Assembly

[Scott Lanning]

On Sat, 29 Jan 2000, Anthony J. Gabrielson wrote:
>	I have two questions.  First can anybody tell me about the Linux
>assembler, I think its called gas - but not sure.  Second does anyone know
>where I can find all the interrupt functions listed?  I am getting into
>assembley and need a few pointers so as always any help is appreciated.

Read the assembly howto. The author has a webpage

    http://lightning.voshod.com/asm/

which is very good on Linux assembly w/o using C library calls
(i.e., using system calls). So it's similar to DOS "int 21h"
in that you set 'eax' and other registers, then "int 80h"
which will call the corresponding system call. On that webpage
is given a nice list of system calls and their corresponding
numbers (which you put in eax). The advantage of using
system calls directly rather than calling C lib functions
is you reduce alot of overhead (which the guy on that webpage
demonstrates), but on the other hand you have to implement
the functions yourself. As an example, I created a version of
the 'cat' utitility which is 444 bytes long vs. 10492 bytes
of the one with Red Hat 6.1 (it only does one file specified
on the command line, though, not accept stdin or multiple files).
The effect is rather dramatic, I think , and besides it's fun
to play with. At the moment, time permitting, I am trying to
be able to use sys_socketcall to connect to another host, but
haven't had much success so far.
    I suggest one method to learn how some assembly in gas works
is to use to make test C programs, say foo.c, then compile with

    as -S foo.c

then examine the resulting foo.s file. For an example, I provide
here my cat.s file; I only just started a while ago, so it's
probably pretty primitive:

-------------------------
	BUF_LEN  = 8192
	O_RDONLY = 0

	.lcomm	buf, BUF_LEN    #buffer for reading

.text
	.globl _start		#entry point declared for linker (ld)
_start:
        pop     %ecx		#pop argument count
        dec     %ecx		#is there anything on command line?
        jz      exit		#no, exit

        pop     %ecx		#skip name of program

	movl	$5, %eax	#sys_open
        pop     %ebx		#filename
	movl	$O_RDONLY, %ecx	#mode
	int	$0x80		#call sys_open

	movl	%eax, %ebp	#fd returned from open()
	
	test	%eax,%eax	#have we opened file?
	jns	.read		#yes, read it
	jmp	exit            #no, exit
	
.read:
	movl	$buf, %ecx	#buffer

.loop:	
	movl	$3, %eax	#sys_read
	movl	%ebp, %ebx	#fd
	movl	$BUF_LEN, %edx	#buffer length
	int	$0x80		#call sys_read

	test	%eax, %eax      #is zero?
	jz	exit            #yes, exit

	movl	%eax, %edx
	movl	$4, %eax	#sys_write
	movl	$1, %ebx	#fd == 1 == stdout
	int	$0x80

	jmp	.loop
exit:
	movl	$1, %eax
	movl	$0, %ebx
	int	$0x80
-----------------------------------

Also, it is useful to have some macros, which you use
the .include command for, like

    .include "macros.s"

then in macros.s, have some things like the following

------------------------
# WRITE OUTPUT TO STDOUT
	#(modifies registers eca,edb)
	#you must set ecx and edx before calling this function
	#ecx = pointer to buffer to output
	#edx = how many bytes to output
	
	.macro  PRINT
	movl $4, %eax		# number of system call == 4
	movl $1, %ebx           # file descriptor == 1 (stdout)
	int  $0x80
        .endm


# EXIT
	.macro  EXIT status
	movl $1, %eax
	movl $\status, %ebx
	int $0x80
        .endm
	
here:
	.ascii "here\n\0"
	STRLEN = . - here

# DEBUG
	.macro	DEBUG
	pushl %eax
	pushl %ebx
	pushl %ecx
	pushl %edx
	movl $here, %ecx
	movl $STRLEN, %edx
	PRINT
	popl %edx
	popl %ecx
	popl %ebx
	popl %eax
	.endm
---------------------------

and call them like

    EXIT 0

would exit your program with a sucessful status.

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