Thread: AT&T Assembler

I'm learning AT&T syntax assembly, and as a little exercise want to write a program that outputs the number of command line arguments [including program name].

As I understand it, the number of args [argc to C gurus] is the first value to be pushed to the stack when a program executes?

Here's what I have so far:

Code:

		.section .data
argnum:
		.ascii   ""

		.section .text
		.globl _start

_start:
		movl $4, %eax
		movl $1, %ebx
		movl (%esp), %edx
		movl %edx, argnum
		movl argnum, %ecx
		movl $2, %edx
		int  $0x80

		movl $1, %eax
		xorl %ebx, %ebx
		int  $0x80

It compiles and runs fine, but nothing visible is output. I think it might have something to do with sys_write not outputting numbers, so that's why I used an ascii var [argnum], with no avail.

Some points
1. .ascii ""
Does this reserve enough space - it seems like it's just 1 byte containing \0 to me

2. movl %edx, argnum
a) you need to convert to ascii, say by adding '0' to it.
b) you need to move a byte, not a long
Note this only works for argc in the range 0..9, for larger numbers, the process gets more complicated.

3. movl argnum, %ecx
I'm guessing you wanted to move the address here, and not the contents?

Thanks for your help I'll have a fiddle and post up any other problems

Hmm still no joy - I movb'd instead, and it [as] warned me it was using %dl instead. I now move the address of argnum into %ecx, but I don't know what you mean by converting it to ascii (I don't know how)

As you can imagine, here it is now:

Code:

		.section .data
argnum:
		.ascii   ""

		.section .text
		.globl _start

_start:
		movl $4, %eax
		movl $1, %ebx
		movl (%esp), %edx
		movb %edx, argnum
		movl (argnum), %ecx
		movl $2, %edx
		int  $0x80

		movl $1, %eax
		xorl %ebx, %ebx
		int  $0x80

Random guesses....

.ascii "0" # space for a single char, which has a \0 following it.

addb %edx,$48 # convert 0 into '0' say
movb %edx, argnum