⚡ Interrupts

assembly

; Conceptual interrupt handling sequence
; Step 1: CPU completes current instruction
; Step 2: Automatically pushes PC and flags to stack
; Step 3: Looks up ISR address from vector table
; Step 4: Jumps to ISR

ISR_HANDLER:
    PUSH R0              ; Save registers used in ISR
    PUSH R1
    IN   R0, DATA_PORT   ; Read data from device
    STORE R0, BUFFER     ; Store in memory buffer
    POP  R1              ; Restore registers
    POP  R0
    IRET                 ; Return: restores PC & flags

; Step 5: CPU pops PC and flags, resumes original program

c

#include <stdint.h>
#include <stdbool.h>

volatile uint8_t buffer[256];
volatile uint16_t head = 0, tail = 0;
volatile bool data_ready = false;

// Simulated Interrupt Service Routine
// Called by hardware when device has data
void __attribute__((interrupt)) device_isr(void) {
    uint8_t data = inb(DEVICE_DATA_PORT);
    
    // Store in circular buffer
    uint16_t next = (head + 1) & 0xFF;
    if (next != tail) {  // Buffer not full
        buffer[head] = data;
        head = next;
    }
    
    // Acknowledge interrupt to PIC
    outb(PIC_EOI, 0x20);
    // ISR returns — CPU restores context automatically
}

// Main program runs concurrently
int main(void) {
    while (1) {
        if (tail != head) {
            // Process data from buffer
            uint8_t data = buffer[tail];
            tail = (tail + 1) & 0xFF;
            process_data(data);
        }
        // Do other work — no polling needed!
    }
    return 0;
}