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Starts to formalise the ST memory map a little.

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This commit is contained in:
Thomas Harte 2019-10-06 17:15:29 -04:00
parent 3f45cd2380
commit fcd2143697
1 changed files with 56 additions and 14 deletions
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70
Machines/AtariST/AtariST.cpp
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@ -45,6 +45,17 @@ class ConcreteMachine:
throw ROMMachine::Error::MissingROMs;
}
Memory::PackBigEndian16(*roms[0], rom_);
// Set up basic memory map.
memory_map_[0] = BusDevice::MostlyRAM;
for(int c = 1; c < 0xf0; ++c) memory_map_[c] = BusDevice::RAM;
// This is appropriate for: TOS 1.x, no cartridge.
for(int c = 0xf0; c < 0xfc; ++c) memory_map_[c] = BusDevice::Unassigned;
for(int c = 0xfc; c < 0xff; ++c) memory_map_[c] = BusDevice::ROM;
memory_map_[0xfa] = memory_map_[0xfb] = BusDevice::Cartridge;
memory_map_[0xff] = BusDevice::IO;
}
// MARK: CRTMachine::Machine
@ -69,22 +80,49 @@ class ConcreteMachine:
// A null cycle leaves nothing else to do.
if(!(cycle.operation & (Microcycle::NewAddress | Microcycle::SameAddress))) return HalfCycles(0);
/* TODO: DTack, bus error, VPA. */
auto address = cycle.word_address();
uint16_t *memory;
if(address < 4) {
memory = rom_.data();
} else if(address < 0x700000) {
memory = ram_.data();
address &= ram_.size() - 1;
// TODO: align with the next access window.
} else if(address < 0x780000) { // TOS 2.0+ address
memory = rom_.data();
address &= rom_.size() - 1;
} else if(address >= 0x7e0000 && address < 0x7f8000) { // TOS 1.0 address
memory = rom_.data();
address &= rom_.size() - 1;
} else {
assert(false);
switch(memory_map_[address >> 15]) {
case BusDevice::MostlyRAM:
if(address < 4) {
memory = rom_.data();
break;
}
case BusDevice::RAM:
memory = ram_.data();
address &= ram_.size() - 1;
// TODO: align with the next access window.
break;
case BusDevice::ROM:
memory = rom_.data();
address %= rom_.size();
break;
case BusDevice::Cartridge:
/*
TOS 1.0 appears to attempt to read from the catridge before it has setup
the bus error vector. Therefore I assume no bus error flows.
*/
switch(cycle.operation & (Microcycle::SelectWord | Microcycle::SelectByte | Microcycle::Read)) {
default: break;
case Microcycle::SelectWord | Microcycle::Read:
*cycle.value = 0xffff;
break;
case Microcycle::SelectByte | Microcycle::Read:
cycle.value->halves.low = 0xff;
break;
}
return HalfCycles(0);
case BusDevice::Unassigned:
return HalfCycles(0);
case BusDevice::IO:
assert(false);
break;
}
// If control has fallen through to here, the access is either a read from ROM, or a read or write to RAM.
@ -119,6 +157,10 @@ class ConcreteMachine:
std::vector<uint16_t> ram_;
std::vector<uint16_t> rom_;
enum class BusDevice {
MostlyRAM, RAM, ROM, Cartridge, IO, Unassigned
};
BusDevice memory_map_[256];
};
}