Interrupts in Assembly

Interrupts are mechanisms used to handle asynchronous events. They can be software-initiated (via the int instruction) or hardware-generated.

Software Interrupts

Software interrupts are invoked using the int instruction followed by an interrupt vector number. For example:

int 0x80    ; Linux system call interrupt
int 0x10    ; BIOS video services
    

Example: Linux System Call

This example demonstrates using int 0x80 for a Linux system call to write a string to the console.

section .data
    message db 'Hello, Interrupt!', 0xA
    msg_len equ $ - message

section .text
    global _start

_start:
    mov eax, 4          ; System call: write
    mov ebx, 1          ; File descriptor: stdout
    mov ecx, message    ; Address of the message
    mov edx, msg_len    ; Length of the message
    int 0x80            ; Trigger interrupt

    mov eax, 1          ; System call: exit
    xor ebx, ebx        ; Exit code: 0
    int 0x80            ; Trigger interrupt
    

Example: BIOS Video Services

This example changes the text color using BIOS interrupt int 0x10.

section .text
    org 0x7C00          ; Code starts at boot sector

_start:
    mov ah, 0x0E        ; Teletype output
    mov al, 'A'         ; Character to display
    int 0x10            ; Trigger BIOS interrupt

    hlt                 ; Halt the CPU
    

Hardware Interrupts

Hardware interrupts are generated by devices (e.g., keyboards, timers). The CPU uses an Interrupt Descriptor Table (IDT) to handle them.

Example: A keyboard interrupt typically uses vector 0x09 (on older systems). Handling these interrupts requires writing an interrupt service routine (ISR).

Key Notes

• Software interrupts are useful for system-level programming.
• Modern operating systems often restrict direct hardware interrupt handling for security and stability.
• Use modern alternatives like syscall or API functions where applicable.