Programming Forums > Application Development > Assembly

Lance · Dec 26th, 2004, 3:06 AM

So I'm fairly new at Assembly, but I get the jist of it. I'm trying to learn AT&T syntax, and the sort, but it's proving a bit difficult to adjust to.

This is supposed to be an infinate loop incrementation, that just prints the numbers as they increment. That's it. Can anyone point out where I went wrong? All criticism and tips would be great, as long as their is an explaination why I did it wrong.

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.bss
count:
    .long 0

.text
.globl main

main:
    movl  $5,%eax
    movl  $1,%ebx
repeat:
    movl  $count,%ecx
    movl  $count,%edx
    int   $0x80

    incl  count
    jmp   repeat

    ret

I'm pretty sure using the BSS segment as a var for a simple incrementation is stupid, but I'm trying to figure out how using variables really works. I've tried this with the %esp and #esi register just to see if my usage of $count is wrong, but it turns out the same.

Any help would be nice.

Lance · Dec 26th, 2004, 2:00 PM

Well that was stupid... I was using syscall 5 (open) the entire time, when I meant 4 (write). Either way, it still doesn't work.

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.section .bss
buf:
 * * * *.long 0

.section .text
.globl main

main:
 * * * *movl * *$4,%eax
 * * * *movl * *$1,%ebx
 * * * *jmp loop
loop:
 * * * *movl * *$buf,%ecx
 * * * *movl * *$1,%edx

 * * * *int * * $0x80

 * * * *incl * *buf
 * * * *jmp * * loop

 * * * *ret

If I remove loop and just jmp main it starts printing random garbage:

123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????

123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????

Over and over, until i ^C'ed it.

So then I wrote it in C...

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#include <stdio.h>

int i;

int main()
{
    while (1)
        write(1, i++, 1);
    return 0;
}

...and ran gcc -S -O0 -o inc_c.s inc.c on it to see the assembly:

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    .file  "inc.c"
    .text
.globl main
    .type  main,@function
main:
    pushl  %ebp
    movl  %esp, %ebp
    subl  $8, %esp
    andl  $-16, %esp
    movl  $0, %eax
    subl  %eax, %esp
.L2:
    subl  $4, %esp
    pushl  $1
    movl  i, %eax
    pushl  %eax
    incl  i
    pushl  $1
    call  write
    addl  $16, %esp
    jmp   .L2
.Lfe1:
    .size  main,.Lfe1-main
    .comm  i,4,4
    .ident "GCC: (GNU) 3.2.2"

Any help here? It apparently stores 'i' in an... automatic variable? I'm a bit confused...

Lance · Dec 26th, 2004, 5:16 PM

Well, so I did it. But it doesn't print just numbers, but text. I forgot write() prints ASCII only.

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.bss
    .comm buf,1,16

.text
.globl main

main:
    movl  $4,%eax
    movl  $1,%ebx
    movl  $buf,%ecx
    movl  $1,%edx

    int   $0x80

    incl  buf
    jmp   main

    ret

So I guess that's as far as I can go without forcing output in the form of the ASCII codes for numbers. :/ If you have a faster way to write this, just tell me. I ponder writing it with the stack instead of common data instead. That would probably be better, but I don't know.

Dec 26th, 2004, 3:06 AM	#1
Lance Programmer Join Date: Oct 2004 Location: Chicago, IL USA Posts: 74 Rep Power: 4	So I'm fairly new at Assembly, but I get the jist of it. I'm trying to learn AT&T syntax, and the sort, but it's proving a bit difficult to adjust to. This is supposed to be an infinate loop incrementation, that just prints the numbers as they increment. That's it. Can anyone point out where I went wrong? All criticism and tips would be great, as long as their is an explaination why I did it wrong. (Toggle Plain Text) .bss count: .long 0 .text .globl main main: movl $5,%eax movl $1,%ebx repeat: movl $count,%ecx movl $count,%edx int $0x80 incl count jmp repeat ret .bss count: .long 0 .text .globl main main: movl $5,%eax movl $1,%ebx repeat: movl $count,%ecx movl $count,%edx int $0x80 incl count jmp repeat ret I'm pretty sure using the BSS segment as a var for a simple incrementation is stupid, but I'm trying to figure out how using variables really works. I've tried this with the %esp and #esi register just to see if my usage of $count is wrong, but it turns out the same. Any help would be nice. __________________ (Toggle Plain Text) /* LANCE / C++; / this makes C bigger but returns the old value / char site = "slackwise.net", home = "lance.slackwise.net", pics = "flickr.com/photos/slackwise"; /* LANCE / C++; / this makes C bigger but returns the old value / char site = "slackwise.net", home = "lance.slackwise.net", pics = "flickr.com/photos/slackwise";

Dec 26th, 2004, 2:00 PM	#2
Lance Programmer Join Date: Oct 2004 Location: Chicago, IL USA Posts: 74 Rep Power: 4	Well that was stupid... I was using syscall 5 (open) the entire time, when I meant 4 (write). Either way, it still doesn't work. (Toggle Plain Text) .section .bss buf: * * * .long 0 .section .text .globl main main: * * movl $4,%eax * * movl $1,%ebx * * jmp loop loop: * * movl $buf,%ecx * * movl $1,%edx * * int * $0x80 * * * incl buf * * jmp * loop * * * ret .section .bss buf: * * .long 0 .section .text .globl main main: * * movl $4,%eax * * movl $1,%ebx * * jmp loop loop: * * movl $buf,%ecx * * movl $1,%edx * * int * $0x80 * * * incl buf * * jmp * loop * * * ret If I remove loop and just jmp main it starts printing random garbage: 123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{\|}~???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????? 123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{\|}~???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????? Over and over, until i ^C'ed it. So then I wrote it in C... (Toggle Plain Text) #include <stdio.h> int i; int main() { while (1) write(1, i++, 1); return 0; } #include <stdio.h> int i; int main() { while (1) write(1, i++, 1); return 0; } ...and ran gcc -S -O0 -o inc_c.s inc.c on it to see the assembly: (Toggle Plain Text) .file "inc.c" .text .globl main .type main,@function main: pushl %ebp movl %esp, %ebp subl $8, %esp andl $-16, %esp movl $0, %eax subl %eax, %esp .L2: subl $4, %esp pushl $1 movl i, %eax pushl %eax incl i pushl $1 call write addl $16, %esp jmp .L2 .Lfe1: .size main,.Lfe1-main .comm i,4,4 .ident "GCC: (GNU) 3.2.2" .file "inc.c" .text .globl main .type main,@function main: pushl %ebp movl %esp, %ebp subl $8, %esp andl $-16, %esp movl $0, %eax subl %eax, %esp .L2: subl $4, %esp pushl $1 movl i, %eax pushl %eax incl i pushl $1 call write addl $16, %esp jmp .L2 .Lfe1: .size main,.Lfe1-main .comm i,4,4 .ident "GCC: (GNU) 3.2.2" Any help here? It apparently stores 'i' in an... automatic variable? I'm a bit confused... __________________ (Toggle Plain Text) / LANCE / C++; / this makes C bigger but returns the old value / char site = "slackwise.net", home = "lance.slackwise.net", pics = "flickr.com/photos/slackwise"; /* LANCE / C++; / this makes C bigger but returns the old value / char site = "slackwise.net", home = "lance.slackwise.net", pics = "flickr.com/photos/slackwise";

Dec 26th, 2004, 5:16 PM	#3
Lance Programmer Join Date: Oct 2004 Location: Chicago, IL USA Posts: 74 Rep Power: 4	Well, so I did it. But it doesn't print just numbers, but text. I forgot write() prints ASCII only. (Toggle Plain Text) .bss .comm buf,1,16 .text .globl main main: movl $4,%eax movl $1,%ebx movl $buf,%ecx movl $1,%edx int $0x80 incl buf jmp main ret .bss .comm buf,1,16 .text .globl main main: movl $4,%eax movl $1,%ebx movl $buf,%ecx movl $1,%edx int $0x80 incl buf jmp main ret So I guess that's as far as I can go without forcing output in the form of the ASCII codes for numbers. :/ If you have a faster way to write this, just tell me. I ponder writing it with the stack instead of common data instead. That would probably be better, but I don't know. __________________ (Toggle Plain Text) /* LANCE / C++; / this makes C bigger but returns the old value / char site = "slackwise.net", home = "lance.slackwise.net", pics = "flickr.com/photos/slackwise"; /* LANCE / C++; / this makes C bigger but returns the old value / char site = "slackwise.net", home = "lance.slackwise.net", pics = "flickr.com/photos/slackwise";

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Programming Forums > Application Development > Assembly

Infinate Loop Incrementation