1 files changed, 114 insertions, 0 deletions
diff --git a/ppu/ppu.go b/ppu/ppu.go
new file mode 100644
index 0000000..cfc3620
--- /dev/null
+++ b/ppu/ppu.go
@@ -0,0 +1,114 @@
+package ppu
+
+import "crecher.net/gones/v2/cart"
+
+const (
+	ctrlNametableMask = 0x03
+	ctrlIncrement32   = 0x04
+	ctrlNMIEnable     = 0x80
+
+	maskShowBackground = 0x08
+	maskShowSprites    = 0x10
+
+	statusOverflow = 0x20
+	statusSprite0  = 0x40
+	statusVBlank   = 0x80
+)
+
+type PPU struct {
+	mapper    cart.Mapper
+	mirroring cart.Mirroring
+
+	vram    [0x800]uint8 // 2KB nametable RAM
+	palette [32]uint8
+
+	Ctrl    uint8 // $2000, latched
+	Mask    uint8 // $2001, latched
+	OAMAddr uint8 // $2003
+	OAM     [256]uint8
+
+	// scroll registers, see https://www.nesdev.org/wiki/PPU_scrolling
+	v, t uint16 // 15 bits used
+	x    uint8  // fine X, 3 bits used
+	w    bool   // write toggle, shared by $2005/$2006
+
+	readBuffer uint8 // internal $2007 read buffer
+	busLatch   uint8 // crude open-bus stand in last byte written to any register
+
+	spriteOverflow bool
+	sprite0Hit     bool
+	vblank         bool // aka nmi_occurred
+
+	Scanline int
+	Dot      int
+	Frame    uint64
+
+	nmiOutput  bool // PPUCTRL bit 7
+	nmiLine    bool // vblank && nmiOutput, latched each tick for edge detection
+	nmiPending bool // set on nmiLine's rising edge, consumed by PollNMI
+
+	// see the $2002 read/vblank race window handled in readStatus
+	suppressVBlankSet   bool
+	suppressNMIForFrame bool
+}
+
+func New(mapper cart.Mapper, mirroring cart.Mirroring) *PPU {
+	return &PPU{mapper: mapper, mirroring: mirroring}
+}
+
+// reports if an NMI edge fired since the last call, clearing it
+func (p *PPU) PollNMI() bool {
+	pending := p.nmiPending
+	p.nmiPending = false
+	return pending
+}
+
+func (p *PPU) renderingEnabled() bool {
+	return p.Mask&(maskShowBackground|maskShowSprites) != 0
+}
+
+func (p *PPU) updateNMILine() {
+	current := p.vblank && p.nmiOutput
+	if current && !p.nmiLine && !p.suppressNMIForFrame {
+		p.nmiPending = true
+	}
+	p.nmiLine = current
+}
+
+// advances PPU by one dot, right now only tracks the dot/scanline position and the vblank flag/NMI line,
+// which is enough to just validate the timings.
+func (p *PPU) Tick() {
+	p.advanceDot()
+
+	switch {
+	case p.Scanline == 241 && p.Dot == 1:
+		if !p.suppressVBlankSet {
+			p.vblank = true
+		}
+		p.suppressVBlankSet = false
+	case p.Scanline == 261 && p.Dot == 1:
+		p.vblank = false
+		p.sprite0Hit = false
+		p.spriteOverflow = false
+		p.suppressVBlankSet = false
+		p.suppressNMIForFrame = false
+	}
+	p.updateNMILine()
+}
+
+func (p *PPU) advanceDot() {
+	maxDot := 340
+	if p.Scanline == 261 && p.renderingEnabled() && p.Frame%2 == 1 {
+		maxDot = 339 // pre-render line is one dot short every odd frame while rendering
+	}
+
+	p.Dot++
+	if p.Dot > maxDot {
+		p.Dot = 0
+		p.Scanline++
+		if p.Scanline > 261 {
+			p.Scanline = 0
+			p.Frame++
+		}
+	}
+}