converting Fasm to gas

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fpga

Joined: 22 Sep 2009
Posts: 36

fpga 07 Oct 2009, 20:03

Although it runs fine I'm struggling to profile my c++ program having linked it with a Fasm.o file. I'm therefore converting it to gas using the '.intel_syntax noprefix' directive in the hope that gprof will like it.

I'm unsure how you ensure that all of "some_string", 0 goes into ebx?
In Fasm I have

Code:

section ".text" ;=================
mov ebx, label_1
section ".data" ;=================
label_1  db "some_string", 0

In Fasm its as if these two statements work out that 'label_1' represents the address and length of the .asciz string "some_string".

I'm unsure of how you'd instruct ebx to take this number of bytes from the correct address using standard intel syntax.

I think 'movzx ecx, byte [some_label]' is done by saying 'movb ecx, [some_label]'

07 Oct 2009, 20:03

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 07 Oct 2009, 21:58

Quote:

In Fasm its as if these two statements work out that 'label_1' represents the address and length of the .asciz string "some_string".

Just to clarify, are you saying that EBX is loaded with the string's address and length?

07 Oct 2009, 21:58

fpga

Joined: 22 Sep 2009
Posts: 36

fpga 08 Oct 2009, 09:13

Thank you for your response.
I did think that last night but not any more.
I couldn't work out how the end of the string "some_string" was detected.
I now think the two statements above just load only the address (label_1) of the asciz string into ebx.
The string is a device file name and last night I was wondering how the program "knew" where the end of that filename, ie the 0, was.

I now think the system call, denoted by 5 in eax, which opens the file, named "some_string", only needs the string's start address in ebx i.e. I think it then moves along from that address until it finds the '0' and then opens the file with that string name.
Here's the Fasm code.

Code:

section ".text" ;=================
   mov eax, 5                 ;open the file
   mov ebx, label_1           ;file name
   mov ecx, 02o                   ;read and write    
   mov edx, 600o         ;read and write for the user
   int   80h                            ;open
section ".data" ;=================
   label_1  db "some_string", 0

I asked on the masm32 forum and Greg very kindly posted this conversion

Code:

.intel_syntax noprefix
.arch pentium
.global _start
.data
    label_1: .asciz "some_string"
.text
    mov ebx, OFFSET label_1
.end

08 Oct 2009, 09:13

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