Question

How can I retrieve data from a .long statement?
For example:

.data 
data_items:
    .long 3,67,34,222,45,75,54,34,44,33,22,11,66,0

.text

.globl _main
    _main:
         movl    $0, %edi
         movl    data_items(,%edi,4), %eax

Gives a large series of errors about absolute addressing not being allowed in x86_64. How can I access this data? I’m fairly new to assembly, so I apologize if my terminology is confusing.

EDIT: I am using GNU Assembler/GCC

Answer 1

The problem is that your data is in the data segment and your code is in the text segment. The linker is set up to require relocatable code, which means you can’t use an absolute address, since you cannot know the absolute address until runtime.

To use relocatable code, you need to access data_items as an offset from the instruction pointer, rip.

_main:
     movl    $0, %edi
     leaq    data_items(%rip), %rax
     movl    (%rax,%rdi,4), %eax

The leaq instruction gets the address of data_items using an offset of the instruction pointer, which can be calculated at link time. Then the movl instruction uses that address as the base for loading the data. Note that I used rdi in the addressing. When you write to edi, the upper 32 bits of rdi are automatically cleared, so this will work unmodified as long as the value in edi is unsigned. You could use edi and eax, but that would truncate addresses which use more than 32 bits, and the compiled code would be larger since the default address size is 64 bits.