ppu: add background rendering

cmd/gones: add sdl2 frontend

1 files changed, 20 insertions, 4 deletions

diff --git a/ppu/ppu.go b/ppu/ppu.go
index cfc3620..322cf3d 100644
--- a/ppu/ppu.go
+++ b/ppu/ppu.go
@@ -5,10 +5,13 @@ import "crecher.net/gones/v2/cart"
 const (
 	ctrlNametableMask = 0x03
 	ctrlIncrement32   = 0x04
+	ctrlBGPatternHigh = 0x10
 	ctrlNMIEnable     = 0x80
 
-	maskShowBackground = 0x08
-	maskShowSprites    = 0x10
+	maskGreyscale          = 0x01
+	maskShowBackgroundLeft = 0x02
+	maskShowBackground     = 0x08
+	maskShowSprites        = 0x10
 
 	statusOverflow = 0x20
 	statusSprite0  = 0x40
@@ -43,6 +46,19 @@ type PPU struct {
 	Dot      int
 	Frame    uint64
 
+	// finished pixels as ARGB8888
+	framebuffer [ScreenWidth * ScreenHeight]uint32
+
+	// background pipeline, see https://www.nesdev.org/wiki/PPU_rendering
+	// next* hold the bytes fetched for the tile after the one being drawn
+	nextTileID    uint8
+	nextAttr      uint8
+	nextPatternLo uint8
+	nextPatternHi uint8
+	// shifters, the top bit (minus fine x) is the pixel being drawn
+	patternLo, patternHi uint16
+	attrLo, attrHi       uint16
+
 	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
@@ -75,10 +91,10 @@ func (p *PPU) updateNMILine() {
 	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.
+// advances PPU by one dot
 func (p *PPU) Tick() {
 	p.advanceDot()
+	p.renderTick()
 
 	switch {
 	case p.Scanline == 241 && p.Dot == 1: