ITS ALIVE (not rly no image yet)

1 files changed, 46 insertions, 0 deletions

diff --git a/ppu/memory.go b/ppu/memory.go
new file mode 100644
index 0000000..3379242
--- /dev/null
+++ b/ppu/memory.go
@@ -0,0 +1,46 @@
+package ppu
+
+import "crecher.net/gones/v2/cart"
+
+// maps a $2000-$3EFF address onto an offset into the 2KB of nametable RAM,
+// according to the cartridge's mirroring mode. four-screen carts will be
+// expected to use their own extra 2KB of RAM via the mapper, but since no
+// implemented mapper does that yet, four-screen just falls back to the vertical
+// mirroring as a placeholder
+
+func (p *PPU) nametableOffset(addr uint16) int {
+	addr &= 0x0FFF
+	table := addr / 0x400
+	offset := int(addr % 0x400)
+
+	var page uint16
+	switch p.mirroring {
+	case cart.Horizontal:
+		page = table / 2
+	case cart.SingleScreenLower:
+		page = 0
+	case cart.SingleScreenUpper:
+		page = 1
+	default: // TODO: add correct four-screen behavior once four-screen carts are added
+		page = table % 2
+	}
+	return int(page)*0x400 + offset
+}
+
+// paletteIndex applies the $3F10/$3F14/$3F18/$3F1C -> $3F00/$3F04/$3F08/$3F0C
+// mirroring
+func paletteIndex(addr uint16) uint16 {
+	idx := addr & 0x1F
+	if idx >= 0x10 && idx%4 == 0 {
+		idx -= 0x10
+	}
+	return idx
+}
+
+func (p *PPU) readPalette(addr uint16) uint8 {
+	return p.palette[paletteIndex(addr)]
+}
+
+func (p *PPU) writePalette(addr uint16, v uint8) {
+	p.palette[paletteIndex(addr)] = v
+}