What is Syscall?

You have written assembly instructions like mov and lea. These instructions run directly on the CPU. But what if you need to do something the CPU cannot do by itself — like print to the screen, read from the keyboard, or exit a program?

For these tasks, you need the operating system (Linux, in this course).

What is a syscall?

A syscall (system call) is a request from your program to the operating system. It is how you ask the OS to do something for you.

Why you need syscalls:

How a syscall works:

1. You put a syscall number in rax (tells the OS what you want)
2. You put arguments in other registers (rdi, rsi, rdx, etc.)
3. You execute the syscall instruction

1. The CPU jumps into the OS kernel
2. The OS performs the request
3. The OS returns control to your program

Analogy:

Think of a syscall like calling a government office:

Syscalls you will learn in this course:

The most important syscall for now: <strong>exit</strong>

Without exit, your program would keep running forever or crash. Every program you write must end with an exit syscall.

mov rax, 60    ; syscall number for exit
mov rdi, 0     ; exit code (0 = success)
syscall        ; ask the OS to exit