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Attempt DBcc.

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This commit is contained in:
Thomas Harte 2022-05-20 11:32:06 -04:00
parent 452dd3ccfd
commit 860cc63e21
2 changed files with 59 additions and 1 deletions
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58
Processors/68000Mk2/Implementation/68000Mk2Implementation.hpp
View File

@ -86,6 +86,11 @@ enum ExecutionState: int {
Scc_Dn,
Scc_Dn_did_not_set,
Scc_Dn_did_set,
DBcc,
DBcc_branch_taken,
DBcc_condition_true,
DBcc_counter_overflow,
};
// MARK: - The state machine.
@ -516,6 +521,8 @@ void Processor<BusHandler, dtack_is_implicit, permit_overrun, signal_will_perfor
}
});
StdCASE(DBcc, perform_state_ = DBcc);
default:
assert(false);
}
@ -1092,6 +1099,44 @@ void Processor<BusHandler, dtack_is_implicit, permit_overrun, signal_will_perfor
next_operand_ = 0;
MoveToState(StoreOperand);
//
// DBcc
//
BeginState(DBcc):
InstructionSet::M68k::perform<
InstructionSet::M68k::Model::M68000,
ProcessorBase,
InstructionSet::M68k::Operation::DBcc
>(
instruction_, operand_[0], operand_[1], status_, *static_cast<ProcessorBase *>(this));
// Next state is set by complete_dbcc
break;
BeginState(DBcc_branch_taken):
IdleBus(1); // n
Prefetch(); // np
Prefetch(); // np
MoveToState(Decode);
BeginState(DBcc_condition_true):
IdleBus(2); // n n
Prefetch(); // np
Prefetch(); // np
MoveToState(Decode);
BeginState(DBcc_counter_overflow):
IdleBus(1); // n
// Yacht lists an extra np here; I'm assuming it's a read from where
// the PC would have gone, had the branch been taken. So do that,
// but then reset the PC to where it would have been.
Prefetch();
program_counter_.l = instruction_address_.l + 4;
Prefetch(); // np
Prefetch(); // np
MoveToState(Decode);
//
// Various states TODO.
//
@ -1145,6 +1190,19 @@ void ProcessorBase::did_scc(bool did_set_ff) {
state_ = did_set_ff ? Scc_Dn_did_set : Scc_Dn_did_not_set;
}
void ProcessorBase::complete_dbcc(bool matched_condition, bool overflowed, int16_t offset) {
// The actual DBcc rule is: branch if !matched_condition and !overflowed; but I think
// that a spurious read from the intended destination PC occurs if overflowed, so update
// the PC for any case of !matched_condition and rely on the DBcc_counter_overflow to
// set it back.
if(!matched_condition) {
state_ = overflowed ? DBcc_counter_overflow : DBcc_branch_taken;
program_counter_.l = instruction_address_.l + uint32_t(offset) + 2;
return;
}
state_ = DBcc_condition_true;
}
// MARK: - External state.
template <class BusHandler, bool dtack_is_implicit, bool permit_overrun, bool signal_will_perform>

2
Processors/68000Mk2/Implementation/68000Mk2Storage.hpp
View File

@ -109,7 +109,7 @@ struct ProcessorBase: public InstructionSet::M68k::NullFlowController {
inline void did_bit_op(int) {}
inline void did_update_status();
template <typename IntT> void complete_bcc(bool, IntT) {}
inline void complete_dbcc(bool, bool, int16_t) {}
inline void complete_dbcc(bool, bool, int16_t);
inline void bsr(uint32_t) {}
inline void jsr(uint32_t) {}
inline void jmp(uint32_t) {}