CODEWITHSUNDEEP
pointersx86globalnasmdword
CarrotSlat
CarrotSlat

Reputation: 59

How to declare a global dword pointer in x86?

I need to declare two global dword pointers that point to two arrays. To my knowledge, global means declare in .data. Is that correct? What is the code to declare these dword pointers so that the are initialized to 0 in x86 assembly with NASM and Intel syntax?

Upvotes: 3

Views: 1295

Answers (1)

James
James

Reputation: 1019

I need to declare two global dword pointers that point to two arrays.

That's simple if i understand correctly, assuming they mean global as in to all files, make a file called pointers.asm or something and type:

[section] .bss
        global _pointer1, _pointer2

        _pointer1   resd 4 ; reserve 4 dwords
        _pointer2   resd 4

We use a .bss section because that memory is set to zero so when you use it your variables are 0 initialized

Or you could just use a .data section if you want and initialize each element to zero yourself:

[section] .data
        global _pointer1, _pointer2

        _pointer1   dd 0,0,0,0
        _pointer2   dd 0,0,0,0

Also still using a .data section, it could be done like this allowing you to specify the size of a buffer like with the .bss section:

[section] .data
    global _pointer1, _pointer2

    _pointer1 times 4 dd 0
    _pointer2 times 4 dd 0

Regardless of the way you decide to do it, to use a pointer to an array declared globally in a separate file:

[section] .text

        global _main
        extern _pointer1

        bytesize equ 4 ; dword = 4 bytes

_main:
        ; write each element of the array
        mov dword [_pointer1 + 0 * bytesize], 0xa
        mov dword [_pointer1 + 1 * bytesize], 0xb
        mov dword [_pointer1 + 2 * bytesize], 0xc
        mov dword [_pointer1 + 3 * bytesize], 0xd

        ; read each element of the array
        mov eax, [_pointer1 + 0 * bytesize]
        mov eax, [_pointer1 + 1 * bytesize]
        mov eax, [_pointer1 + 2 * bytesize]
        mov eax, [_pointer1 + 3 * bytesize]

        ret

This main program returns with 0xd or 13 stored in eax, hopefully by looking at this you can get a grasp what is going on.

Upvotes: 3

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