CODEWITHSUNDEEP
assembly
Chris G.
Chris G.

Reputation: 3981

Simple x86 linux assembly program returning unexpected value

Here is a simple program that should find the minimum of data_items. a 0 is used to terminate the block of memory. %ebx is tracking the current minimum and 0 should never get copied into it because of the je instruction. And yet, this program is returning 0 to the OS and not 3. 

.section .data

data_items:
 .long 3,67,34,222,45,75,54,34,44,222,11,66,0

.section .text

.globl _start

_start:
 movl $0, %edi          #move 0 into the index register
 movl data_items(,%edi,4), %eax  #load the first byte of data
 movl %eax, %ebx        #first item, so its smalles, ebx tracks smallest

 start_loop:
  cmpl $0, %eax         #check if we hit the end
  je loop_exit
  incl %edi
  movl data_items(,%edi,4), %eax
  cmpl %ebx, %eax
  jge start_loop

  movl %eax, %ebx
  jmp start_loop

 loop_exit:
  movl $1, %eax
  int $0x80

Edit: Correct code here: 

.section .data

data_items:
 .long 3,67,34,222,45,75,54,34,44,222,11,66,0

.section .text

.globl _start

_start:
 movl $0, %edi          #move 0 into the index register
 movl data_items(,%edi,4), %eax  #load the first byte of data
 movl %eax, %ebx        #first item, so its biggest, ebx tracks biggest

 start_loop:
  incl %edi
  movl data_items(,%edi,4), %eax
  cmpl $0, %eax
  je loop_exit

  cmpl %ebx, %eax
  jge start_loop

  movl %eax, %ebx
  jmp start_loop

 loop_exit:
  movl $1, %eax
  int $0x80

Upvotes: 0

Views: 57

Answers (1)

nneonneo
nneonneo

Reputation: 179687

The logic isn't right.

In C code, your program looks like

edi = 0;
eax = data_items[edi];
ebx = eax;
while(eax != 0) {
    eax = data_items[++edi];
    if(eax < ebx) ebx = eax;
}
exit(ebx);

The problem is that you load eax then immediately store it as the minimum. If eax = 0, you store it as the minimum (because it is), and only then do you break.

Upvotes: 1

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