Question

I am working on a simple routine that will loop through a list of numbers and return the max. It is always returning 11 and I cannot see what’s wrong with my logic. To test the routine I have a list of numbers (data_items) that I am looping through. What am I doing wrong here?

 .section .data

data_items:    #these are the data items

.long 3,67,34,222,45,75,857,858,983,11,55,43,23,123,785,4356,0

.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                     #since this is the first item, %eax is the biggest


start_loop:
cmpl $0, %eax                       #check to see if we've hit the end
je loop_exit
incl %edi                           #load the next value
movl data_items(,%edi,4), %eax
cmpl %ebx, %eax                     #compare values
jle start_loop                      #jump to the start of the loop if the value is not larger

movl %eax, %ebx                     #move the value as the largest
jmp start_loop                      #jump to the loop beginning

loop_exit:
movl $1, %eax                       #1 is the exit() syscall
int $0x80

Answer 1

There’s nothing wrong with your logic at all. When I enter that code into qq.s and execute the following:

pax$ as -o qq.o qq.s

pax$ ld -o qq qq.o

pax$ gdb qq
GNU gdb (Ubuntu/Linaro 7.2-1ubuntu11) 7.2
Copyright (C) 2010 Free Software Foundation, Inc.
... blah blah blah ...
Reading symbols from /home/pax/qq...(no debugging symbols found)...done.

(gdb) break loop_exit
Breakpoint 1 at 0x8048097

(gdb) run
Starting program: /home/pax/qq 
Breakpoint 1, 0x08048097 in loop_exit ()

(gdb) info reg ebx
ebx            0x1104   4356

(gdb) _

In other words, the correct value is being loaded into ebx.