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Implements (arguably: fixes) BSR.

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This commit is contained in:
Thomas Harte 2019-04-15 23:20:36 -04:00
parent 1f0e3b157a
commit dec5535e54
3 changed files with 25 additions and 7 deletions
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14
Processors/68000/Implementation/68000Implementation.hpp
View File

@ -190,15 +190,21 @@ template <class T, bool dtack_is_implicit> void Processor<T, dtack_is_implicit>:
active_step_->microcycle.length = HalfCycles(8 + ((active_program_->source->full & 31) / 16) * 4);
break;
// Bcc: evaluates the relevant condition and displacement size and then:
// Bcc: ordinarily evaluates the relevant condition and displacement size and then:
// if condition is false, schedules bus operations to get past this instruction;
// otherwise applies the offset and schedules bus operations to refill the prefetch queue.
//
// Special case: the condition code is 1, which is ordinarily false. In that case this
// is the trailing step of a BSR.
case Operation::Bcc: {
// Grab the 8-bit offset.
const int8_t byte_offset = int8_t(prefetch_queue_.halves.high.halves.low);
// Test the conditional.
const bool should_branch = evaluate_condition(prefetch_queue_.halves.high.halves.high);
// Check whether this is secretly BSR.
const bool is_bsr = ((decoded_instruction_ >> 8) & 0xf) == 1;
// Test the conditional, treating 'false' as true.
const bool should_branch = is_bsr || evaluate_condition(prefetch_queue_.halves.high.halves.high);
// Schedule something appropriate, by rewriting the program for this instruction temporarily.
if(should_branch) {
@ -208,7 +214,7 @@ template <class T, bool dtack_is_implicit> void Processor<T, dtack_is_implicit>:
program_counter_.full += int16_t(prefetch_queue_.halves.low.full);
}
program_counter_.full -= 2;
bus_program = branch_taken_bus_steps_;
bus_program = is_bsr ? bsr_bus_steps_ : branch_taken_bus_steps_;
} else {
if(byte_offset) {
bus_program = branch_byte_not_taken_bus_steps_;

17
Processors/68000/Implementation/68000Storage.cpp
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@ -332,7 +332,7 @@ struct ProcessorStorageConstructor {
ADDASUBA, // Maps a destination register and a source mode and register to an ADDA or SUBA.
BRA, // Maps to a BRA. All fields are decoded at runtime.
Bcc, // Maps to a Bcc. All fields are decoded at runtime.
BccBSR, // Maps to a Bcc or BSR. Other than determining the type of operation, fields are decoded at runtime.
BTST, // Maps a source register and a destination register and mode to a BTST.
BTSTIMM, // Maps a destination mode and register to a BTST #.
@ -427,7 +427,7 @@ struct ProcessorStorageConstructor {
{0xf1f8, 0xb188, Operation::CMPl, Decoder::CMPM}, // 4-81 (p185)
// {0xff00, 0x6000, Operation::BRA, Decoder::BRA}, // 4-55 (p159) TODO: confirm that this really, really is just a special case of Bcc.
{0xf000, 0x6000, Operation::Bcc, Decoder::Bcc}, // 4-25 (p129)
{0xf000, 0x6000, Operation::Bcc, Decoder::BccBSR}, // 4-25 (p129) and 4-59 (p163)
{0xf1c0, 0x41c0, Operation::MOVEAl, Decoder::LEA}, // 4-110 (p214)
{0xf100, 0x7000, Operation::MOVEq, Decoder::MOVEq}, // 4-134 (p238)
@ -848,7 +848,16 @@ struct ProcessorStorageConstructor {
} break;
// This decoder actually decodes nothing; it just schedules a PerformOperation followed by an empty step.
case Decoder::Bcc: {
case Decoder::BccBSR: {
const int condition = (instruction >> 8) & 0xf;
if(condition == 1) {
// This is BSR, which is unconditional and means pushing a return address to the stack first.
// Push the return address to the stack.
op(Action::PrepareJSR, seq("n nW+ nw", { ea(1), ea(1) }));
}
// This is Bcc.
op(Action::PerformOperation);
op(); // The above looks terminal, but will be dynamically reprogrammed.
} break;
@ -2511,6 +2520,7 @@ CPU::MC68000::ProcessorStorage::ProcessorStorage() {
const size_t branch_taken_offset = constructor.assemble_program("n np np");
const size_t branch_byte_not_taken_offset = constructor.assemble_program("nn np");
const size_t branch_word_not_taken_offset = constructor.assemble_program("nn np np");
const size_t bsr_offset = constructor.assemble_program("np np");
const size_t dbcc_condition_true_offset = constructor.assemble_program("nn np np");
const size_t dbcc_condition_false_no_branch_offset = constructor.assemble_program("n nr np np", { &dbcc_false_address_ });
@ -2539,6 +2549,7 @@ CPU::MC68000::ProcessorStorage::ProcessorStorage() {
branch_taken_bus_steps_ = &all_bus_steps_[branch_taken_offset];
branch_byte_not_taken_bus_steps_ = &all_bus_steps_[branch_byte_not_taken_offset];
branch_word_not_taken_bus_steps_ = &all_bus_steps_[branch_word_not_taken_offset];
bsr_bus_steps_ = &all_bus_steps_[bsr_offset];
dbcc_condition_true_steps_ = &all_bus_steps_[dbcc_condition_true_offset];
dbcc_condition_false_no_branch_steps_ = &all_bus_steps_[dbcc_condition_false_no_branch_offset];

1
Processors/68000/Implementation/68000Storage.hpp
View File

@ -297,6 +297,7 @@ class ProcessorStorage {
BusStep *branch_taken_bus_steps_;
BusStep *branch_byte_not_taken_bus_steps_;
BusStep *branch_word_not_taken_bus_steps_;
BusStep *bsr_bus_steps_;
uint32_t dbcc_false_address_;
BusStep *dbcc_condition_true_steps_;