Silicon to Scripting: 32 and 64 bit assembly

We have been writing assembly that looks like

A = 100
D = 32
*A = D

For a simple game, that works to only have a few instructions, but more complex CPUs have more complex instruction sets.

We will be writing some assembly that can run on your computer (hopefully). Most likely you are on a x86 64 bit operating system (opposed to an ARM one). If you are on ARM, you don’t need to follow along. Read through this, and you’ll be able to follow along again in the next post.

https://en.wikipedia.org/wiki/X86#Purpose

To run assembly on linux, you will need an assembler and a linker. In this tutorial, we will be using nasm and ld. To install it on Ubuntu, run

sudo apt update && sudo apt upgrade
sudo apt install nasm binutils

Assembly on x86 systems is split up into multiple sections, each with different purposes. The .text sections is where the code for the program goes.

To inform the OS on where to start running the program,

section .text
    global _start

_start:
    mov eax, 1          ; syscall number for sys_exit
    mov ebx, 42         ; exit code 42
    int 0x80            ; call kernel

• https://www.gridbugs.org/if-you-use-a-custom-linker-script-_start-is-not-necessarily-the-entry-point/