57 lines
1.8 KiB
C
57 lines
1.8 KiB
C
#ifndef INTERRUPT_H
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#define INTERRUPT_H
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// reserve locations 0,1,2 used by the APPLE1 IRQ jump vector
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// these will contain a three byte instruction "JUMP <interrupt_handler>"
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// in order to make the interrupt routine not reside in zero page
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#pragma zp_reserve(0,1,2)
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// acknowledge the interrupt by reading the status register on the TMS9918 (see manual section 2.3.1)
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inline void acknowledge_interrupt() {
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asm { lda VDP_REG };
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}
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export __address(0) byte IRQ_JUMP_OPCODE; // location 0 contains the opcode for "JMP"
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export __address(1) word IRQ_JUMP_ADDRESS; // location 1 and 2 contain the jump address
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// storage for a simple watch timer
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export volatile byte _ticks;
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export volatile byte _seconds;
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export volatile byte _minutes;
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export volatile byte _hours;
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export volatile byte _irq_trigger;
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// interrupt routine called every 1/60th by the CPU after TMS9918 sets the /INT pin
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export __interrupt(hardware_all) void interrupt_handler() {
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// update the watch
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if(++_ticks == 60) {
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_ticks = 0;
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if(++_seconds == 60) {
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_seconds = 0;
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if(++_minutes == 60) {
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_minutes = 0;
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_hours++;
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}
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}
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}
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_irq_trigger = 1; // signals that an interrupt has been triggered
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acknowledge_interrupt(); // acknowledge interrupt by reading the status register
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}
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// safely installs the interrupt routine
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void install_interrupt() {
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asm { sei }; // disable 6502 interrupts
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IRQ_JUMP_OPCODE = 0x4C; // $4C = JUMP opcode
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IRQ_JUMP_ADDRESS = (word) &interrupt_handler; // JUMP interrupt_handler
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asm { cli }; // re-enable 6502 interrupts
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}
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// waits until IRQ is triggered
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void wait_interrupt() {
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// _irq_trigger = 0;
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// while(_irq_trigger == 0); // waits until it's set to 1 from the interrupt handler
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}
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#endif
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