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#define PROGRAM_NAME "stadium"
#define USAGE_OPTS "pokegold.gbc"
#include "common.h"
// The Game Boy cartridge header stores a global checksum at 0x014E-0x014F
#define GLOBALOFF 0x014E
// ASCII "N64PS3" header
#define N64PS3SIZE 6
// N64PS3 + CRC
#define HEADERSIZE (N64PS3SIZE + 2)
// Checksum every half-bank
#define CHECKSIZE 0x2000
// The CRC polynomial value
#define CRC_POLY 0xC387
// The CRC initial value (also used for checksums)
#define CRC_INIT 0xFEFE
uint8_t n64ps3[N64PS3SIZE] = {'N', '6', '4', 'P', 'S', '3'};
#define SET_U16BE(file, off, v) do { \
file[(off) + 0] = (uint8_t)(((v) & 0xFF00) >> 8); \
file[(off) + 1] = (uint8_t)(((v) & 0x00FF) >> 0); \
} while (0)
void calculate_checksums(uint8_t *file, int filesize) {
int NUMCHECKS = filesize / CHECKSIZE;
int DATASIZE = HEADERSIZE + NUMCHECKS * 2; // 2 bytes per checksum
int ORIGIN = filesize - DATASIZE; // Stadium data goes at the end of the file
// Initialize the CRC table
uint16_t crc_table[256];
for (int i = 0; i < 256; i++) {
uint16_t c = i;
uint16_t rem = 0;
for (int y = 0; y < 8; y++) {
rem = (rem >> 1) ^ ((rem ^ c) & 1 ? CRC_POLY : 0);
c >>= 1;
}
crc_table[i] = rem;
}
// Clear the global checksum
SET_U16BE(file, GLOBALOFF, 0);
// Initialize the Stadium data (this should be free space anyway)
memset(file + ORIGIN, 0, DATASIZE);
memcpy(file + ORIGIN, n64ps3, N64PS3SIZE);
// Calculate the half-bank checksums
for (int i = 0; i < NUMCHECKS; i++) {
uint16_t checksum = CRC_INIT;
for (int j = 0; j < CHECKSIZE; j++) {
checksum += file[i * CHECKSIZE + j];
}
SET_U16BE(file, ORIGIN + HEADERSIZE + i * 2, checksum);
}
// Calculate the CRC of the half-bank checksums
uint16_t crc = CRC_INIT;
for (int i = ORIGIN + HEADERSIZE; i < ORIGIN + DATASIZE; i++) {
crc = (crc >> 8) ^ crc_table[(crc & 0xFF) ^ file[i]];
}
SET_U16BE(file, ORIGIN + HEADERSIZE - 2, crc);
// Calculate the global checksum
uint16_t globalsum = 0;
for (int i = 0; i < filesize; i++) {
globalsum += file[i];
}
SET_U16BE(file, GLOBALOFF, globalsum);
}
int main(int argc, char *argv[]) {
if (argc < 2) {
usage_exit(1);
}
char *filename = argv[1];
long filesize;
uint8_t *file = read_u8(filename, &filesize);
calculate_checksums(file, filesize);
write_u8(filename, file, filesize);
return 0;
}
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