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Use less branchy inner loop.

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This commit is contained in:
Thomas Harte 2023-11-21 22:42:53 -05:00
parent bcd4a2216a
commit 1828a10885
1 changed files with 60 additions and 61 deletions
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121
Machines/PCCompatible/PCCompatible.cpp
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@ -578,10 +578,6 @@ class ConcreteMachine:
public MachineTypes::ScanProducer
{
public:
// i.e. CPU clock rate is 1/3 * ~1.19Mhz ~= 0.4 MIPS.
static constexpr int CPUMultiplier = 1;
static constexpr int CPUDivisor = 3;
ConcreteMachine(
[[maybe_unused]] const Analyser::Static::Target &target,
const ROMMachine::ROMFetcher &rom_fetcher
@ -618,77 +614,80 @@ class ConcreteMachine:
// bool log = false;
// std::string previous;
void run_for(const Cycles duration) override {
auto pit_ticks = duration.as_integral();
while(pit_ticks--) {
const auto pit_ticks = duration.as_integral();
cpu_divisor_ += pit_ticks;
int ticks = cpu_divisor_ / 3;
cpu_divisor_ %= 3;
while(ticks--) {
//
// First draft: all hardware runs in lockstep, as a multiple or divisor of the PIT frequency.
//
// Advance the PIT.
//
// Advance the PIT and audio.
//
pit_.run_for(1);
++speaker_.cycles_since_update;
pit_.run_for(1);
++speaker_.cycles_since_update;
pit_.run_for(1);
// Advance audio clock.
++speaker_.cycles_since_update;
// Advance the CPU.
cpu_divisor_ += CPUMultiplier;
int cycles = cpu_divisor_ / CPUDivisor;
cycles %= CPUDivisor;
//
// Perform one CPU instruction every three PIT cycles.
// i.e. CPU instruction rate is 1/3 * ~1.19Mhz ~= 0.4 MIPS.
//
// To consider: a Duff-esque switch table calling into a function templated on clock phase
// might alleviate a large part of the conditionality here?
while(cycles--) {
// Query for interrupts and apply if pending.
if(pic_.pending() && context.flags.flag<InstructionSet::x86::Flag::Interrupt>()) {
// Regress the IP if a REP is in-progress so as to resume it later.
if(context.flow_controller.should_repeat()) {
context.registers.ip() = decoded_ip_;
context.flow_controller.begin_instruction();
}
// Signal interrupt.
InstructionSet::x86::interrupt(
pic_.acknowledge(),
context
);
}
// Get the next thing to execute.
if(!context.flow_controller.should_repeat()) {
// Decode from the current IP.
decoded_ip_ = context.registers.ip();
const auto remainder = context.memory.next_code();
decoded = decoder.decode(remainder.first, remainder.second);
// If that didn't yield a whole instruction then the end of memory must have been hit;
// continue from the beginning.
if(decoded.first <= 0) {
const auto all = context.memory.all();
decoded = decoder.decode(all.first, all.second);
}
context.registers.ip() += decoded.first;
// log |= decoded.second.operation() == InstructionSet::x86::Operation::STI;
} else {
// Query for interrupts and apply if pending.
if(pic_.pending() && context.flags.flag<InstructionSet::x86::Flag::Interrupt>()) {
// Regress the IP if a REP is in-progress so as to resume it later.
if(context.flow_controller.should_repeat()) {
context.registers.ip() = decoded_ip_;
context.flow_controller.begin_instruction();
}
// if(log) {
// const auto next = to_string(decoded, InstructionSet::x86::Model::i8086);
// if(next != previous) {
// std::cout << next << std::endl;
// previous = next;
// }
// }
// Execute it.
InstructionSet::x86::perform(
decoded.second,
// Signal interrupt.
InstructionSet::x86::interrupt(
pic_.acknowledge(),
context
);
}
// Get the next thing to execute.
if(!context.flow_controller.should_repeat()) {
// Decode from the current IP.
decoded_ip_ = context.registers.ip();
const auto remainder = context.memory.next_code();
decoded = decoder.decode(remainder.first, remainder.second);
// If that didn't yield a whole instruction then the end of memory must have been hit;
// continue from the beginning.
if(decoded.first <= 0) {
const auto all = context.memory.all();
decoded = decoder.decode(all.first, all.second);
}
context.registers.ip() += decoded.first;
// log |= decoded.second.operation() == InstructionSet::x86::Operation::STI;
} else {
context.flow_controller.begin_instruction();
}
// if(log) {
// const auto next = to_string(decoded, InstructionSet::x86::Model::i8086);
// if(next != previous) {
// std::cout << next << std::endl;
// previous = next;
// }
// }
// Execute it.
InstructionSet::x86::perform(
decoded.second,
context
);
}
}