
The f–number is a very useful measure of how much light reaches the
film. A light meter can determine that, for example, to expose a film of
specific sensitivity with f5.6 may require the exposure to last
1/1000 sec.
It does not matter whether it is a 35–mm camera, or a 6x9cm camera,
etc. As long as we know the f–number, we can determine the proper
exposure.
The f–number is easy to calculate:
F = FL / D
In other words, the f–number equals the focal length divided by the
diameter of the pinhole. It also means a higher f–number either implies
a smaller pinhole or a larger focal distance, or both. That, in turn,
implies, the higher the f–number, the longer the exposure has to be.
Furthermore, while pinhole diameter and focal distance are one–
dimensional measurements, both, the film and the pinhole, are two–
dimensional. That means that if you have measured the exposure at f–
number A as t, then the exposure at f–number B is:
t * (B / A)2
12.2.5. Normalized F–Number
While many modern cameras can change the diameter of their pinhole,
and thus their f–number, quite smoothly and gradually, such was not
always the case.
To allow for different f–numbers, cameras typically contained a metal
plate with several holes of different sizes drilled to them.
Their sizes were chosen according to the above formula in such a way
that the resultant f–number was one of standard f–numbers used on
all cameras everywhere. For example, a very old Kodak Duaflex IV
camera in my possession has three such holes for f–numbers 8, 11, and
16.
A more recently made camera may offer f–numbers of 2.8, 4, 5.6, 8, 11,
16, 22, and 32 (as well as others). These numbers were not chosen
arbitrarily: They all are powers of the square root of 2, though they
may be rounded somewhat.
12.2.6. The F–Stop
A typical camera is designed in such a way that setting any of the
normalized f–numbers changes the feel of the dial. It will naturally
stop in that position. Because of that, these positions of the dial
are called f–stops.
Since the f–numbers at each stop are powers of the square root of 2,
moving the dial by 1 stop will double the amount of light required for
proper exposure. Moving it by 2 stops will quadruple the required
exposure. Moving the dial by 3 stops will require the increase in
exposure 8 times, etc.
12.3. Designing the Pinhole Software
We are now ready to decide what exactly we want our pinhole software
to do.
12.3.1. Processing Program Input
Since its main purpose is to help us design a working pinhole camera, we
will use the focal length as the input to the program. This is
something we can determine without software: Proper focal length is
determined by the size of the film and by the need to shoot “regular”
pictures, wide angle pictures, or telephoto pictures.
Most of the programs we have written so far worked with individual
characters, or bytes, as their input: The hex program converted
individual bytes into a hexadecimal number, the csv program either let a
character through, or deleted it, or changed it to a different
character, etc.
One program, ftuc used the state machine to consider at most two input
bytes at a time.
But our pinhole program cannot just work with individual characters,
it has to deal with larger syntactic units.
For example, if we want the program to calculate the pinhole diameter
(and other values we will discuss later) at the focal lengths of 100 mm,
150 mm, and 210 mm, we may want to enter something like this:
100, 150, 210
Our program needs to consider more than a single byte of input at a
time. When it sees the first 1, it must understand it is seeing the
first digit of a decimal number. When it sees the 0 and the other 0,
it must know it is seeing more digits of the same number.
When it encounters the first comma, it must know it is no longer
receiving the digits of the first number. It must be able to convert the
digits of the first number into the value of 100. And the digits of the
second number into the value of 150. And, of course, the digits of
the third number into the numeric value of 210.
We need to decide what delimiters to accept: Do the input numbers have
to be separated by a comma? If so, how do we treat two numbers separated
by something else?
Personally, I like to keep it simple. Something either is a number, so I
process it. Or it is not a number, so I discard it. I don’t like the
computer complaining about me typing in an extra character when it is
obvious that it is an extra character. Duh!
Plus, it allows me to break up the monotony of computing and type in a
query instead of just a number:
What is the best pinhole diameter for the focal length of 150?
There is no reason for the computer to spit out a number of complaints:
Syntax error: What
Syntax error: is
Syntax error: the
Syntax error: best
Et cetera, et cetera, et cetera.
Secondly, I like the # character to denote the start of a comment
which extends to the end of the line. This does not take too much effort
to code, and lets me treat input files for my software as executable
scripts.
In our case, we also need to decide what units the input should come in:
We choose millimeters because that is how most photographers measure
the focus length.
Finally, we need to decide whether to allow the use of the decimal point
(in which case we must also consider the fact that much of the world
uses a decimal comma).
In our case allowing for the decimal point/comma would offer a false
sense of precision: There is little if any noticeable difference between
the focus lengths of 50 and 51, so allowing the user to input something
like 50.5 is not a good idea. This is my opinion, mind you, but I am
the one writing this program. You can make other choices in yours, of
course.
12.3.2. Offering Options
The most important thing we need to know when building a pinhole
camera is the diameter of the pinhole. Since we want to shoot sharp
images, we will use the above formula to calculate the pinhole
diameter from focal length. As experts are offering several different
values for the PC constant, we will need to have the choice.
It is traditional in Unix programming to have two main ways of
choosing program parameters, plus to have a default for the time the
user does not make a choice.
Why have two ways of choosing?
One is to allow a (relatively) permanent choice that applies
automatically each time the software is run without us having to tell it
over and over what we want it to do.
The permanent choices may be stored in a configuration file, typically
found in the user’s home directory. The file usually has the same
name as the application but is started with a dot. Often “rc” is added
to the file name. So, ours could be ~/.pinhole or ~/.pinholerc. (The ~/
means current user’s home directory.)
The configuration file is used mostly by programs that have many
configurable parameters. Those that have only one (or a few) often use a
different method: They expect to find the parameter in an environment
variable. In our case, we might look at an environment variable named
PINHOLE.
Usually, a program uses one or the other of the above methods.
Otherwise, if a configuration file said one thing, but an environment
variable another, the program might get confused (or just too
complicated).
Because we only need to choose one such parameter, we will go with the
second method and search the environment for a variable named PINHOLE.
The other way allows us to make ad hoc decisions: “Though I usually
want you to use 0.039, this time I want 0.03872.” In other words, it
allows us to override the permanent choice.
This type of choice is usually done with command line parameters.
Finally, a program always needs a default. The user may not make any
choices. Perhaps he does not know what to choose. Perhaps he is “just
browsing.” Preferably, the default will be the value most users would
choose anyway. That way they do not need to choose. Or, rather, they can
choose the default without an additional effort.
Given this system, the program may find conflicting options, and
handle them this way:
If it finds an ad hoc choice (e.g., command line parameter), it should
accept that choice. It must ignore any permanent choice and any
default.
Otherwise, if it finds a permanent option (e.g., an environment
variable), it should accept it, and ignore the default.
Otherwise, it should use the default.
We also need to decide what format our PC option should have.
At first site, it seems obvious to use the PINHOLE=0.04 format for the
environment variable, and -p0.04 for the command line.
Allowing that is actually a security risk. The PC constant is a very
small number. Naturally, we will test our software using various small
values of PC. But what will happen if someone runs the program
choosing a huge value?
It may crash the program because we have not designed it to handle
huge numbers.
Or, we may spend more time on the program so it can handle huge numbers.
We might do that if we were writing commercial software for computer
illiterate audience.
Or, we might say, “Tough! The user should know better."”
Or, we just may make it impossible for the user to enter a huge number.
This is the approach we will take: We will use an implied 0. prefix.
In other words, if the user wants 0.04, we will expect him to type -p04,
or set PINHOLE=04 in his environment. So, if he says -p9999999, we will
interpret it as 0.9999999—still ridiculous but at least safer.
Secondly, many users will just want to go with either Bender’s constant
or Connors’ constant. To make it easier on them, we will interpret
-b as identical to -p04, and -c as identical to -p037.
12.3.3. The Output
We need to decide what we want our software to send to the output, and
in what format.
Since our input allows for an unspecified number of focal length
entries, it makes sense to use a traditional database–style output of
showing the result of the calculation for each focal length on a
separate line, while separating all values on one line by a tab
character.
Optionally, we should also allow the user to specify the use of the
CSV format we have studied earlier. In this case, we will print out a
line of comma–separated names describing each field of every line, then
show our results as before, but substituting a comma for the tab.
We need a command line option for the CSV format. We cannot use -c
because that already means use Connors’ constant. For some strange
reason, many web sites refer to CSV files as “Excel spreadsheet”
(though the CSV format predates Excel). We will, therefore, use the -e
switch to inform our software we want the output in the CSV format.
We will start each line of the output with the focal length. This may
sound repetitious at first, especially in the interactive mode: The user
types in the focal length, and we are repeating it.
But the user can type several focal lengths on one line. The input can
also come in from a file or from the output of another program. In
that case the user does not see the input at all.
By the same token, the output can go to a file which we will want to
examine later, or it could go to the printer, or become the input of
another program.
So, it makes perfect sense to start each line with the focal length as
entered by the user.
No, wait! Not as entered by the user. What if the user types in
something like this:
00000000150
Clearly, we need to strip those leading zeros.
So, we might consider reading the user input as is, converting it to
binary inside the FPU, and printing it out from there.
But...
What if the user types something like this:
17459765723452353453534535353530530534563507309676764423
Ha! The packed decimal FPU format lets us input 18–digit numbers. But
the user has entered more than 18 digits. How do we handle that?
Well, we could modify our code to read the first 18 digits, enter it
to the FPU, then read more, multiply what we already have on the TOS
by 10 raised to the number of additional digits, then add to it.
Yes, we could do that. But in this program it would be ridiculous (in
a different one it may be just the thing to do): Even the
circumference of the Earth expressed in millimeters only takes 11
digits. Clearly, we cannot build a camera that large (not yet, anyway).
So, if the user enters such a huge number, he is either bored, or
testing us, or trying to break into the system, or playing games—
doing anything but designing a pinhole camera.
What will we do?
We will slap him in the face, in a manner of speaking:
17459765723452353453534535353530530534563507309676764423 ??? ???
??? ???
???
To achieve that, we will simply ignore any leading zeros. Once we find a
non–zero digit, we will initialize a counter to 0 and start taking
three steps:
Send the digit to the output.
Append the digit to a buffer we will use later to produce the packed
decimal we can send to the FPU.
Increase the counter.
Now, while we are taking these three steps, we also need to watch out
for one of two conditions:
If the counter grows above 18, we stop appending to the buffer. We
continue reading the digits and sending them to the output.
If, or rather when, the next input character is not a digit, we are done
inputting for now.
Incidentally, we can simply discard the non–digit, unless it is a #,
which we must return to the input stream. It starts a comment, so we
must see it after we are done producing output and start looking for
more input.
That still leaves one possibility uncovered: If all the user enters is a
zero (or several zeros), we will never find a non–zero to display.
We can determine this has happened whenever our counter stays at 0. In
that case we need to send 0 to the output, and perform another “slap in
the face”:
0 ??? ??? ??? ??? ???
Once we have displayed the focal length and determined it is valid
(greater than 0 but not exceeding 18 digits), we can calculate the
pinhole diameter.
It is not by coincidence that pinhole contains the word pin. Indeed,
many a pinhole literally is a pin hole, a hole carefully punched with
the tip of a pin.
That is because a typical pinhole is very small. Our formula gets the
result in millimeters. We will multiply it by 1000, so we can output the
result in microns.
At this point we have yet another trap to face: Too much precision.
Yes, the FPU was designed for high precision mathematics. But we are not
dealing with high precision mathematics. We are dealing with physics
(optics, specifically).
Suppose we want to convert a truck into a pinhole camera (we would not
be the first ones to do that!). Suppose its box is 12 meters long, so we
have the focal length of 12000. Well, using Bender’s constant, it
gives us square root of 12000 multiplied by 0.04, which is 4.381780460
millimeters, or 4381.780460 microns.
Put either way, the result is absurdly precise. Our truck is not exactly
12000 millimeters long. We did not measure its length with such a
precision, so stating we need a pinhole with the diameter of 4.381780460
millimeters is, well, deceiving. 4.4 millimeters would do just fine.
N.B.: I “only” used ten digits in the above example. Imagine the
absurdity of going for all 18!
We need to limit the number of significant digits of our result. One way
of doing it is by using an integer representing microns. So, our
truck would need a pinhole with the diameter of 4382 microns. Looking at
that number, we still decide that 4400 microns, or 4.4 millimeters is
close enough.
Additionally, we can decide that no matter how big a result we get, we
only want to display four siginificant digits (or any other number of
them, of course). Alas, the FPU does not offer rounding to a specific
number of digits (after all, it does not view the numbers as decimal but
as binary).
We, therefore, must devise an algorithm to reduce the number of
significant digits.
Here is mine (I think it is awkward—if you know a better one, please,
let me know):
Initialize a counter to 0.
While the number is greater than or equal to 10000, divide it by 10
and increase the counter.
Output the result.
While the counter is greater than 0, output 0 and decrease the counter.
N.B.: The 10000 is only good if you want four significant digits. For
any other number of significant digits, replace 10000 with 10 raised
to the number of significant digits.
We will, then, output the pinhole diameter in microns, rounded off to
four significant digits.
At this point, we know the focal length and the pinhole diameter. That
means we have enough information to also calculate the f–number.
We will display the f–number, rounded to four significant digits.
Chances are the f–number will tell us very little. To make it more
meaningful, we can find the nearest normalized f–number, i.e., the
nearest power of the square root of 2.
We do that by multiplying the actual f–number by itself, which, of
course, will give us its square. We will then calculate its base–2
logarithm, which is much easier to do than calculating the base–square
–root–of–2 logarithm! We will round the result to the nearest
integer. Next, we will raise 2 to the result. Actually, the FPU gives us
a good shortcut to do that: We can use the fscale op code to “scale”
1, which is analogous to shifting an integer left. Finally, we
calculate the square root of it all, and we have the nearest
normalized f–number.
If all that sounds overwhelming—or too much work, perhaps—it may
become much clearer if you see the code. It takes 9 op codes
altogether:
fmul st0, st0
fld1
fld st1
fyl2x
frndint
fld1
fscale
fsqrt
fstp st1
The first line, fmul st0, st0, squares the contents of the TOS (top of
the stack, same as st, called st0 by nasm). The fld1 pushes 1 on the
TOS.
The next line, fld st1, pushes the square back to the TOS. At this point
the square is both in st and st(2) (it will become clear why we leave a
second copy on the stack in a moment). st(1) contains 1.
Next, fyl2x calculates base–2 logarithm of st multiplied by st(1). That
is why we placed 1 on st(1) before.
At this point, st contains the logarithm we have just calculated,
st(1) contains the square of the actual f–number we saved for later.
frndint rounds the TOS to the nearest integer. fld1 pushes a 1. fscale
shifts the 1 we have on the TOS by the value in st(1), effectively
raising 2 to st(1).
Finally, fsqrt calculates the square root of the result, i.e., the
nearest normalized f–number.
We now have the nearest normalized f–number on the TOS, the base–2
logarithm rounded to the nearest integer in st(1), and the square of the
actual f–number in st(2). We are saving the value in st(2) for later.
But we do not need the contents of st(1) anymore. The last line, fstp
st1, places the contents of st to st(1), and pops. As a result, what was
st(1) is now st, what was st(2) is now st(1), etc. The new st
contains the normalized f–number. The new st(1) contains the square
of the actual f–number we have stored there for posterity.
At this point, we are ready to output the normalized f–number.
Because it is normalized, we will not round it off to four significant
digits, but will send it out in its full precision.
The normalized f-number is useful as long as it is reasonably small
and can be found on our light meter. Otherwise we need a different
method of determining proper exposure.
Earlier we have figured out the formula of calculating proper exposure
at an arbitrary f–number from that measured at a different f–number.
Every light meter I have ever seen can determine proper exposure at f5.
6. We will, therefore, calculate an “f5.6 multiplier,” i.e., by how
much we need to multiply the exposure measured at f5.6 to determine
the proper exposure for our pinhole camera.
From the above formula we know this factor can be calculated by dividing
our f–number (the actual one, not the normalized one) by 5.6, and
squaring the result.
Mathematically, dividing the square of our f–number by the square of
5.6 will give us the same result.
Computationally, we do not want to square two numbers when we can only
square one. So, the first solution seems better at first.
But...
5.6 is a constant. We do not have to have our FPU waste precious cycles.
We can just tell it to divide the square of the f–number by whatever
5.62 equals to. Or we can divide the f–number by 5.6, and then square
the result. The two ways now seem equal.
But, they are not!
Having studied the principles of photography above, we remember that the
5.6 is actually square root of 2 raised to the fifth power. An
irrational number. The square of this number is exactly 32.
Not only is 32 an integer, it is a power of 2. We do not need to
divide the square of the f–number by 32. We only need to use fscale
to shift it right by five positions. In the FPU lingo it means we will
fscale it with st(1) equal to -5. That is much faster than a division.
So, now it has become clear why we have saved the square of the f–
number on the top of the FPU stack. The calculation of the f5.6
multiplier is the easiest calculation of this entire program! We will
output it rounded to four significant digits.
There is one more useful number we can calculate: The number of stops
our f–number is from f5.6. This may help us if our f–number is just
outside the range of our light meter, but we have a shutter which lets
us set various speeds, and this shutter uses stops.
Say, our f–number is 5 stops from f5.6, and the light meter says we
should use 1/1000 sec. Then we can set our shutter speed to 1/1000
first, then move the dial by 5 stops.
This calculation is quite easy as well. All we have to do is to
calculate the base-2 logarithm of the f5.6 multiplier we had just
calculated (though we need its value from before we rounded it off).
We then output the result rounded to the nearest integer. We do not need
to worry about having more than four significant digits in this one:
The result is most likely to have only one or two digits anyway.
12.4. FPU Optimizations
In assembly language we can optimize the FPU code in ways impossible
in high languages, including C.
Whenever a C function needs to calculate a floating–point value, it
loads all necessary variables and constants into FPU registers. It
then does whatever calculation is required to get the correct result.
Good C compilers can optimize that part of the code really well.
It “returns” the value by leaving the result on the TOS. However,
before it returns, it cleans up. Any variables and constants it used
in its calculation are now gone from the FPU.
It cannot do what we just did above: We calculated the square of the f–
number and kept it on the stack for later use by another function.
We knew we would need that value later on. We also knew we had enough
room on the stack (which only has room for 8 numbers) to store it
there.
A C compiler has no way of knowing that a value it has on the stack will
be required again in the very near future.
Of course, the C programmer may know it. But the only recourse he has is
to store the value in a memory variable.
That means, for one, the value will be changed from the 80-bit precision
used internally by the FPU to a C double (64 bits) or even single (32
bits).
That also means that the value must be moved from the TOS into the
memory, and then back again. Alas, of all FPU operations, the ones
that access the computer memory are the slowest.
So, whenever programming the FPU in assembly language, look for the ways
of keeping intermediate results on the FPU stack.
We can take that idea even further! In our program we are using a
constant (the one we named PC).
It does not matter how many pinhole diameters we are calculating: 1, 10,
20, 1000, we are always using the same constant. Therefore, we can
optimize our program by keeping the constant on the stack all the time.
Early on in our program, we are calculating the value of the above
constant. We need to divide our input by 10 for every digit in the
constant.
It is much faster to multiply than to divide. So, at the start of our
program, we divide 10 into 1 to obtain 0.1, which we then keep on the
stack: Instead of dividing the input by 10 for every digit, we
multiply it by 0.1.
By the way, we do not input 0.1 directly, even though we could. We
have a reason for that: While 0.1 can be expressed with just one decimal
place, we do not know how many binary places it takes. We, therefore,
let the FPU calculate its binary value to its own high precision.
We are using other constants: We multiply the pinhole diameter by 1000
to convert it from millimeters to microns. We compare numbers to 10000
when we are rounding them off to four significant digits. So, we keep
both, 1000 and 10000, on the stack. And, of course, we reuse the 0.1
when rounding off numbers to four digits.
Last but not least, we keep -5 on the stack. We need it to scale the
square of the f–number, instead of dividing it by 32. It is not by
coincidence we load this constant last. That makes it the top of the
stack when only the constants are on it. So, when the square of the f–
number is being scaled, the -5 is at st(1), precisely where fscale
expects it to be.
It is common to create certain constants from scratch instead of loading
them from the memory. That is what we are doing with -5:
fld1 ; TOS = 1
fadd st0, st0 ; TOS = 2
fadd st0, st0 ; TOS = 4
fld1 ; TOS = 1
faddp st1, st0 ; TOS = 5
fchs ; TOS = -5
We can generalize all these optimizations into one rule: Keep repeat
values on the stack!
TIP: PostScript is a stack–oriented programming language. There are
many more books available about PostScript than about the FPU assembly
language: Mastering PostScript will help you master the FPU.
12.5. pinhole—The Code
;;;;;;; pinhole.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;
;
; Find various parameters of a pinhole camera construction and use
;
; Started: 9-Jun-2001
; Updated: 10-Jun-2001
;
; Copyright (c) 2001 G. Adam Stanislav
; All rights reserved.
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;
%include 'system.inc'
%define BUFSIZE 2048
section .data
align 4
ten dd 10
thousand dd 1000
tthou dd 10000
fd.in dd stdin
fd.out dd stdout
envar db 'PINHOLE=' ; Exactly 8 bytes, or 2 dwords long
pinhole db '04,', ; Bender's constant (0.04)
connors db '037', 0Ah ; Connors' constant
usg db 'Usage: pinhole [-b] [-c] [-e] [-p <value>] [-o <outfile>] [-i
<infile>]', 0Ah
usglen equ $-usg
iemsg db "pinhole: Can't open input file", 0Ah
iemlen equ $-iemsg
oemsg db "pinhole: Can't create output file", 0Ah
oemlen equ $-oemsg
pinmsg db "pinhole: The PINHOLE constant must not be 0", 0Ah
pinlen equ $-pinmsg
toobig db "pinhole: The PINHOLE constant may not exceed 18 decimal
places", 0Ah
biglen equ $-toobig
huhmsg db 9, '???'
separ db 9, '???'
sep2 db 9, '???'
sep3 db 9, '???'
sep4 db 9, '???', 0Ah
huhlen equ $-huhmsg
header db 'focal length in millimeters,pinhole diameter in microns,'
db 'F-number,normalized F-number,F-5.6 multiplier,stops '
db 'from F-5.6', 0Ah
headlen equ $-header
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
dbuffer resb 20 ; decimal input buffer
bbuffer resb 10 ; BCD buffer
section .text
align 4
huh:
call write
push dword huhlen
push dword huhmsg
push dword [fd.out]
sys.write
add esp, byte 12
ret
align 4
perr:
push dword pinlen
push dword pinmsg
push dword stderr
sys.write
push dword 4 ; return failure
sys.exit
align 4
consttoobig:
push dword biglen
push dword toobig
push dword stderr
sys.write
push dword 5 ; return failure
sys.exit
align 4
ierr:
push dword iemlen
push dword iemsg
push dword stderr
sys.write
push dword 1 ; return failure
sys.exit
align 4
oerr:
push dword oemlen
push dword oemsg
push dword stderr
sys.write
push dword 2
sys.exit
align 4
usage:
push dword usglen
push dword usg
push dword stderr
sys.write
push dword 3
sys.exit
align 4
global _start
_start:
add esp, byte 8 ; discard argc and argv[0]
sub esi, esi
.arg:
pop ecx
or ecx, ecx
je near .getenv ; no more arguments
; ECX contains the pointer to an argument
cmp byte [ecx], '-'
jne usage
inc ecx
mov ax, [ecx]
inc ecx
.o:
cmp al, 'o'
jne .i
; Make sure we are not asked for the output file twice
cmp dword [fd.out], stdout
jne usage
; Find the path to output file - it is either at [ECX+1],
; i.e., -ofile --
; or in the next argument,
; i.e., -o file
or ah, ah
jne .openoutput
pop ecx
jecxz usage
.openoutput:
push dword 420 ; file mode (644 octal)
push dword 0200h | 0400h | 01h
; O_CREAT | O_TRUNC | O_WRONLY
push ecx
sys.open
jc near oerr
add esp, byte 12
mov [fd.out], eax
jmp short .arg
.i:
cmp al, 'i'
jne .p
; Make sure we are not asked twice
cmp dword [fd.in], stdin
jne near usage
; Find the path to the input file
or ah, ah
jne .openinput
pop ecx
or ecx, ecx
je near usage
.openinput:
push dword 0 ; O_RDONLY
push ecx
sys.open
jc near ierr ; open failed
add esp, byte 8
mov [fd.in], eax
jmp .arg
.p:
cmp al, 'p'
jne .c
or ah, ah
jne .pcheck
pop ecx
or ecx, ecx
je near usage
mov ah, [ecx]
.pcheck:
cmp ah, '0'
jl near usage
cmp ah, '9'
ja near usage
mov esi, ecx
jmp .arg
.c:
cmp al, 'c'
jne .b
or ah, ah
jne near usage
mov esi, connors
jmp .arg
.b:
cmp al, 'b'
jne .e
or ah, ah
jne near usage
mov esi, pinhole
jmp .arg
.e:
cmp al, 'e'
jne near usage
or ah, ah
jne near usage
mov al, ','
mov [huhmsg], al
mov [separ], al
mov [sep2], al
mov [sep3], al
mov [sep4], al
jmp .arg
align 4
.getenv:
; If ESI = 0, we did not have a -p argument,
; and need to check the environment for "PINHOLE="
or esi, esi
jne .init
sub ecx, ecx
.nextenv:
pop esi
or esi, esi
je .default ; no PINHOLE envar found
; check if this envar starts with 'PINHOLE='
mov edi, envar
mov cl, 2 ; 'PINHOLE=' is 2 dwords long
rep cmpsd
jne .nextenv
; Check if it is followed by a digit
mov al, [esi]
cmp al, '0'
jl .default
cmp al, '9'
jbe .init
; fall through
align 4
.default:
; We got here because we had no -p argument,
; and did not find the PINHOLE envar.
mov esi, pinhole
; fall through
align 4
.init:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
sub edx, edx
mov edi, dbuffer+1
mov byte [dbuffer], '0'
; Convert the pinhole constant to real
.constloop:
lodsb
cmp al, '9'
ja .setconst
cmp al, '0'
je .processconst
jb .setconst
inc dl
.processconst:
inc cl
cmp cl, 18
ja near consttoobig
stosb
jmp short .constloop
align 4
.setconst:
or dl, dl
je near perr
finit
fild dword [tthou]
fld1
fild dword [ten]
fdivp st1, st0
fild dword [thousand]
mov edi, obuffer
mov ebp, ecx
call bcdload
.constdiv:
fmul st0, st2
loop .constdiv
fld1
fadd st0, st0
fadd st0, st0
fld1
faddp st1, st0
fchs
; If we are creating a CSV file,
; print header
cmp byte [separ], ','
jne .bigloop
push dword headlen
push dword header
push dword [fd.out]
sys.write
.bigloop:
call getchar
jc near done
; Skip to the end of the line if you got '#'
cmp al, '#'
jne .num
call skiptoeol
jmp short .bigloop
.num:
; See if you got a number
cmp al, '0'
jl .bigloop
cmp al, '9'
ja .bigloop
; Yes, we have a number
sub ebp, ebp
sub edx, edx
.number:
cmp al, '0'
je .number0
mov dl, 1
.number0:
or dl, dl ; Skip leading 0's
je .nextnumber
push eax
call putchar
pop eax
inc ebp
cmp ebp, 19
jae .nextnumber
mov [dbuffer+ebp], al
.nextnumber:
call getchar
jc .work
cmp al, '#'
je .ungetc
cmp al, '0'
jl .work
cmp al, '9'
ja .work
jmp short .number
.ungetc:
dec esi
inc ebx
.work:
; Now, do all the work
or dl, dl
je near .work0
cmp ebp, 19
jae near .toobig
call bcdload
; Calculate pinhole diameter
fld st0 ; save it
fsqrt
fmul st0, st3
fld st0
fmul st5
sub ebp, ebp
; Round off to 4 significant digits
.diameter:
fcom st0, st7
fstsw ax
sahf
jb .printdiameter
fmul st0, st6
inc ebp
jmp short .diameter
.printdiameter:
call printnumber ; pinhole diameter
; Calculate F-number
fdivp st1, st0
fld st0
sub ebp, ebp
.fnumber:
fcom st0, st6
fstsw ax
sahf
jb .printfnumber
fmul st0, st5
inc ebp
jmp short .fnumber
.printfnumber:
call printnumber ; F number
; Calculate normalized F-number
fmul st0, st0
fld1
fld st1
fyl2x
frndint
fld1
fscale
fsqrt
fstp st1
sub ebp, ebp
call printnumber
; Calculate time multiplier from F-5.6
fscale
fld st0
; Round off to 4 significant digits
.fmul:
fcom st0, st6
fstsw ax
sahf
jb .printfmul
inc ebp
fmul st0, st5
jmp short .fmul
.printfmul:
call printnumber ; F multiplier
; Calculate F-stops from 5.6
fld1
fxch st1
fyl2x
sub ebp, ebp
call printnumber
mov al, 0Ah
call putchar
jmp .bigloop
.work0:
mov al, '0'
call putchar
align 4
.toobig:
call huh
jmp .bigloop
align 4
done:
call write ; flush output buffer
; close files
push dword [fd.in]
sys.close
push dword [fd.out]
sys.close
finit
; return success
push dword 0
sys.exit
align 4
skiptoeol:
; Keep reading until you come to cr, lf, or eof
call getchar
jc done
cmp al, 0Ah
jne .cr
ret
.cr:
cmp al, 0Dh
jne skiptoeol
ret
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
clc
ret
read:
jecxz .read
call write
.read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword [fd.in]
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .empty
sub eax, eax
ret
align 4
.empty:
add esp, byte 4
stc
ret
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
jecxz .ret ; nothing to write
sub edi, ecx ; start of buffer
push ecx
push edi
push dword [fd.out]
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
.ret:
ret
align 4
bcdload:
; EBP contains the number of chars in dbuffer
push ecx
push esi
push edi
lea ecx, [ebp+1]
lea esi, [dbuffer+ebp-1]
shr ecx, 1
std
mov edi, bbuffer
sub eax, eax
mov [edi], eax
mov [edi+4], eax
mov [edi+2], ax
.loop:
lodsw
sub ax, 3030h
shl al, 4
or al, ah
mov [edi], al
inc edi
loop .loop
fbld [bbuffer]
cld
pop edi
pop esi
pop ecx
sub eax, eax
ret
align 4
printnumber:
push ebp
mov al, [separ]
call putchar
; Print the integer at the TOS
mov ebp, bbuffer+9
fbstp [bbuffer]
; Check the sign
mov al, [ebp]
dec ebp
or al, al
jns .leading
; We got a negative number (should never happen)
mov al, '-'
call putchar
.leading:
; Skip leading zeros
mov al, [ebp]
dec ebp
or al, al
jne .first
cmp ebp, bbuffer
jae .leading
; We are here because the result was 0.
; Print '0' and return
mov al, '0'
jmp putchar
.first:
; We have found the first non-zero.
; But it is still packed
test al, 0F0h
jz .second
push eax
shr al, 4
add al, '0'
call putchar
pop eax
and al, 0Fh
.second:
add al, '0'
call putchar
.next:
cmp ebp, bbuffer
jb .done
mov al, [ebp]
push eax
shr al, 4
add al, '0'
call putchar
pop eax
and al, 0Fh
add al, '0'
call putchar
dec ebp
jmp short .next
.done:
pop ebp
or ebp, ebp
je .ret
.zeros:
mov al, '0'
call putchar
dec ebp
jne .zeros
.ret:
ret
The code follows the same format as all the other filters we have seen
before, with one subtle exception:
We are no longer assuming that the end of input implies the end of
things to do, something we took for granted in the character–oriented
filters.
This filter does not process characters. It processes a language (albeit
a very simple one, consisting only of numbers).
When we have no more input, it can mean one of two things:
We are done and can quit. This is the same as before.
The last character we have read was a digit. We have stored it at the
end of our ASCII–to–float conversion buffer. We now need to convert
the contents of that buffer into a number and write the last line of our
output.
For that reason, we have modified our getchar and our read routines to
return with the carry flag clear whenever we are fetching another
character from the input, or the carry flag set whenever there is no
more input.
Of course, we are still using assembly language magic to do that! Take a
good look at getchar. It always returns with the carry flag clear.
Yet, our main code relies on the carry flag to tell it when to quit—and
it works.
The magic is in read. Whenever it receives more input from the system,
it just returns to getchar, which fetches a character from the input
buffer, clears the carry flag and returns.
But when read receives no more input from the system, it does not return
to getchar at all. Instead, the add esp, byte 4 op code adds 4 to ESP,
sets the carry flag, and returns.
So, where does it return to? Whenever a program uses the call op code,
the microprocessor pushes the return address, i.e., it stores it on
the top of the stack (not the FPU stack, the system stack, which is in
the memory). When a program uses the ret op code, the microprocessor
pops the return value from the stack, and jumps to the address that
was stored there.
But since we added 4 to ESP (which is the stack pointer register), we
have effectively given the microprocessor a minor case of amnesia: It no
longer remembers it was getchar that called read.
And since getchar never pushed anything before calling read, the top
of the stack now contains the return address to whatever or whoever
called getchar. As far as that caller is concerned, he called getchar,
which returned with the carry flag set!
Other than that, the bcdload routine is caught up in the middle of a
Lilliputian conflict between the Big–Endians and the Little–Endians.
It is converting the text representation of a number into that number:
The text is stored in the big–endian order, but the packed decimal is
little–endian.
To solve the conflict, we use the std op code early on. We cancel it
with cld later on: It is quite important we do not call anything that
may depend on the default setting of the direction flag while std is
active.
Everything else in this code should be quite clear, providing you have
read the entire chapter that precedes it.
It is a classical example of the adage that programming requires a lot
of thought and only a little coding. Once we have thought through
every tiny detail, the code almost writes itself.
12.6. Using pinhole
Because we have decided to make the program ignore any input except
for numbers (and even those inside a comment), we can actually perform
textual queries. We do not have to, but we can.
In my humble opinion, forming a textual query, instead of having to
follow a very strict syntax, makes software much more user friendly.
Suppose we want to build a pinhole camera to use the 4x5 inch film.
The standard focal length for that film is about 150mm. We want to fine
–tune our focal length so the pinhole diameter is as round a number
as possible. Let us also suppose we are quite comfortable with cameras
but somewhat intimidated by computers. Rather than just have to type
in a bunch of numbers, we want to ask a couple of questions.
Our session might look like this:
% pinhole
Computer,
What size pinhole do I need for the focal length of 150?
150 490 306 362 2930 12
Hmmm... How about 160?
160 506 316 362 3125 12
Let's make it 155, please.
155 498 311 362 3027 12
Ah, let's try 157...
157 501 313 362 3066 12
156?
156 500 312 362 3047 12
That's it! Perfect! Thank you very much!
^D
We have found that while for the focal length of 150, our pinhole
diameter should be 490 microns, or 0.49 mm, if we go with the almost
identical focal length of 156 mm, we can get away with a pinhole
diameter of exactly one half of a millimeter.
12.7. Scripting
Because we have chosen the # character to denote the start of a comment,
we can treat our pinhole software as a scripting language.
You have probably seen shell scripts that start with:
#! /bin/sh
...or...
#!/bin/sh
...because the blank space after the #! is optional.
Whenever Unix is asked to run an executable file which starts with the
#!, it assumes the file is a script. It adds the command to the rest
of the first line of the script, and tries to execute that.
Suppose now that we have installed pinhole in /usr/local/bin/, we can
now write a script to calculate various pinhole diameters suitable for
various focal lengths commonly used with the 120 film.
The script might look something like this:
#! /usr/local/bin/pinhole -b -i
# Find the best pinhole diameter
# for the 120 film
### Standard
80
### Wide angle
30, 40, 50, 60, 70
### Telephoto
100, 120, 140
Because 120 is a medium size film, we may name this file medium.
We can set its permissions to execute, and run it as if it were a
program:
% chmod 755 medium
% ./medium
Unix will interpret that last command as:
% /usr/local/bin/pinhole -b -i ./medium
It will run that command and display:
80 358 224 256 1562 11
30 219 137 128 586 9
40 253 158 181 781 10
50 283 177 181 977 10
60 310 194 181 1172 10
70 335 209 181 1367 10
100 400 250 256 1953 11
120 438 274 256 2344 11
140 473 296 256 2734 11
Now, let us enter:
% ./medium -c
Unix will treat that as:
% /usr/local/bin/pinhole -b -i ./medium -c
That gives it two conflicting options: -b and -c (Use Bender’s constant
and use Connors’ constant). We have programmed it so later options
override early ones—our program will calculate everything using
Connors’ constant:
80 331 242 256 1826 11
30 203 148 128 685 9
40 234 171 181 913 10
50 262 191 181 1141 10
60 287 209 181 1370 10
70 310 226 256 1598 11
100 370 270 256 2283 11
120 405 296 256 2739 11
140 438 320 362 3196 12
We decide we want to go with Bender’s constant after all. We want to
save its values as a comma–separated file:
% ./medium -b -e > bender
% cat bender
focal length in millimeters,pinhole diameter in microns,F-number,
normalized F-number,F-5.6 multiplier,stops from F-5.6
80,358,224,256,1562,11
30,219,137,128,586,9
40,253,158,181,781,10
50,283,177,181,977,10
60,310,194,181,1172,10
70,335,209,181,1367,10
100,400,250,256,1953,11
120,438,274,256,2344,11
140,473,296,256,2734,11
%
Chapter 13 – Caveats
Assembly language programmers who “grew up” under MS DOS and Windows
often tend to take shortcuts. Reading the keyboard scan codes and
writing directly to video memory are two classical examples of practices
which, under MS DOS are not frowned upon but considered the right thing
to do.
The reason? Both the PC BIOS and MS DOS are notoriously slow when
performing these operations.
You may be tempted to continue similar practices in the Unix
environment. For example, I have seen a web site which explains how to
access the keyboard scan codes on a popular Unix clone.
That is generally a very bad idea in Unix environment! Let me explain
why.
13.1. Unix Is Protected
For one thing, it may simply not be possible. Unix runs in protected
mode. Only the kernel and device drivers are allowed to access
hardware directly. Perhaps a particular Unix clone will let you read the
keyboard scan codes, but chances are a real Unix operating system
will not. And even if one version may let you do it, the next one may
not, so your carefully crafted software may become a dinosaur
overnight.
13.2. Unix Is an Abstraction
But there is a much more important reason not to try accessing the
hardware directly (unless, of course, you are writing a device driver),
even on the Unix-like systems that let you do it:
Unix is an abstraction!
There is a major difference in the philosophy of design between MS DOS
and Unix. MS DOS was designed as a single-user system. It is run on a
computer with a keyboard and a video screen attached directly to that
computer. User input is almost guaranteed to come from that keyboard.
Your program’s output virtually always ends up on that screen.
This is NEVER guaranteed under Unix. It is quite common for a Unix
user to pipe and redirect program input and output:
% program1 | program2 | program3 > file1
If you have written program2, your input does not come from the keyboard
but from the output of program1. Similarly, your output does not go
to the screen but becomes the input for program3 whose output, in turn,
goes to file1.
But there is more! Even if you made sure that your input comes from, and
your output goes to, the terminal, there is no guarantee the terminal
is a PC: It may not have its video memory where you expect it, nor may
its keyboard be producing PC-style scan codes. It may be a Macintosh, or
any other computer.
Now you may be shaking your head: My software is in PC assembly
language, how can it run on a Macintosh? But I did not say your software
would be running on a Macintosh, only that its terminal may be a
Macintosh.
Under Unix, the terminal does not have to be directly attached to the
computer that runs your software, it can even be on another continent,
or, for that matter, on another planet. It is perfectly possible that
a Macintosh user in Australia connects to a Unix system in North America
(or anywhere else) via telnet. The software then runs on one computer,
while the terminal is on a different computer: If you try to read the
scan codes, you will get the wrong input!
Same holds true about any other hardware: A file you are reading may
be on a disk you have no direct access to. A camera you are reading
images from may be on a space shuttle, connected to you via satellites.
That is why under Unix you must never make any assumptions about where
your data is coming from and going to. Always let the system handle
the physical access to the hardware.
N.B.: These are caveats, not absolute rules. Exceptions are possible.
For example, if a text editor has determined it is running on a local
machine, it may want to read the scan codes directly for improved
control. I am not mentioning these caveats to tell you what to do or
what not to do, just to make you aware of certain pitfalls that await
you if you have just arrived to Unix form MS DOS. Of course, creative
people often break rules, and it is OK as long as they know they are
breaking them and why.
------------------------------------------------------------------------
--------
Appendix A – Assembly Language Pearls
Here are some other assembly language articles I have written since
starting this tutorial. They are on separate pages mostly to keep this
tutorial down to a manageable size.
String Length – learn how to calculate the length of a text string in
assembly language.
Smallest Unix Program – see how we can shrink the smallest Unix
program.
Appendix B – BSD Style Copyright
If you wish to release your software with a BSD-style copyright, you can
find the file bsd-style-copyright in several places on your FreeBSD
computer. But it uses C-style comments.
I have edited it for inclusion in assembly language programs. If you
want it, download the nasm-compatible BSD-style copyright, insert your
name, and include it in your source code.
Appendix C – Change Log
Originally, this tutorial used .code as the name of the code section.
Traditionally, the name is .text. I used .code because it seemed more
intuitive, and it worked anyway.
Alas, as the newer versions of FreeBSD have upgraded to a new version of
ld, it does not work right anymore. That is why I have now changed
all occurances of .code to .text in this tutorial. Please, do so in your
own code as well.
Appendix D – Acknowledgements
This tutorial would never have been possible without the help of many
experienced FreeBSD programmers from the FreeBSD hackers mailing list,
many of whom have patiently answered my questions, and pointed me in the
right direction in my attempts to explore the inner workings of Unix
system programming in general and FreeBSD in particular.
Thomas M. Sommers opened the door for me. His How do I write “Hello,
world” in FreeBSD assembler? web page was my first encounter with an
example of assembly language programming under FreeBSD.
Jake Burkholder has kept the door open by willingly answering all of
my questions and supplying me with example assembly language source
code.
Copyright ? 2000-2001 G. Adam Stanislav.
All rights reserved.
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