mirror of https://github.com/bsnes-emu/bsnes.git
103 lines
2.6 KiB
C++
103 lines
2.6 KiB
C++
//called once every four clock cycles;
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//as NMI steps by scanlines (divisible by 4) and IRQ by PPU 4-cycle dots.
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//
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//ppu.(vh)counter(n) returns the value of said counters n-clocks before current time;
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//it is used to emulate hardware communication delay between opcode and interrupt units.
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auto CPU::pollInterrupts() -> void {
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//NMI hold
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if(status.nmiHold) {
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status.nmiHold = false;
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if(io.nmiEnabled) status.nmiTransition = true;
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}
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//NMI test
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bool nmiValid = vcounter(2) >= ppu.vdisp();
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if(!status.nmiValid && nmiValid) {
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//0->1 edge sensitive transition
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status.nmiLine = true;
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status.nmiHold = true; //hold /NMI for four cycles
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} else if(status.nmiValid && !nmiValid) {
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//1->0 edge sensitive transition
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status.nmiLine = false;
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}
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status.nmiValid = nmiValid;
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//IRQ hold
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status.irqHold = false;
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if(status.irqLine) {
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if(io.virqEnabled || io.hirqEnabled) status.irqTransition = true;
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}
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//IRQ test
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bool irqValid = io.virqEnabled || io.hirqEnabled;
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if(irqValid) {
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if((io.virqEnabled && vcounter(10) != (io.virqPos))
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|| (io.hirqEnabled && hcounter(10) != (io.hirqPos + 1) * 4)
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|| (io.virqPos && vcounter(6) == 0) //IRQs cannot trigger on last dot of field
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) irqValid = false;
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}
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if(!status.irqValid && irqValid) {
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//0->1 edge sensitive transition
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status.irqLine = true;
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status.irqHold = true; //hold /IRQ for four cycles
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}
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status.irqValid = irqValid;
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}
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auto CPU::nmitimenUpdate(uint8 data) -> void {
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bool nmiEnabled = io.nmiEnabled;
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bool virqEnabled = io.virqEnabled;
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bool hirqEnabled = io.hirqEnabled;
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io.nmiEnabled = data & 0x80;
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io.virqEnabled = data & 0x20;
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io.hirqEnabled = data & 0x10;
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//0->1 edge sensitive transition
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if(!nmiEnabled && io.nmiEnabled && status.nmiLine) {
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status.nmiTransition = true;
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}
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//?->1 level sensitive transition
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if(io.virqEnabled && !io.hirqEnabled && status.irqLine) {
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status.irqTransition = true;
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}
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if(!io.virqEnabled && !io.hirqEnabled) {
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status.irqLine = false;
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status.irqTransition = false;
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}
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status.irqLock = true;
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}
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auto CPU::rdnmi() -> bool {
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bool result = status.nmiLine;
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if(!status.nmiHold) {
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status.nmiLine = false;
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}
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return result;
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}
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auto CPU::timeup() -> bool {
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bool result = status.irqLine;
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if(!status.irqHold) {
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status.irqLine = false;
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status.irqTransition = false;
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}
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return result;
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}
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auto CPU::nmiTest() -> bool {
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if(!status.nmiTransition) return false;
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status.nmiTransition = false;
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r.wai = false;
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return true;
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}
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auto CPU::irqTest() -> bool {
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if(!status.irqTransition && !r.irq) return false;
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status.irqTransition = false;
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r.wai = false;
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return !r.p.i;
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}
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