1 files changed, 141 insertions, 0 deletions
diff --git a/ppu/registers.go b/ppu/registers.go
new file mode 100644
index 0000000..5445619
--- /dev/null
+++ b/ppu/registers.go
@@ -0,0 +1,141 @@
+package ppu
+
+func (p *PPU) ReadRegister(addr uint16) uint8 {
+	switch addr & 0x7 {
+	case 2:
+		return p.readStatus()
+	case 4:
+		return p.OAM[p.OAMAddr]
+	case 7:
+		return p.readData()
+	default:
+		return p.busLatch // $2000,2001,2003,2005,2006 are write only: open bus
+	}
+}
+
+// handles cpu write to $2000-2007
+func (p *PPU) WriteRegister(addr uint16, v uint8) {
+	p.busLatch = v
+	switch addr & 0x7 {
+	case 0:
+		p.Ctrl = v
+		p.t = (p.t &^ 0x0C00) | (uint16(v&ctrlNametableMask) << 10)
+		p.nmiOutput = v&ctrlNMIEnable != 0
+		p.updateNMILine()
+	case 1:
+		p.Mask = v
+	case 3:
+		p.OAMAddr = v
+	case 4:
+		p.OAM[p.OAMAddr] = v
+		p.OAMAddr++
+	case 5:
+		p.writeScroll(v)
+	case 6:
+		p.writeAddr(v)
+	case 7:
+		p.writeData(v)
+	}
+}
+
+// implements the $2002 read, including the hardware race around the dot
+// vblank gets set (see https://www.nesdev.org/wiki/NMI)
+func (p *PPU) readStatus() uint8 {
+	result := p.busLatch & 0x1F
+	if p.vblank {
+		result |= statusVBlank
+	}
+	if p.sprite0Hit {
+		result |= statusSprite0
+	}
+	if p.spriteOverflow {
+		result |= statusOverflow
+	}
+
+	if p.Scanline == 241 {
+		switch p.Dot {
+		case 0:
+			p.suppressVBlankSet = true
+			p.suppressNMIForFrame = true
+		case 1, 2:
+			p.suppressNMIForFrame = true
+			p.nmiPending = false
+		}
+	}
+
+	p.vblank = false
+	p.w = false
+	p.updateNMILine()
+	return result
+}
+
+func (p *PPU) writeScroll(v uint8) {
+	if !p.w {
+		p.t = (p.t &^ 0x001F) | uint16(v>>3)
+		p.x = v & 0x07
+	} else {
+		p.t = (p.t &^ 0x73E0) | (uint16(v&0x07) << 12) | (uint16(v&0xF8) << 2)
+	}
+	p.w = !p.w
+}
+
+func (p *PPU) writeAddr(v uint8) {
+	if !p.w {
+		p.t = (p.t & 0x00FF) | (uint16(v&0x3F) << 8)
+	} else {
+		p.t = (p.t & 0xFF00) | uint16(v)
+		p.v = p.t
+	}
+	p.w = !p.w
+}
+
+// implements buffered $2007 and a quirk which i am too lazy to write down
+// right now :D
+// https://www.nesdev.org/wiki/PPU_registers#The_PPUDATA_data_port
+func (p *PPU) readData() uint8 {
+	addr := p.v & 0x3FFF
+	var result uint8
+	if addr >= 0x3F00 {
+		result = p.readPalette(addr)
+		p.readBuffer = p.readVRAM(addr & 0x2FFF)
+	} else {
+		result = p.readBuffer
+		p.readBuffer = p.readVRAM(addr)
+	}
+	p.incrementV()
+	return result
+}
+
+func (p *PPU) writeData(v uint8) {
+	addr := p.v & 0x3FFF
+	if addr >= 0x3F00 {
+		p.writePalette(addr, v)
+	} else {
+		p.writeVRAM(addr, v)
+	}
+	p.incrementV()
+}
+
+func (p *PPU) incrementV() {
+	if p.Ctrl&ctrlIncrement32 != 0 {
+		p.v += 32
+	} else {
+		p.v++
+	}
+	p.v &= 0x7FFF
+}
+
+func (p *PPU) readVRAM(addr uint16) uint8 {
+	if addr < 0x2000 {
+		return p.mapper.PPURead(addr)
+	}
+	return p.vram[p.nametableOffset(addr)]
+}
+
+func (p *PPU) writeVRAM(addr uint16, v uint8) {
+	if addr < 0x2000 {
+		p.mapper.PPUWrite(addr, v)
+		return
+	}
+	p.vram[p.nametableOffset(addr)] = v
+}