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Introduces an explicit area of floating bus, starts adding bus errors.

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This commit is contained in:
Thomas Harte 2019-12-08 11:52:43 -05:00
parent 332f0d6167
commit 0ed87c61bd
1 changed files with 39 additions and 15 deletions
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54
Machines/Atari/ST/AtariST.cpp
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@ -83,7 +83,8 @@ class ConcreteMachine:
// Set up basic memory map.
memory_map_[0] = BusDevice::MostlyRAM;
int c = 1;
for(; c < 0x08; ++c) memory_map_[c] = BusDevice::RAM;
for(; c < int(ram_.size() >> 15); ++c) memory_map_[c] = BusDevice::RAM;
for(; c < 0x40; ++c) memory_map_[c] = BusDevice::Floating;
for(; c < 0xff; ++c) memory_map_[c] = BusDevice::Unassigned;
const bool is_early_tos = true;
@ -174,19 +175,30 @@ class ConcreteMachine:
// If this is a new strobing of the address signal, test for bus error and pre-DTack delay.
//
// DTack delay rule: if accessing RAM or the shifter, align with the two cycles next available
// for the CPU to access that side of the bus.
HalfCycles delay(0);
if((cycle.operation & Microcycle::NewAddress) && (address < ram_.size() || (address == (0xff8260 >> 1)))) {
// DTack will be implicit; work out how long until that should be,
// and apply bus error constraints.
const int i_phase = bus_phase_.as<int>() & 7;
if(i_phase < 4) {
delay = HalfCycles(4 - i_phase);
advance_time(delay);
if(cycle.operation & Microcycle::NewAddress) {
if(
// Anything unassigned should generate a bus error.
(memory_map_[address >> 15] == BusDevice::Unassigned) ||
// Bus errors also apply to unprivileged access to the first 0x800 bytes, or the IO area.
(!is_supervisor && (address < 0x400 || memory_map_[address >> 15] == BusDevice::IO))
) {
mc68000_.set_bus_error(true);
return delay; // TODO: there should be an extra delay here.
}
// TODO: presumably test is if(after declared memory size and (not supervisor or before hardware space)) bus_error?
// DTack delay rule: if accessing RAM or the shifter, align with the two cycles next available
// for the CPU to access that side of the bus.
if(address < ram_.size() || (address == (0xff8260 >> 1))) {
// DTack will be implicit; work out how long until that should be,
// and apply bus error constraints.
const int i_phase = bus_phase_.as<int>() & 7;
if(i_phase < 4) {
delay = HalfCycles(4 - i_phase);
advance_time(delay);
}
}
}
uint16_t *memory = nullptr;
@ -199,8 +211,6 @@ class ConcreteMachine:
}
case BusDevice::RAM:
memory = ram_.data();
address &= ram_.size() - 1;
// TODO: align with the next access window.
break;
case BusDevice::ROM:
@ -208,8 +218,9 @@ class ConcreteMachine:
address %= rom_.size();
break;
case BusDevice::Floating:
// TODO: provide vapour reads here. But: will these always be of the last video fetch?
case BusDevice::Unassigned:
// TODO: figure out the rules about bus errors.
case BusDevice::Cartridge:
/*
TOS 1.0 appears to attempt to read from the catridge before it has setup
@ -443,7 +454,20 @@ class ConcreteMachine:
std::vector<uint16_t> rom_;
enum class BusDevice {
MostlyRAM, RAM, ROM, Cartridge, IO, Unassigned
/// 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.
Floating,
/// Allows reading from rom_; writes do nothing.
ROM,
/// Allows interaction with a cartrige_.
Cartridge,
/// Marks the IO page, in which finer decoding will occur.
IO,
/// An unassigned page has nothing below it, in a way that triggers exceptions.
Unassigned
};
BusDevice memory_map_[256];