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added latch to ppu

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Lauchmelder 2022-02-28 17:14:32 +01:00
parent 7e50290634
commit 2b00f1ecb2
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GPG key ID: C2403C69D78F011D
2 changed files with 54 additions and 12 deletions
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28
src/PPU.cpp
View file

@ -35,8 +35,10 @@ void PPU::Reset()
void PPU::Tick()
{
// On this cycle the VBlankStarted bit is set in the ppustatus
if (y == 241 && x == 1)
{
// Set flag and send NMI if necessary
ppustatus.Flag.VBlankStarted = 1;
if (ppuctrl.Flag.VBlankNMI)
bus->NMI();
@ -44,9 +46,11 @@ void PPU::Tick()
isFrameDone = true;
}
// This cycle resets the VBlankStarted flag
if (y == 261 && x == 1)
ppustatus.Flag.VBlankStarted = 0;
// Advance pixel counters
x++;
if (x > 340)
{
@ -59,34 +63,33 @@ void PPU::Tick()
Byte PPU::ReadRegister(Byte id)
{
Byte returnVal = 0x00;
// Reading from a register fills the latch with the contents of the register
// Write-only regs don't fill the latch
// But in any case, the latch contents are returned
switch (id)
{
case 0:
returnVal = ppuctrl.Raw;
latch = ppuctrl.Raw;
break;
case 1:
returnVal = ppumask.Raw;
latch = ppumask.Raw;
break;
case 2:
returnVal = ppustatus.Raw;
latch = ppustatus.Raw;
ppustatus.Flag.VBlankStarted = 0;
addressLatch = 0;
break;
case 5:
returnVal = 0x00;
break;
case 6:
returnVal = 0x00;
break;
case 7:
returnVal = bus->ReadPPU(ppuaddr.Raw);
latch = bus->ReadPPU(ppuaddr.Raw);
ppuaddr.Raw += (ppuctrl.Flag.VRAMAddrIncrement ? 32 : 1);
break;
@ -95,7 +98,7 @@ Byte PPU::ReadRegister(Byte id)
break;
}
return returnVal;
return latch;
}
void PPU::WriteRegister(Byte id, Byte val)
@ -114,13 +117,16 @@ void PPU::WriteRegister(Byte id, Byte val)
ppustatus.Raw = val;
break;
// PPUADDR and PPUSCROLL both take 2 accesses to fully set
// When writing to them the address latch is switched. The latch
// determines whether the hi or lo byte should be written next
case 5:
if (addressLatch == 0)
ppuscroll.x = val;
else
ppuscroll.y = val;
addressLatch = 1 - addressLatch;
addressLatch = !addressLatch;
break;
case 6:
@ -129,7 +135,7 @@ void PPU::WriteRegister(Byte id, Byte val)
else
ppuaddr.Bytes.lo = val;
addressLatch = 1 - addressLatch;
addressLatch = !addressLatch;
break;
case 7:

38
src/PPU.hpp
View file

@ -4,6 +4,9 @@
class Bus;
/**
* @brief The PPU of the NES.
*/
class PPU
{
friend class PPUWatcher;
@ -11,18 +14,49 @@ class PPU
public:
PPU(Bus* bus);
/**
* @brief Powerup PPU.
* Internal state corresponds to powerup state
*/
void Powerup();
/**
* @brief Powerup PPU.
* Internal state corresponds to reset state
*/
void Reset();
/**
* @brief Tick PPU forward once.
*/
void Tick();
/**
* @brief Read from memory mapped PPU regs.
*/
Byte ReadRegister(Byte id);
/**
* @brief Write to memory mapped PPU regs.
*/
void WriteRegister(Byte id, Byte val);
/**
* @brief Check whether the PPU finished rendering a frame.
* Returns true if the VBlankStart cycle was hit previously. The function resets
* the boolearn when it is called
*/
inline bool IsFrameDone() { bool returnVal = isFrameDone; isFrameDone = false; return returnVal; }
private:
/**
* @brief Wraps Bus::ReadPPU.
*/
Byte Read(Word addr);
/**
* @brief Wraps Bus::WritePPU.
*/
void Write(Word addr, Byte val);
private: // Registers
@ -81,8 +115,10 @@ private: // Registers
Address ppuaddr;
// Current x, y position the PPU operates on
uint16_t x, y;
Byte addressLatch = 0;
Byte latch = 0;
bool addressLatch = false;
private:
bool isFrameDone = false;