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Mildly reduce cost of 8-byte ROM overlay.

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This commit is contained in:
Thomas Harte 2023-11-27 15:48:30 -05:00
parent 2a0375e9c2
commit 87eec47b79
1 changed files with 16 additions and 10 deletions
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26
Machines/Atari/ST/AtariST.cpp
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@ -100,8 +100,7 @@ class ConcreteMachine:
Memory::PackBigEndian16(roms.find(rom_name)->second, rom_);
// Set up basic memory map.
memory_map_[0] = BusDevice::MostlyRAM;
int c = 1;
int c = 0;
for(; c < int(ram_.size() >> 16); ++c) memory_map_[c] = BusDevice::RAM;
for(; c < 0x40; ++c) memory_map_[c] = BusDevice::Floating;
for(; c < 0xff; ++c) memory_map_[c] = BusDevice::Unassigned;
@ -118,6 +117,9 @@ class ConcreteMachine:
memory_map_[0xfa] = memory_map_[0xfb] = BusDevice::Cartridge;
memory_map_[0xff] = BusDevice::IO;
// Copy the first 8 bytes of ROM into RAM.
reinstall_rom_vector();
midi_acia_->set_interrupt_delegate(this);
keyboard_acia_->set_interrupt_delegate(this);
@ -249,18 +251,15 @@ class ConcreteMachine:
uint8_t *memory = nullptr;
switch(memory_map_[address >> 16]) {
default:
case BusDevice::MostlyRAM:
if(address < 8) {
memory = rom_.data();
break;
}
[[fallthrough]];
case BusDevice::RAM:
memory = ram_.data();
break;
case BusDevice::ROM:
memory = rom_.data();
if(!(operation & Microcycle::Read)) {
return delay;
}
address -= rom_start_;
break;
@ -408,6 +407,9 @@ class ConcreteMachine:
}
// If control has fallen through to here, the access is either a read from ROM, or a read or write to RAM.
//
// In both write cases, immediately reinstall the first eight bytes of RAM from ROM, so that any write to
// that area is in effect a no-op. This is cheaper than the conditionality of actually checking.
switch(operation & (Microcycle::SelectWord | Microcycle::SelectByte | Microcycle::Read)) {
default:
break;
@ -422,17 +424,23 @@ class ConcreteMachine:
if(address >= video_range_.low_address && address < video_range_.high_address)
video_.flush();
*reinterpret_cast<uint16_t *>(&memory[address]) = cycle.value->w;
reinstall_rom_vector();
break;
case Microcycle::SelectByte:
if(address >= video_range_.low_address && address < video_range_.high_address)
video_.flush();
memory[address] = cycle.value->b;
reinstall_rom_vector();
break;
}
return HalfCycles(0);
}
void reinstall_rom_vector() {
std::copy(rom_.begin(), rom_.begin() + 8, ram_.begin());
}
void flush_output(int outputs) final {
dma_.flush();
mfp_.flush();
@ -520,8 +528,6 @@ class ConcreteMachine:
uint32_t rom_start_ = 0;
enum class BusDevice {
/// A mostly RAM page is one that returns ROM for the first 8 bytes, RAM elsewhere.
MostlyRAM,
/// Allows reads and writes to ram_.
RAM,
/// Nothing is mapped to this area, and it also doesn't trigger an exception upon access.