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.

1 files changed, 56 insertions, 0 deletions

diff --git a/rom.c b/rom.c
new file mode 100644
index 0000000..d432cc2
--- /dev/null
+++ b/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);
+}