NASM Coding: Basics

Linux programs interact with the operating system and its resources (specifically, by asking the OS to do some service for the program), such as writing content to a file or checking the current time, using the

syscall

NASM instruction.

Before calling

syscall

, we first prepare the necessary data (service/syscall number, and arguments) for the call to run correctly: we put the syscall number inside the

rax

register, and the arguments inside the following registers:

rdi

,

rsi

,

rdx

,

r10

,

r8

, and

r9

(in this order).

This means that we can call any OS service (function/syscall) that needs up to $6$ arguments. Most OS syscalls, however, usually use much fewer than $6$ argument (the average number of arguments is $1$ to $3$ arguments.)

NASM Linux syscalls are easy to understand using code of the C language. On the next slides, we will see examples of Linux syscalls written in C, and their easy translation into NASM.