apple1-videocard-lib/lib/interrupt.h

#ifndef INTERRUPT_H
#define INTERRUPT_H

// reserve locations 0,1,2 used by the APPLE1 IRQ jump vector
// these will contain a three byte instruction "JUMP <interrupt_handler>"
// in order to make the interrupt routine not reside in zero page
#pragma zp_reserve(0,1,2)

// acknowledge the interrupt by reading the status register on the TMS9918 (see manual section 2.3.1)
inline void acknowledge_interrupt() {
   asm { lda VDP_REG };
}

EXPORT __address(0) byte IRQ_JUMP_OPCODE;    // location 0 contains the opcode for "JMP"
EXPORT __address(1) word IRQ_JUMP_ADDRESS;   // location 1 and 2 contain the jump address

// storage for a simple watch timer
EXPORT volatile byte _ticks;
EXPORT volatile byte _seconds;
EXPORT volatile byte _minutes;
EXPORT volatile byte _hours;
EXPORT volatile byte _irq_trigger;

// interrupt routine called every 1/60th by the CPU after TMS9918 sets the /INT pin
EXPORT __interrupt(hardware_all) void interrupt_handler() {
   // update the watch
   if(++_ticks == 60) {
      _ticks = 0;
      if(++_seconds == 60) {
         _seconds = 0;
         if(++_minutes == 60) {
            _minutes = 0;
            _hours++;
         }
      }
   }
   _irq_trigger = 1;         // signals that an interrupt has been triggered

   acknowledge_interrupt();  // acknowledge interrupt by reading the status register
}

// safely installs the interrupt routine
void install_interrupt() {
   asm { sei };                                   // disable 6502 interrupts
   IRQ_JUMP_OPCODE  = 0x4C;                       // $4C = JUMP opcode
   IRQ_JUMP_ADDRESS = (word) &interrupt_handler;  // JUMP interrupt_handler
   asm { cli };                                   // re-enable 6502 interrupts
}

// waits until IRQ is triggered
void wait_interrupt() {
   // _irq_trigger = 0;
   // while(_irq_trigger == 0);  // waits until it's set to 1 from the interrupt handler
}

#endif