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Allow that final Z80 cycle to start anywhere in the first three of Nick's window of six.

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Thomas Harte 2021-07-02 22:29:35 -04:00
parent 850aa2b23a
commit 4fffb3cf19
1 changed files with 9 additions and 5 deletions
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14
Machines/Enterprise/Nick.cpp
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@ -96,15 +96,19 @@ uint8_t Nick::read([[maybe_unused]] uint16_t address) {
}
Cycles Nick::get_time_until_z80_slot(Cycles after_period) const {
// Place Z80 accesses as the first six cycles in each sixteen-cycle window.
// Place Z80 accesses in the first six cycles in each sixteen-cycle window.
// That models video accesses as being the final ten. Which has the net effect
// of responding to the line parameter table interrupt flag as soon as it's
// loaded.
// i.e. 0 -> 0, 1 -> 15 ... 15 -> 1.
return Cycles(
15 ^ ((horizontal_counter_ + 15 + after_period.as<int>()) & 15)
);
// Assumed below: the Z80 can start its final cycle anywhere in the first three
// of the permitted six.
const int offset = (horizontal_counter_ + after_period.as<int>()) & 15;
if(offset < 3) {
return 0;
} else {
return 16 - offset;
}
}
void Nick::run_for(Cycles duration) {