A Sega Genesis/Megadrive ROM checksum validator.

mdchek.c

/*
** A Sega Genesis/Megadrive ROM checksum validator.
** Reads existing checksum and then calculates the valid one.
** Attempts to fix if existing was not valid.
*/

#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>

#define CS_OFFS 398 // 256 + 142

int main( int argc, char** argv ) {
	if ( argc < 2 ) {
		fprintf( stderr, "No rom file specified!\n" );
		return EXIT_FAILURE;
	}

	FILE* romFile = fopen( argv[1], "rb+" );

	if ( romFile == NULL ) {
		fprintf( stderr, "File not found!\n" );
		return EXIT_FAILURE;
	}

	fseek( romFile, 0, SEEK_END );
	uint32_t chekSize = ftell( romFile );

	// check for valid size
	// 128K - 4M and 128K aligned
	if ( chekSize == 0 || chekSize > 0x00400000 || chekSize % (128*1024) != 0 ) {
		fprintf( stderr, "Invalid file size!\n" );
		fclose( romFile ); romFile = NULL;
		return EXIT_FAILURE;
	}

	// read existing cheksum
	uint16_t prev_cs = 0;
	fseek( romFile, CS_OFFS, SEEK_SET );
	fread( &prev_cs, 2, 1, romFile );
	// swap
	prev_cs = (uint16_t)((prev_cs<<8)|(prev_cs>>8));
	printf( "Existing checksum: 0x%04x\n", prev_cs );

	// skip header
	fseek( romFile, 512, SEEK_SET );
	chekSize -= 512;
	//printf( "%u\n", chekSize );

	uint16_t checksum = 0; // actual

	// calculate valid
	while ( chekSize > 0 ) {
		uint8_t cc;
		fread( &cc, 1, 1, romFile );
		checksum += (uint16_t)(cc * 256);
		fread( &cc, 1, 1, romFile );
		checksum += cc;
		chekSize -= 2;
	}

	printf( "Valid checksum: 0x%04x\n", checksum );
	//return 0;

	// fix if existing not valid
	if ( prev_cs != checksum ) {
		printf( "Existing checksum was not valid! Fixing...\n" );

		// swap
		checksum = (uint16_t)((checksum<<8)|(checksum>>8));

		fseek( romFile, CS_OFFS, SEEK_SET );
		fwrite( &checksum, 2, 1, romFile );
	} else {
		printf( "Existing checksum is valid! Nothing to do here!\n" );
	}

	fclose( romFile ); romFile = NULL;

	return EXIT_SUCCESS;
}