public PpuMemoryRegisters(IPpu ppu)
{
this.ppu = ppu;
LCDC = new LcdControlRegister(ppu);
STAT = new LcdStatusRegister(ppu);
}
// We clock our CPU at 1mhz so we use MCycles
// The GPU state machine works ayncronously to the CPU (in HW but not in the emulator). It works like this, measured in CPU cycles:
// [OAM Search 20 cycles] -> [Pixel Transfer 43 cycles] -> [HBlank 51] = 114 clocks per line
// [VBlank 1140 cycles]
// 144 lines on screen + 10 lines vblank = 154 lines
// 114 * 154 = 17,556 clocks per screen
// 1,048,576 / 17,556 = 59.72hz refresh rate
public void Step()
{
UInt32 cpuTickCount = dmg.cpu.Ticks;
// Here we monitor how many cycles the CPU has executed and we map the GPU state to match how the real hardware behaves. This allows
// us to generate interupts at the right time
if (MemoryRegisters.LCDC.LcdEnable == 0)
{
return;
}
UInt32 tickCount = (cpuTickCount - lastCpuTickCount);
// Track how many cycles the CPU has done since we last changed states
elapsedTicks += tickCount;
lastCpuTickCount = cpuTickCount;
LcdStatusRegister lcdStat = MemoryRegisters.STAT;
switch (Mode)
{
// See the enable function for some details on this
case PpuMode.Glitched_OAM:
if (elapsedTicks >= Glitched_OAM_Length)
{
Mode = PpuMode.PixelTransfer;
elapsedTicks -= Glitched_OAM_Length;
}
break;
case PpuMode.OamSearch:
if (elapsedTicks >= OAM_Length)
{
OamSearch();
Mode = PpuMode.PixelTransfer;
elapsedTicks -= OAM_Length;
}
break;
// Transfers one scanline of pixels to the screen
// The emulator must also work this way as the cpu is still running and many graphical effects change the state of the system between scanlines
case PpuMode.PixelTransfer:
if (elapsedTicks >= PixelTransfer_Length)
{
RenderScanline();
Mode = PpuMode.HBlank;
if (lcdStat.HBlankInterruptEnable)
{
dmg.interrupts.RequestInterrupt(Interrupts.Interrupt.INTERRUPTS_LCDSTAT);
}
elapsedTicks -= PixelTransfer_Length;
}
break;
// PPU is idle during hblank
case PpuMode.HBlank:
// 51 machine cycles ticks (mcycles)
if (elapsedTicks >= HBlank_Length)
{
CurrentScanline++;
// Coincidence interrupt checks if the scanline is equal to the value in FF45
if (lcdStat.LycLyCoincidenceInterruptEnable &&
CurrentScanline == lcdStat.LYC)
{
dmg.interrupts.RequestInterrupt(Interrupts.Interrupt.INTERRUPTS_LCDSTAT);
}
if (CurrentScanline == 144)
{
Mode = PpuMode.VBlank;
vblankTicks = (elapsedTicks - HBlank_Length);
//This will only fire the interrupt if IE for vblank is set
dmg.interrupts.RequestInterrupt(Interrupts.Interrupt.INTERRUPTS_VBLANK);
if (lcdStat.VBlankInterruptEnable)
{
dmg.interrupts.RequestInterrupt(Interrupts.Interrupt.INTERRUPTS_LCDSTAT);
}
// We can set the renderer drawing the frame as soon as we enter vblank
lock (FrameBuffer)
{
// Flip frames
if (FrameBuffer == frameBuffer0)
{
FrameBuffer = frameBuffer1;
drawBuffer = frameBuffer0;
}
else
{
FrameBuffer = frameBuffer0;
drawBuffer = frameBuffer1;
}
}
// lock to 60fps
double fps60 = 1000 / 60.0;
while (dmg.EmulatorTimer.Elapsed.TotalMilliseconds - lastFrameTime < fps60)
{
}
lastFrameTime = dmg.EmulatorTimer.Elapsed.TotalMilliseconds;
if (dmg.OnFrame != null)
{
dmg.OnFrame();
}
}
else
{
Mode = PpuMode.OamSearch;
if (lcdStat.OamInterruptEnable)
{
dmg.interrupts.RequestInterrupt(Interrupts.Interrupt.INTERRUPTS_LCDSTAT);
}
}
// Don't lose any ticks, cannot set to zero
elapsedTicks -= HBlank_Length;
}
break;
// PPU is idle during vblank
// The vblank takes the equivilant of 10 scanlines
case PpuMode.VBlank:
vblankTicks += tickCount;
if (elapsedTicks >= ScanLine_Length)
{
CurrentScanline++;
// Coincidence interrupt checks if the scanline is equal to the value in FF45
if (lcdStat.LycLyCoincidenceInterruptEnable &&
CurrentScanline == lcdStat.LYC)
{
dmg.interrupts.RequestInterrupt(Interrupts.Interrupt.INTERRUPTS_LCDSTAT);
}
elapsedTicks -= ScanLine_Length;
}
if (CurrentScanline == 154)
{
CurrentScanline = 0;
// Coincidence interrupt checks if the scanline is equal to the value in FF45
if (lcdStat.LycLyCoincidenceInterruptEnable &&
CurrentScanline == lcdStat.LYC)
{
dmg.interrupts.RequestInterrupt(Interrupts.Interrupt.INTERRUPTS_LCDSTAT);
}
if ((vblankTicks - elapsedTicks) != VBlank_Length)
{
throw new Exception("PPU out of sync");
}
Mode = PpuMode.OamSearch;
// Debugger hooks
if (dmg.OnFrameEnd != null)
{
dmg.OnFrameEnd(frame, false);
}
frame++;
// Debugger hooks
if (dmg.OnFrameStart != null)
{
dmg.OnFrameStart(frame);
}
}
break;
}
}
