add basic ppu register reading

1 files changed, 62 insertions, 0 deletions

diff --git a/ppu.c b/ppu.c
index 5dc20a3..52837ad 100644
--- a/ppu.c
+++ b/ppu.c
@@ -2,6 +2,38 @@
 
 struct ppu ppu = {0};
 
+static void
+vram_addr_inc(void)
+{
+	ppu.regs.v += (ppu.ctrl.inc_mode) ? 32 : 1;
+}
+
+static uint16_t
+vram_addr_mirror(uint16_t addr)
+{
+	uint8_t nametable;
+
+	addr &= 0x2fff;
+	addr -= 0x2000;
+	nametable = addr / 0x400;
+
+	switch (ppu.rom->mirror) {
+	case M_HORIZONTAL:
+		if (nametable == 1 || nametable == 2)
+			return addr - 0x400;
+		else if (nametable == 3)
+			return addr - 0x800;
+		break;
+	case M_VERTICAL:
+		if (nametable == 2 || nametable == 3)
+			return addr - 0x800;
+	default:
+		break;
+	};
+
+	return addr;
+}
+
 void
 ppu_tick(void)
 {
@@ -17,6 +49,36 @@ ppu_tick(void)
 uint8_t
 ppu_read(uint16_t addr)
 {
+	uint8_t tmp;
+
+	switch (addr) {
+	case 0x2002:
+		tmp = (ppu.status.vblank << 7)
+		    | (ppu.status.sprite0_hit << 6)
+		    | (ppu.status.sprite_overflow << 5);
+
+		ppu.status.vblank = 0;
+		ppu.regs.w = 0;
+
+		return tmp;
+	case 0x2004:
+		return ppu.oam[ppu.oam_addr];
+	case 0x2007:
+		vram_addr_inc();
+
+		if (addr <= 0x1fff) {
+			tmp = ppu.last_read;
+			ppu.last_read = ppu.rom->chr_rom[ppu.regs.v];
+			return tmp;
+		} else if (addr <= 0x2fff) {
+			tmp = ppu.last_read;
+			ppu.last_read = ppu.vram[vram_addr_mirror(addr)];
+			return tmp;
+		}
+	default:
+		fprintf(stderr, "Invalid PPU read at address $%04\n", addr);
+		return 0;
+	}
 }
 
 void