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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
}
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