add basic implementation of ppu write registers

Makefile

@@ -1,5 +1,7 @@
+CFLAGS = -O2 -Wall -Wextra -pedantic -Wno-unused-parameter
+
 nes:
-	${CC} -o nes cpu.c rom.c
+	${CC} ${CFLAGS} -o nes cpu.c rom.c ppu.c apu.c
 
 test: nes
 	./nes ${HOME}/src/other/nes-test-roms/other/nestest.nes

README.md

@@ -1,6 +1,29 @@
-emu_nes
-=======
+# emu_nes
 A Nintendo Entertainment System (NES) emulator written for the purpose of
 understanding how the NES and similar 8-bit computers of that era worked.
 
-Makes heavy use of https://www.nesdev.org/.
+The NES used a variant of the MOS 6502 8-bit microcontroller called a
+Ricoh 2A03 that contained the 6502 without the BCD mode for arithmetic,
+an audio signal generator (APU), controlling the gamepads, and for DMA.
+([Source: NESdev](https://www.nesdev.org/wiki/DMA))
+
+A major goal for the emulator is finishing the software implementation
+in C and later implement this in hardware with either an FPGA or another
+CPU like a RISC-V. Later, I want to generalize the 2A03 simulator into
+a full 6502 for use with a Commodore 64 and Apple II.
+
+Makes heavy use of the [NESdev wiki](https://www.nesdev.org/wiki).
+
+## Current State
+- [x] CPU (Ricoh 2A03, variant of MOS 6502)
+- [ ] PPU (Graphics)
+- [ ] APU (Audio)
+
+Currently, running the emulator with nestest.nes results in incorrect
+results in the memory space of the APU registers. This is due to the
+APU not being implemented yet.
+
+Also, the only ROM mapper that is implemented is Mapper 0 to
+run the test ROM, but it also supports many other ROMs listed
+[here](https://nesdir.github.io/mapper0.html) such as Donkey Kong and
+Super Mario Bros.

apu.c

@@ -0,0 +1,55 @@
+#include <stdio.h>
+#include <string.h>
+
+#include "apu.h"
+
+struct apu apu = {0};
+
+void
+apu_init(void)
+{
+	memset(&apu, 0, sizeof(apu));
+}
+
+uint8_t
+apu_read(uint16_t addr)
+{
+	switch (addr) {
+	/* pulse1 */
+	case 0x4000:
+		return (apu.pulse1.duty << 6) | (apu.pulse1.envelope << 5)
+		    | (apu.pulse1.const_vol << 4) | apu.pulse1.envelope_vol;
+	case 0x4001:
+		return (apu.pulse1.sweep_enable << 7) | (apu.pulse1.sweep_period << 4)
+		    | (apu.pulse1.sweep_negative << 3) | apu.pulse1.sweep_shift_count;
+	case 0x4002:
+		return apu.pulse1.timer_low;
+	case 0x4003:
+		return (apu.pulse1.length_counter_load << 3) | apu.pulse1.timer_high;
+	/* pulse2 */
+	case 0x4004:
+		return (apu.pulse2.duty << 6) | (apu.pulse2.envelope << 5)
+		    | (apu.pulse2.const_vol << 4) | apu.pulse2.envelope_vol;
+	case 0x4005:
+		return (apu.pulse1.sweep_enable << 7) | (apu.pulse1.sweep_period << 4)
+		    | (apu.pulse1.sweep_negative << 3) | apu.pulse1.sweep_shift_count;
+	case 0x4006:
+		return apu.pulse1.timer_low;
+	case 0x4007:
+		return (apu.pulse1.length_counter_load << 3) | apu.pulse1.timer_high;
+	/* status */
+	case 0x4015:
+		return (apu.status.dmc_int << 7) | (apu.status.frame_int << 6)
+		    | (apu.status.dmc_active << 4) | (apu.status.lc_noise << 3)
+		    | (apu.status.lc_triangle << 2) | (apu.status.lc_pulse2 << 1)
+		    | apu.status.lc_pulse1;
+	default:
+		fprintf(stderr, "Invalid APU read at $%04X!\n", addr);
+		return 0;
+	};
+}
+
+void
+apu_write(uint16_t addr, uint8_t byte)
+{
+}

apu.h

@@ -0,0 +1,105 @@
+#ifndef APU_H
+#define APU_H
+
+#include <stdint.h>
+
+struct pulse {
+	/* $4000/$4004 */
+	uint8_t duty : 2;
+	uint8_t envelope : 1;
+	uint8_t const_vol : 1;
+	uint8_t envelope_vol : 4;
+	/* $4001/$4005 */
+	uint8_t sweep_enable : 1;
+	uint8_t sweep_period : 3;
+	uint8_t sweep_negative : 1;
+	uint8_t sweep_shift_count : 3;
+	/* $4002/$4006 */
+	uint8_t timer_low;
+	/* $4003/$4007 */
+	uint8_t length_counter_load : 5;
+	uint8_t timer_high : 3;
+};
+
+struct triangle {
+	/* $4008 */
+	uint8_t counter_disable : 1;
+	uint8_t counter_reload : 7;
+	/* $400A */
+	uint8_t timer_low;
+	/* $400B */
+	uint8_t counter_load : 5;
+	uint8_t timer_high : 3;
+};
+
+struct noise {
+	/* $400C */
+	uint8_t padding1 : 2;
+	uint8_t loop_envelope : 1;
+	uint8_t constant_volume : 1;
+	uint8_t envelope_volume : 4;
+	/* $400E */
+	uint8_t loop_noise : 1;
+	uint8_t padding2 : 3;
+	uint8_t noise_period : 4;
+	/* $400F */
+	uint8_t length_counter_load : 5;
+	uint8_t padding3 : 3;
+};
+
+struct dmc {
+	/* $4010 */
+	uint8_t irq_enable : 1;
+	uint8_t loop : 1;
+	uint8_t padding1 : 2;
+	uint8_t frequency : 4;
+	/* $4011 */
+	uint8_t padding2 : 1;
+	uint8_t load_counter : 7;
+	/* $4012 */
+	uint8_t sample_addr;
+	/* $4013 */
+	uint8_t sample_length;
+};
+
+struct apu {
+	/* $4000-$4013 (write) */
+	struct pulse pulse1;
+	struct pulse pulse2;
+	struct triangle triangle;
+	struct noise noise;
+	struct dmc dmc;
+	/* $4015 (write) */
+	struct control {
+		uint8_t padding : 3;
+		uint8_t dmc_enable : 1;
+		uint8_t noise_enable : 1;
+		uint8_t triangle_enable : 1;
+		uint8_t pulse2_enable : 1;
+		uint8_t pulse1_enable : 1;
+	} control;
+	/* $4015 (read) */
+	struct status {
+		uint8_t dmc_int : 1;
+		uint8_t frame_int : 1;
+		uint8_t padding : 1;
+		uint8_t dmc_active : 1;
+		uint8_t lc_noise : 1;
+		uint8_t lc_triangle : 1;
+		uint8_t lc_pulse2 : 1;
+		uint8_t lc_pulse1 : 1;
+	} status;
+	/* $4017 (write) */
+	struct frame_counter {
+		uint8_t mode : 1;
+		uint8_t irq_inhibit : 1;
+		uint8_t padding : 6;
+	} frame_counter;
+};
+extern struct apu apu;
+
+void apu_tick(void);
+uint8_t apu_read(uint16_t addr);
+void apu_write(uint16_t addr, uint8_t byte);
+
+#endif /* APU_H */

cpu.c

@@ -5,8 +5,10 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "apu.h"
 #include "cpu.h"
 #include "opcodes.h"
+#include "ppu.h"
 #include "rom.h"
 
 #define MAX(a, b)	((a > b) ? a : b)
@@ -26,16 +28,14 @@
 
 #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))
 #define PULL()	\
 	(peek(0x0100 + ++regs.sp))
 
-struct Rom rom = {0};
+struct rom rom = {0};
 uint32_t cycles = 0;
 
 bool page_crossed = false;
@@ -52,10 +52,11 @@ peek(uint16_t addr)
 			return rom.prg_rom[addr - 0x8000];
 		else
 			fprintf(stderr, "PRG ROG size is not 0x4000 nor 0x8000\n"), exit(1);
-	} else {
+	} else if ((addr >= 0x4000 && addr <= 0x4013) || addr == 0x4015 || addr == 0x4017)
+		return apu_read(addr);
+	else
 		return memory[addr];
 }
-}
 
 static uint16_t
 peek16(uint16_t addr)
@@ -67,6 +68,11 @@ peek16(uint16_t addr)
 static void
 memwrite(uint16_t addr, uint8_t byte)
 {
+	if (addr >= 0x4000 && addr <= 0x4017) {
+		apu_write(addr, byte);
+		return;
+	}
+
 	MEMORY_MIRROR(addr);
 
 	memory[addr] = byte;
@@ -145,15 +151,24 @@ opcode_mem(enum addressing_mode mode)
 	return val;
 }
 
+static void
+tick(void)
+{
+	cycles++;
+	ppu_tick();
+	ppu_tick();
+	ppu_tick();
+}
+
 static void
 branch(uint16_t addr, bool cond)
 {
 	if (!cond)
 		return;
-	cycles++;
+	tick();
 
 	if (((regs.pc + 1) & 0xFF00) != (addr & 0xFF00))
-		cycles++;
+		tick();
 
 	regs.pc = addr;
 }
@@ -946,18 +961,21 @@ interpret(void)
 
 		while (spaces--) putchar(' ');
 
-		printf("A:%02X X:%02X Y:%02X P:%02X SP:%02X CYC:%d\n",
-		    regs.a, regs.x, regs.y, STATUS_TO_INT(), regs.sp, cycles);
+		printf("A:%02X X:%02X Y:%02X P:%02X SP:%02X PPU:%3d,%3d CYC:%d\n",
+		    regs.a, regs.x, regs.y, STATUS_TO_INT(), regs.sp,
+		    ppu.scanlines, ppu.cycles, cycles);
 
 		if (opcodes[op].memread)
 			opcodes[op].instr(peek(arg));
 		else
 			opcodes[op].instr(arg);
 
-		cycles += opcodes[op].cycles;
+		for (uint8_t i = 0; i < opcodes[op].cycles; i++)
+			tick();
+
 		if (page_crossed) {
 			if (opcodes[op].page_cross)
-				cycles++;
+				tick();
 			page_crossed = false;
 		}
 	}
@@ -976,6 +994,8 @@ cpu_init(void)
 	regs.status.unused = 1;
 
 	cycles += 7;
+	for (uint8_t i = 0; i < 7 * 3; i++)
+		ppu_tick();
 }
 
 int
@@ -1005,6 +1025,7 @@ main(int argc, char *argv[])
 
 	parse_rom(buf, buflen, &rom);
 	free(buf);
+	ppu.rom = &rom;
 
 	cpu_init();
 

ppu.c

@@ -0,0 +1,156 @@
+#include "ppu.h"
+
+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)
+{
+	ppu.cycles++;
+
+	if (ppu.cycles > 256 && ppu.cycles <= 320)
+		ppu.oam_addr = 0;
+
+	if (ppu.cycles >= 341) {
+		ppu.cycles -= 341;
+		ppu.scanlines++;
+	}
+	if (ppu.scanlines >= 262)
+		ppu.scanlines -= 262;
+}
+
+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.write_toggle = 0;
+
+		return tmp;
+	case 0x2004:
+		return ppu.oam[ppu.oam_addr];
+	case 0x2007:
+		tmp = ppu.last_read;
+
+		if (addr <= 0x1fff)
+			ppu.last_read = ppu.rom->chr_rom[ppu.regs.v];
+		else if (addr <= 0x2fff)
+			ppu.last_read = ppu.vram[vram_addr_mirror(addr)];
+
+		vram_addr_inc();
+
+		return tmp;
+	default:
+		fprintf(stderr, "Invalid PPU read at address $%04\n", addr);
+		return 0;
+	}
+}
+
+void
+ppu_write(uint16_t addr, uint8_t byte)
+{
+	switch (addr) {
+	case 0x2000:
+		ppu.ctrl.nametable_base = byte & 3;
+		ppu.ctrl.inc_mode = byte & (1 << 2);
+		ppu.ctrl.sprite_tile_sel = byte & (1 << 3);
+		ppu.ctrl.bg_tile_sel = byte & (1 << 4);
+		ppu.ctrl.sprite_height = byte & (1 << 5);
+		ppu.ctrl.master_slave = byte & (1 << 6);
+		ppu.ctrl.nmi_enable = byte & (1 << 7);
+
+		if (ppu.ctrl.nametable_base & 1)
+			ppu.regs.scroll_x += 256;
+		if (ppu.ctrl.nametable_base & 2)
+			ppu.regs.scroll_y += 240;
+
+		/*
+		 * TODO: send NMI if vblank flag still set and nmi_enable
+		 * changes from 0 to 1
+		 */
+		break;
+	case 0x2001:
+		ppu.mask.grayscale = byte & 1;
+		ppu.mask.bg_left_enable = byte & (1 << 1);
+		ppu.mask.sprite_left_enable = byte & (1 << 2);
+		ppu.mask.bg_enable = byte & (1 << 3);
+		ppu.mask.sprite_enable = byte & (1 << 4);
+		ppu.mask.colour_emphasis = byte & (1 << 5);
+
+		break;
+	case 0x2003:
+		ppu.oam_addr = byte;
+
+		break;
+	case 0x2004:
+		ppu.oam[ppu.oam_addr++] = byte;
+
+		break;
+	case 0x2005:
+		if (ppu.regs.write_toggle == 0)
+			ppu.regs.scroll_x = byte;
+		else
+			ppu.regs.scroll_y = byte;
+		ppu.regs.write_toggle = !ppu.regs.write_toggle;
+
+		break;
+	case 0x2006:
+		if (ppu.regs.write_toggle == 0)
+			ppu.regs.v = (byte << 8);
+		else {
+			ppu.regs.v |= byte;
+			ppu.regs.v &= 0x3fff;
+		}
+		ppu.regs.write_toggle = !ppu.regs.write_toggle;
+
+		break;
+	case 0x2007:
+		ppu.vram[ppu.regs.v] = byte;
+		vram_addr_inc();
+
+		break;
+	case 0x4014:
+		break;
+	default:
+		fprintf(stderr, "Invalid PPU write at address $%04X\n", addr);
+		return;
+	}
+}

ppu.h

@@ -0,0 +1,55 @@
+#ifndef PPU_H
+#define PPU_H
+
+#include <stdio.h>
+#include <stdint.h>
+
+#include "rom.h"
+
+struct ppu {
+	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 */
+		uint16_t scroll_x : 9;    /* x scroll position */
+		uint16_t scroll_y : 9;    /* y scroll position */
+		uint8_t write_toggle : 1; /* first/second write toggle */
+	} regs;
+
+	struct ppu_ctrl {	/* $2000 */
+		uint8_t nmi_enable : 1;
+		uint8_t master_slave : 1;
+		uint8_t sprite_height : 1;
+		uint8_t bg_tile_sel : 1;
+		uint8_t sprite_tile_sel : 1;
+		uint8_t inc_mode : 1;
+		uint8_t nametable_base : 2;
+	} ctrl;
+	struct ppu_mask {	/* $2001 */
+		uint8_t colour_emphasis : 3;
+		uint8_t sprite_enable : 1;
+		uint8_t bg_enable : 1;
+		uint8_t sprite_left_enable : 1;
+		uint8_t bg_left_enable : 1;
+		uint8_t grayscale : 1;
+	} mask;
+	struct ppu_status {	/* $2002 */
+		uint8_t vblank : 1;
+		uint8_t sprite0_hit : 1;
+		uint8_t sprite_overflow : 1;
+		uint8_t pad : 5;
+	} status;
+};
+extern struct ppu ppu;
+
+void ppu_tick(void);
+
+#endif /* PPU_H */

rom.c

@@ -6,7 +6,7 @@
 #include "rom.h"
 
 void
-parse_rom(const uint8_t *data, size_t data_len, struct Rom *rom)
+parse_rom(const uint8_t *data, size_t data_len, struct rom *rom)
 {
 	size_t prg_rom_offset = 16;
 	size_t chr_rom_offset = 0;
@@ -48,7 +48,7 @@ parse_rom(const uint8_t *data, size_t data_len, struct Rom *rom)
 }
 
 void
-free_rom(struct Rom *rom)
+free_rom(struct rom *rom)
 {
 	free(rom->prg_rom);
 	if (rom->chr_rom_size > 0)

rom.h

@@ -1,10 +1,13 @@
+#ifndef ROM_H
+#define ROM_H
+
 enum screen_mirroring {
 	M_HORIZONTAL,
 	M_VERTICAL,
 	M_FOUR,
 };
 
-struct Rom {
+struct rom {
 	char *prg_rom;
 	char *chr_rom;
 	size_t prg_rom_size;
@@ -13,5 +16,7 @@ struct Rom {
 	enum screen_mirroring mirror;
 };
 
-void parse_rom(const uint8_t *data, size_t data_len, struct Rom *rom);
-void free_rom(struct Rom *rom);
+void parse_rom(const uint8_t *data, size_t data_len, struct rom *rom);
+void free_rom(struct rom *rom);
+
+#endif /* ROM_H */

shell.nix

@@ -0,0 +1,6 @@
+with import <nixpkgs> {}; mkShellNoCC {
+	nativeBuildInputs = [
+		bmake
+		clang
+	];
+}