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Expands documentation of NMI setting.

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Given that it was previously incorrect, explains logic behind request_status_ and last_request_status_ setting. Also takes the opportunity to ensure that NMI is 'sampled' at the same time as IRQ; whether the next thing should be the NMI routine now occurs one cycle before the end of any instruction. That's an assumption for now. Testing to come.
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Thomas Harte 2018-03-02 11:10:02 -05:00 committed by GitHub
parent 6cce9aa54e
commit 74dfe56d2b
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1 changed files with 14 additions and 2 deletions
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16
Processors/Z80/Implementation/Z80Implementation.hpp
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@ -1002,12 +1002,24 @@ bool ProcessorBase::get_interrupt_line() {
}
void ProcessorBase::set_non_maskable_interrupt_line(bool value, HalfCycles offset) {
// NMIs are edge triggered and cannot be masked.
// NMIs are edge triggered, so react to changes only, particularly changes to active.
if(nmi_line_ != value) {
nmi_line_ = value;
if(value) {
// request_status_ holds a bit mask of interrupt requests pending; since
// this was a new transition, an NMI is now definitely pending.
request_status_ |= Interrupt::NMI;
if(offset.as_int() < 0) {
// If the caller is indicating that this request actually happened in the
// recent past, and it happened long enough ago that it should have been
// spotted during this instruction* then change history to ensure that an
// NMI happens next.
//
// * it actually doesn't matter what sort of bus cycle is currently ongoing;
// either it is that which terminates a whole instruction, in which case this
// test is correct, or it isn't, in which case setting request_status_ above
// was sufficient and the below is merely redundant.
if(offset <= HalfCycles(-2)) {
last_request_status_ |= Interrupt::NMI;
}
}