system

System Calls in Assembly

System calls allow interaction with the operating system to perform tasks such as file operations, input/output, and process control.

Steps for Making a System Call

Load the system call number into the rax register.
Load arguments into specific registers (e.g., rdi, rsi, etc.).
Trigger the system call using the syscall instruction.

Example: Writing to the Screen

section .data
    msg db 'Hello, World!', 0xA  ; Message to display
    len equ $ - msg              ; Calculate message length

section .text
    global _start

_start:
    mov rax, 1        ; System call: write
    mov rdi, 1        ; File descriptor: stdout
    mov rsi, msg      ; Address of the message
    mov rdx, len      ; Length of the message
    syscall           ; Perform the system call

    mov rax, 60       ; System call: exit
    xor rdi, rdi      ; Exit code: 0
    syscall

Example: Reading from the Keyboard

section .bss
    input resb 10      ; Reserve 10 bytes for input

section .text
    global _start

_start:
    mov rax, 0        ; System call: read
    mov rdi, 0        ; File descriptor: stdin
    mov rsi, input    ; Address to store input
    mov rdx, 10       ; Maximum number of bytes to read
    syscall           ; Perform the system call

    mov rax, 60       ; System call: exit
    xor rdi, rdi      ; Exit code: 0
    syscall

System calls are highly platform-dependent. The above examples are specific to Linux on x86-64 architecture.