As you might guess, I’m new to this (both nasm and assembly, though I’ve done some basic assembly before).
I’m trying to create a function that prints integers to standard output. Using non-reusable code (where the number to be printed is static), I’ve succeeded… However, for obvious reasons, I want it to take the number to print as an argument.
I’m unsure whether a macro or a function is best for me, and I can’t find much on the topic of NASM macros at all. I’ve read the manual quite carefully, but it’s not enough.
Anyway, I’ve tried to do this with a NASM macro, as I created another one that prints strings with success that way.
I’ve narrowed down the problematic code to this:
%macro crash 1
jmp %%endstr
%%str: db %1,0x0a
%%endstr:
mov [%%str], byte 0x16 <<< this crashes (segmentation fault)
%endmacro
section .text
global _start
_start:
crash "abc"
It looks like anything that uses brackets on the buffer crashes, and I can only assume I’m doing it wrong.
What I want the above to do is to overwrite the first byte in %%str with another byte value. More precisely, I need to write a string to a buffer byte-by-byte backwards; I (try to) do this with a loop, where I do
mov [%%str+rcx], dl
dec rcx
until rcx is 0.
If I shouldn’t use macros for this, please enlighten me!
I intend to save the function in a mini-library for later use as well, so it should be easy to pop it in to any NASM project.
As the topic and tags say, all this is under Linux/amd64.
You can’t do that in the code segment because it’s read-only. You should declare str in the @data segment, then you’ll be fine. And, just like @user786653 said, “You should make this a function, having macros spread internal state all around your code is bad style (even for assembler!)”.