implement basic iNES and Mapper 0 ROM loading

It seems like the test ROM loads fine but the instructions are not,
but that's exactly what the test ROM is for I suppose.

cpu.c

@@ -1,8 +1,11 @@
+#include <libgen.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
+#include "rom.h"
+
 #define MAX(a, b)	((a > b) ? a : b)
 
 #define STATUS_UPDATE_ZERO(r)		\
@@ -37,7 +40,9 @@ struct registers {
 	struct cpu_flags status;
 	uint16_t pc;
 };
-struct registers regs;
+struct registers regs = {0};
+
+struct Rom rom = {0};
 
 enum addressing_mode {
 	AM_IMM,
@@ -56,9 +61,6 @@ enum addressing_mode {
 /* 64K address space, 16bit words */
 uint8_t memory[0x16000];
 
-/* example program taken from https://bugzmanov.github.io/nes_ebook/chapter_3_1.html */
-uint8_t program[] = { 0xa9, 0xc0, 0xaa, 0xe8, 0x00 };
-
 uint32_t cycles = 0;
 
 static uint8_t
@@ -66,7 +68,14 @@ peek(uint16_t addr)
 {
 	MEMORY_MIRROR(addr);
 
-	return memory[addr];
+	if (addr > 0x7FFF) {
+		if (rom.prg_rom_size == 0x4000)
+			return rom.prg_rom[(addr - 0x8000) % 0x4000];
+		else if (rom.prg_rom_size == 0x8000)
+			return rom.prg_rom[addr - 0x8000];
+	} else {
+		return memory[addr];
+	}
 }
 
 static uint16_t
@@ -738,7 +747,7 @@ interpret(void)
 	for (;;) {
 		opcode = peek(regs.pc++);
 
-		printf("opcode: $%02X\n", opcode);
+		printf("opcode: $%02X @ $%04X\n", opcode, regs.pc - 1);
 
 		switch (opcode) {
 		case 0x69:
@@ -1403,7 +1412,7 @@ interpret(void)
 		    regs.status.decimal_mode, regs.status.brk,
 		    regs.status.unused, regs.status.overflow,
 		    regs.status.negative);
-		printf("A: $%02X, X: $%02X, Y: $%02X, SP: $%02X, PC: $%02X\n",
+		printf("A: $%02X, X: $%02X, Y: $%02X, SP: $%02X, PC: $%04X\n",
 		    regs.a, regs.x, regs.y, regs.sp, regs.pc);
 	}
 }
@@ -1416,26 +1425,51 @@ cpu_init(void)
 	regs.pc = 0xFFFC;
 	regs.sp = 0xFD;
 
-	memset(&regs.status, 0, sizeof(regs.status));
+	//memset(&regs.status, 0, sizeof(regs.status));
 	regs.status.unused = 1;
 
 	cycles += 7;
 }
 
 int
-main(void)
+main(int argc, char *argv[])
 {
-	cpu_init();
+	FILE *fp;
+	uint8_t *buf;
+	size_t buflen;
 
-	if (sizeof(program) > (0x10000 - 0x8000)) {
-		fprintf(stderr, "program is too big for memory\n");
+	if (argc != 2) {
+		fprintf(stderr, "Usage: %s rom.nes\n", basename(argv[0]));
 		return 1;
 	}
 
-	memcpy(memory + 0x8000, program, sizeof(program));
+	fp = fopen(argv[1], "r");
+	fseek(fp, 0, SEEK_END);
+	buflen = ftell(fp);
+	buf = calloc(1, buflen);
+	fseek(fp, 0, SEEK_SET);
+	if (fread(buf, 1, buflen, fp) != buflen && ferror(fp)) {
+		fprintf(stderr, "file %s was not read properly\n", argv[1]);
+		clearerr(fp);
+		fclose(fp);
+		return 1;
+	}
+	fclose(fp);
+
+	parse_rom(buf, buflen, &rom);
+	free(buf);
+
+	cpu_init();
+
+	/* TODO: move to separate file? */
+	if (rom.mapper != 0) {
+		fprintf(stderr, "Only iNES ROMs using Mapper 0 are supported for now.\n");
+		return 1;
+	}
+
+	memwrite16(0xFFFC, 0x8000);
 	regs.pc = 0x8000;
 
-	printf("Initial State:\n");
 	printf("status: %i%i%i%i%i%i%i%i\n", regs.status.carry,
 	    regs.status.zero, regs.status.interrupt_disable,
 	    regs.status.decimal_mode, regs.status.brk,
@@ -1448,5 +1482,7 @@ main(void)
 
 	interpret();
 
+	free_rom(&rom);
+
 	return 0;
 }

rom.c

@@ -0,0 +1,56 @@
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "rom.h"
+
+void
+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;
+
+	if (data_len < 16) {
+		fprintf(stderr, "Invalid ROM: size less than 16 bytes\n");
+		exit(1);
+	}
+
+	if (data[0] != 'N' || data[1] != 'E' || data[2] != 'S' || data[3] != 0x1A) {
+		fprintf(stderr, "Invalid ROM: not in iNES format\n");
+		exit(1);
+	}
+
+	/* trainer data present */
+	if (data[6] & (1 << 2))
+		prg_rom_offset += 512;
+
+	rom->prg_rom_size = data[4] * 0x4000;
+	rom->prg_rom = calloc(rom->prg_rom_size, 1);
+	memcpy(rom->prg_rom, data + prg_rom_offset, rom->prg_rom_size);
+
+	rom->chr_rom_size = data[5] * 0x2000;
+	if (rom->chr_rom_size > 0) {
+		rom->chr_rom = calloc(rom->chr_rom_size, 1);
+		chr_rom_offset = prg_rom_offset + rom->prg_rom_size;
+		memcpy(rom->chr_rom, data + chr_rom_offset, rom->chr_rom_size);
+	}
+
+	if ((data[6] & (1 << 3)) != 0)
+		rom->mirror = M_FOUR;
+	else if ((data[6] & 1) == 1)
+		rom->mirror = M_VERTICAL;
+	else if ((data[6] & 1) == 0)
+		rom->mirror = M_HORIZONTAL;
+
+	rom->mapper = (data[6] & (0xF << 4)) >> 4;
+	rom->mapper |= data[7] & (0xF << 4);
+}
+
+void
+free_rom(struct Rom *rom)
+{
+	free(rom->prg_rom);
+	if (rom->chr_rom_size > 0)
+		free(rom->chr_rom);
+}

rom.h

@@ -0,0 +1,17 @@
+enum screen_mirroring {
+	M_HORIZONTAL,
+	M_VERTICAL,
+	M_FOUR,
+};
+
+struct Rom {
+	char *prg_rom;
+	char *chr_rom;
+	size_t prg_rom_size;
+	size_t chr_rom_size;
+	uint8_t mapper;
+	enum screen_mirroring mirror;
+};
+
+void parse_rom(const uint8_t *data, size_t data_len, struct Rom *rom);
+void free_rom(struct Rom *rom);