From 2084552c536a8299a3578c8b9b687cff98e6d9a3 Mon Sep 17 00:00:00 2001
From: vin <git@vineetk.net>
Date: Sun, 1 Sep 2024 15:39:32 -0400
Subject: [PATCH] add basic ppu register reading

---
 cpu.c |  4 +---
 ppu.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 ppu.h | 15 ++++++++++++---
 3 files changed, 75 insertions(+), 6 deletions(-)

diff --git a/cpu.c b/cpu.c
index 97ece60..a04c603 100644
--- a/cpu.c
+++ b/cpu.c
@@ -28,9 +28,7 @@
 
 #define MEMORY_MIRROR(addr)	\
 	if (addr < 0x2000)	\
-		addr &= 0x07FF;	\
-	else if (addr < 0x4000)	\
-		addr &= 0x2007;
+		addr &= 0x07FF;
 
 #define PUSH(b)	\
 	(memwrite(0x0100 + regs.sp--, b))
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
diff --git a/ppu.h b/ppu.h
index ffd66d7..c5483e1 100644
--- a/ppu.h
+++ b/ppu.h
@@ -7,12 +7,23 @@
 #include "rom.h"
 
 struct ppu {
-	uint8_t oam[64*4];
 	uint8_t vram[2048];
+	uint8_t oam[64*4];
+	uint8_t oam_addr;	/* $2003 */
 	uint8_t palette[16*2];
 	uint16_t cycles, scanlines;
 	struct rom *rom;
 
+	uint8_t last_read;
+
+	struct ppu_regs {
+		uint16_t v : 15; /* current vram address */
+		uint16_t t : 15; /* temporary vram address */
+		uint8_t x : 3;   /* fine x scroll */
+		uint8_t w : 1;   /* first/second write toggle */
+		uint8_t padding : 6;
+	} regs;
+
 	struct ppu_ctrl {	/* $2000 */
 		uint8_t nmi_enable : 1;
 		uint8_t master_slave : 1;
@@ -36,8 +47,6 @@ struct ppu {
 		uint8_t sprite_overflow : 1;
 		uint8_t pad : 5;
 	} status;
-	uint8_t oam_addr;	/* $2003 */
-	uint8_t oam_data;	/* $2004 */
 };
 extern struct ppu ppu;