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Made a first attempt at paging.

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This commit is contained in:
Thomas Harte 2015-08-13 13:24:02 +01:00
parent 9ca57b80a7
commit 9838e01cc1
2 changed files with 39 additions and 7 deletions
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42
Machines/Atari2600.cpp
View File

@ -23,6 +23,7 @@ Machine::Machine()
_crt = new Outputs::CRT(228, 262, 1, 4);
_piaTimerStatus = 0xff;
memset(_collisions, 0xff, sizeof(_collisions));
_rom = nullptr;
setup6502();
set_reset_line(true);
@ -30,6 +31,7 @@ Machine::Machine()
Machine::~Machine()
{
delete[] _rom;
delete _crt;
}
@ -267,9 +269,30 @@ int Machine::perform_bus_operation(CPU6502::BusOperation operation, uint16_t add
if(operation != CPU6502::BusOperation::Ready) {
// check for a ROM access
// check for a ROM or paging access
if ((address&0x1000) && isReadOperation(operation)) {
returnValue &= _rom[address&_romMask];
if(_rom_size > 4096 && ((address & 0x1f00) == 0x1f00)) {
uint8_t *base_ptr = _romPages[0];
uint8_t first_paging_register = 0xf8 - (_rom_size >> 14)*2;
const uint8_t paging_register = address&0xff;
if(paging_register >= first_paging_register) {
const uint16_t selected_page = paging_register - first_paging_register;
if(selected_page * 4096 < _rom_size) {
base_ptr = &_rom[selected_page * 4096];
}
}
if(base_ptr != _romPages[0]) {
_romPages[0] = base_ptr;
_romPages[1] = base_ptr + 1024;
_romPages[2] = base_ptr + 2048;
_romPages[3] = base_ptr + 3072;
}
}
returnValue &= _romPages[(address >> 10)&3][address&1023];
}
// check for a RAM access
@ -458,7 +481,16 @@ int Machine::perform_bus_operation(CPU6502::BusOperation operation, uint16_t add
void Machine::set_rom(size_t length, const uint8_t *data)
{
length = std::min((size_t)4096, length);
memcpy(_rom, data, length);
_romMask = length - 1;
_rom_size = 1024;
while(_rom_size < length && _rom_size < 32768) _rom_size <<= 1;
delete[] _rom;
_rom = new uint8_t[_rom_size];
memcpy(_rom, data, std::min(_rom_size, length));
size_t romMask = _rom_size - 1;
_romPages[0] = _rom;
_romPages[1] = &_rom[1024 & romMask];
_romPages[2] = &_rom[2048 & romMask];
_romPages[3] = &_rom[3072 & romMask];
}

4
Machines/Atari2600.hpp
View File

@ -30,8 +30,8 @@ class Machine: public CPU6502::Processor<Machine> {
Outputs::CRT *get_crt() { return _crt; }
private:
uint8_t _rom[4096], _ram[128];
uint16_t _romMask;
uint8_t *_rom, *_romPages[4], _ram[128];
size_t _rom_size;
uint64_t _timestamp;