Can someone help me find what is wrong with the following program? I'm reading 'Programming from the Ground Up' and attempting to translate the examples into x86-64 assembly. The following program finds the largest number in a set of data. But when I assemble, link, and run it I get 0. Which is obviously not the largest number. It runs fine with 32-bit registers/instructions, but not 64-bit.
# PURPOSE: This program finds the largest value in a set of data.
#
#
# VARIABLES: %rax holds the current value. %rdi holds the largest
# value. %rbx holds the current index. data_items is the
# actual set of data. The data is terminated with a 0.
#
.section .data
data_items:
.long 76, 38, 10, 93, 156, 19, 73, 84, 109, 12, 21, 0
.section .text
.globl _start
_start:
movq $0, %rbx
movq data_items(, %rbx, 4), %rax
movq %rax, %rdi
loop_start:
cmpq $0, %rax # Have we reached the end?
je loop_end
incq %rbx # Increment the index.
movq data_items(, %rbx, 4), %rax # Load the next value.
cmpq %rdi, %rax # Is new value larger?
jle loop_start
movq %rax, %rdi # New val is larger, store
# it.
jmp loop_start
loop_end:
# The largest value is already in %rdi and will be returned as
# exit status code.
movq $60, %rax
syscall
You are using movq which moves 64-bit values from your list that contains 32-bit values. That will give you wrong results. Define your list to hold 64-bit values instead: replace .long with .quad and replace 4 with 8 in the movs.