#include <stdio.h>
int main(int argc, char **argv)
{
    if (argc == 1) {
        printf("usage: cm3_checksum \n");
        return 1;
    }

    FILE *file;
    if ((file = fopen(argv[1], "r+")) == NULL) {
        return 1;
    }

    /* The checksum of the exception vector */
    unsigned int i, n, checksum=0;
    for (i=0; i<7; i++) {
        fread(&n, 4, 1, file);
        checksum += n;
    }

    /* The 2's complement of the checksum */
    checksum = -checksum;
    printf("checksum: 0x%X\n", checksum);

    /* write back the checksum to location 7 */
    fwrite(&checksum, 4, 1, file);
    fclose(file);
    return 0;
}