CODEWITHSUNDEEP
cassembly
user1106831
user1106831

Reputation:

Understanding x86 Assembly Code from C code

C code:

#include <stdio.h>

main() {
    int i;
    for (i = 0; i < 10; i++) {
        printf("%s\n", "hello");
    }
}

ASM:

    .file   "simple_loop.c"
    .section    .rodata
.LC0:
    .string "hello"
    .text
    .globl  main
    .type   main, @function
main:
.LFB0:
    .cfi_startproc
    pushl   %ebp # push ebp onto stack
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp # setup base pointer or stack ?
    .cfi_def_cfa_register 5
    andl    $-16, %esp # ? 
    subl    $32, %esp # ?
    movl    $0, 28(%esp) # i = 0
    jmp .L2
.L3:
    movl    $.LC0, (%esp) # point stack pointer to "hello" ?
    call    puts # print "hello"
    addl    $1, 28(%esp) # i++
.L2:
    cmpl    $9, 28(%esp) # if i < 9
    jle .L3              # goto l3
    leave
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    ret

So I am trying to improve my understanding of x86 assembly code. For the above code, I marked off what I believe I understand. As for the question marked content, could someone share some light? Also, if any of my comments are off, please let me know.

Upvotes: 3

Views: 934

Answers (1)

Greg Hewgill
Greg Hewgill

Reputation: 994351

andl    $-16, %esp # ? 
subl    $32, %esp # ?

This reserves some space on the stack. First, the andl instruction rounds the %esp register down to the next lowest multiple of 16 bytes (exercise: find out what the binary value of -16 is to see why). Then, the subl instruction moves the stack pointer down a bit further (32 bytes), reserving some more space (which it will use next). I suspect this rounding is done so that access through the %esp register is slightly more efficient (but you'd have to inspect your processor data sheets to figure out why).

movl    $.LC0, (%esp) # point stack pointer to "hello" ?

This places the address of the string "hello" onto the stack (this instruction doesn't change the value of the %esp register itself). Apparently your compiler considers it more efficient to move data onto the stack directly, rather than to use the push instruction.

Upvotes: 2

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