apple1-videocard-lib/interrupt.h
2021-12-08 12:53:13 +01:00

43 lines
1.5 KiB
C

// 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;
// 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++;
}
}
}
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
}