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 TMS9918 interrupt by reading the status register (see manual section 2.3.1)
#define TMS_ACK_INTERRUPT asm { lda VDP_REG }

// acknowledge the VIA 6522 interrupt by reading the T1CL register
#define VIA_ACK_INTERRUPT asm { bit VIA_T1CL }

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

#pragma zp_reserve(1)
#pragma zp_reserve(2)
#pragma zp_reserve(0) 

// 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 time_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   

   #ifdef TMS9918
   TMS_ACK_INTERRUPT;
   #endif

   #ifdef VIA6522
   VIA_ACK_INTERRUPT;
   #endif
}

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