setmac.c

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

static void print_usage()
{
    printf("Usage:\n");
    printf("    setmac [file] [MAC address]\n");
    printf("MAC addresses should be one of two forms:\n");
    printf("    xx:yy:zz:ss:tt:ww\n");
    printf("    xxyyzzssttww\n");
}

static int is_hex_value(char value)
{
    if (value >= '0' && value <= '9') return 1;
    if (value >= 'a' && value <= 'f') return 1;
    if (value >= 'A' && value <= 'F') return 1;
    return 0;
}

static unsigned char get_hex_value(char value)
{
    if (value >= '0' && value <= '9') return value - '0';
    if (value >= 'a' && value <= 'f') return (value - 'a') + 10;
    if (value >= 'A' && value <= 'F') return (value - 'A') + 10;
    return 0xff;
}

static unsigned char get_eeprom_checksum(unsigned short *eeprom)
{
    int checksum = (int)0xffffffa5;
    checksum ^= eeprom[0] & 0x00ff;
    checksum <<= 24;
    checksum >>= 24;
    checksum = (checksum << 1) | ((checksum < 0) ? 1 : 0);
    checksum <<= 24;
    checksum >>= 24;
    for (int i = 1; i < 0x80; i++)
    {
        unsigned char upper = eeprom[i] >> 8;
        unsigned char lower = eeprom[i] & 0x00ff;
        unsigned char xorval = upper ^ lower;
        checksum ^= xorval;
        checksum <<= 24;
        checksum >>= 24;
        checksum = (checksum << 1) | ((checksum < 0) ? 1 : 0);
        checksum <<= 24;
        checksum >>= 24;
    }

    return (unsigned char)checksum;
}

int main(int argc, char **argv)
{
    if (argc < 3)
    {
        print_usage();
        return 1;
    }

    FILE *input = fopen(argv[1], "rb");
    if (!input)
    {
        fprintf(stderr, "Unable to open file %s for reading.\n", argv[1]);
        return 1;
    }

    fseek(input, 0, SEEK_END);
    long file_size = ftell(input);
    fseek(input, 0, SEEK_SET);
    if (file_size != 0x100)
    {
        fprintf(stderr, "File %s does not appear to be a 93C56 EEPROM dump.\n", argv[1]);
        fclose(input);
        return 1;
    }

    int length = strlen(argv[2]);
    if (length != 12 && length != 17)
    {
        fprintf(stderr, "'%s' does not appear to be a MAC address.\n", argv[2]);
        print_usage();
        fclose(input);
        return 1;
    }

    unsigned char mac[6];
    int mac_pos = 0;
    int upper_nybble = 1;
    for (int i = 0; i < length; i++)
    {
        if (argv[2][i] == ':') continue;

        if (!is_hex_value(argv[2][i]))
        {
            fprintf(stderr, "'%c' is not a hexadecimal value.\n", argv[2][i]);
            print_usage();
            fclose(input);
            return 1;
        }

        if (upper_nybble)
        {
            mac[mac_pos] = get_hex_value(argv[2][i]) << 4;
        }
        else
        {
            mac[mac_pos] |= get_hex_value(argv[2][i]);
            mac_pos++;
        }

        upper_nybble = !upper_nybble;
    }

    unsigned short eeprom[0x80];
    for (int i = 0; i < 0x80; i++)
    {
        unsigned char upper = 0;
        unsigned char lower = 0;
        fread(&lower, 1, 1, input);
        fread(&upper, 1, 1, input);
        eeprom[i] = (upper << 8) | lower;
    }
    fclose(input);

    eeprom[0x7d] = (mac[0] << 8) | mac[1];
    eeprom[0x7e] = (mac[2] << 8) | mac[3];
    eeprom[0x7f] = (mac[4] << 8) | mac[5];

    unsigned char checksum = get_eeprom_checksum(eeprom);
    eeprom[0x00] &= 0x00ff;
    eeprom[0x00] |= checksum << 8;

    input = fopen(argv[1], "wb");
    if (!input)
    {
        fprintf(stderr, "Unable to open file %s for writing. Check that it is not write-protected.\n", argv[1]);
        return 1;
    }
    for (int i = 0; i < 0x80; i++)
    {
        unsigned char upper = eeprom[i] >> 8;
        unsigned char lower = (unsigned char)eeprom[i];
        fwrite(&lower, 1, 1, input);
        fwrite(&upper, 1, 1, input);
    }
    fclose(input);

    printf("MAC set successfully.\n");
    return 0;
}