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Imposes a three-cycle penalty for SN76489 access.

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This is my reading of (i) the SN76489 data sheet; plus (ii) the ColecoVision schematic.
This commit is contained in:
Thomas Harte 2018-05-16 19:06:03 -04:00
parent bfdd3468ea
commit b3f4d0ed8c
1 changed files with 15 additions and 4 deletions
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19
Machines/ColecoVision/ColecoVision.cpp
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@ -198,8 +198,19 @@ class ConcreteMachine:
// MARK: Z80::BusHandler
forceinline HalfCycles perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {
time_since_vdp_update_ += cycle.length;
time_since_sn76489_update_ += cycle.length;
// The SN76489 will use its ready line to trigger the Z80's wait for three
// cycles when accessed. Everything else runs at full speed. Short-circuit
// that whole piece of communications by just accruing the time here if applicable.
const HalfCycles penalty(
(
cycle.operation == CPU::Z80::PartialMachineCycle::Output &&
((*cycle.address >> 5) & 7) == 7
) ? 6 : 0
);
const HalfCycles length = cycle.length + penalty;
time_since_vdp_update_ += length;
time_since_sn76489_update_ += length;
uint16_t address = cycle.address ? *cycle.address : 0x0000;
switch(cycle.operation) {
@ -329,13 +340,13 @@ class ConcreteMachine:
}
if(time_until_interrupt_ > 0) {
time_until_interrupt_ -= cycle.length;
time_until_interrupt_ -= length;
if(time_until_interrupt_ <= HalfCycles(0)) {
z80_.set_non_maskable_interrupt_line(true, time_until_interrupt_);
}
}
return HalfCycles(0);
return penalty;
}
void flush() {