How do I move the VALUE in a register to a memory variable in NASM?

Question

I'm trying to move the associated VALUE (not memory address) contained in a register to a memory variable, but it is not working.

section .data
        regValue dq 0


section .text
        global main

main:
        push rbp
        mov rbp, rsp
        mov rax, 844
        mov rdi, 9393
        mov [regValue], [rdi]

I get error: error: operation size not specified

Answer 1

Well, for a start, the value of rdi is actually rdi rather than [rdi]. The latter, even assuming it's valid (I'm more of a gas man than a nasm man), would be the value stored in the memory that rdi points to.

And, to specify sizes of operands (where gas uses the more succinct movl/movb/etc operations), you specify the size with the operands, such as:

mov qword [regValue], 9393

However, I don't think that's necessary when you're using a 64-bit register like rdi for the source since the size can be inferred from that. I think you can just do:

mov [regValue], rdi

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BTW, in NASM you should use default rel at the top of your file to prefer RIP-relative addressing modes for static data (like [regValue]), instead of 32-bit absolute.