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Add documentation, honour signal_will_perform.

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This commit is contained in:
Thomas Harte 2022-05-17 15:05:11 -04:00
parent 84071ac6d0
commit eed2672db5
2 changed files with 34 additions and 1 deletions
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25
Processors/68000Mk2/68000Mk2.hpp
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@ -361,7 +361,30 @@ struct State {
InstructionSet::M68k::RegisterSet registers;
};
template <class BusHandler, bool dtack_is_implicit = true, bool permit_overrun = true, bool signal_will_perform = false> class Processor: private ProcessorBase {
/*!
Provides an emulation of the 68000 with accurate bus logic via the @c BusHandler, subject to the following template parameters:
@c dtack_is_implicit means that the 68000 won't wait around for DTACK during any data access. BERR or VPA may still be
signalled at the appropriate moment and will override the implicit DTACK, but the processor won't spin if nothing is explicitly
signalled. Enabling this simplifies the internal state machine and therefore improves performance; bus handlers can still indicate
that time was spent waiting for DTACK by returning an appropriate value from @c perform_bus_operation.
@c permit_overrun allows the 68000 to be relaxed in how it interprets the constraint specified by the @c duration parameter to
@c run_for. If this is @c false, @c run_for will always return as soon as it has called @c perform_bus_operation with whichever
operation is ongoing at the requested stopping time. If it is @c true then the 68000 is granted leeway to overrun the requested stop
time by 'a small amount' as and when it is a benefit to do so. Any overrun will be subtracted from the next @c run_for.
In practice this allows the implementation to avoid a bunch of conditional checks by considering whether it needs to exit less frequently.
Teleologically, it's expected that most — if not all — single-processor machines can permit overruns for a performance boost with
no user-visible difference.
@c signal_will_perform indicates whether the 68000 will call the bus handler's @c will_perform. Unlike the popular 8-bit CPUs,
the 68000 doesn't offer an indication of when instruction dispatch will occur so this is provided *for testing purposes*. It allows test cases
to track execution and inspect internal state in a wholly unrealistic fashion.
*/
template <class BusHandler, bool dtack_is_implicit = true, bool permit_overrun = true, bool signal_will_perform = false>
class Processor: private ProcessorBase {
public:
Processor(BusHandler &bus_handler) : ProcessorBase(), bus_handler_(bus_handler) {}

10
Processors/68000Mk2/Implementation/68000Mk2Implementation.hpp
View File

@ -15,6 +15,8 @@
namespace CPU {
namespace MC68000Mk2 {
// MARK: - The state machine.
template <class BusHandler, bool dtack_is_implicit, bool permit_overrun, bool signal_will_perform>
void Processor<BusHandler, dtack_is_implicit, permit_overrun, signal_will_perform>::run_for(HalfCycles duration) {
// Accumulate the newly paid-in cycles. If this instance remains in deficit, exit.
@ -197,6 +199,12 @@ void Processor<BusHandler, dtack_is_implicit, permit_overrun, signal_will_perfor
// Obtain operand flags and pick a perform pattern.
setup_operation();
// Signal the bus handler if requested.
if constexpr (signal_will_perform) {
bus_handler_.will_perform(instruction_address_, opcode_);
}
// Ensure the first parameter is next fetched.
next_operand_ = 0;
[[fallthrough]];
@ -270,6 +278,8 @@ void Processor<BusHandler, dtack_is_implicit, permit_overrun, signal_will_perfor
}
// MARK: - Operation specifications.
template <class BusHandler, bool dtack_is_implicit, bool permit_overrun, bool signal_will_perform>
void Processor<BusHandler, dtack_is_implicit, permit_overrun, signal_will_perform>::setup_operation() {