Mario Kart 64 uses a 512 byte EEPROM unit to save time trial records and the selected sound mode. A mirror of EEPROM data is kept at 8018EB90 in RAM.

For each course, there is a 24 byte structure that contains the five best completion records and the best lap record. There is an additional byte which indicates whether or not any records have been saved for the set, and another byte which could be a checksum.

Record entries are 24 bits each and are stored with little endian byte ordering:

tttttttt tttttttt cccctttt

Where t is a 20 bit value for the completion time in centiseconds and c is a 4 bit ID of the character used in the trial. If a record has a time greater than or equal to 100 minutes, it is not displayed.

Valid character IDs for c are as follows:

The default value applied to record entries is 0xC02709, which is 100 minutes and Mario as the character selection.

Each record contains a 1-byte checksum at offset 0x17 and is computed as follows:

unsigned char RecordChecksum(unsigned char data[], int offset)
{
   int checksum = 0;
   for (int rec = 0; rec < 7; rec++) {
      for (int i = 0; i < 3; i++) {
         unsigned char val = data[offset + rec * 3 + i];
         checksum += (val * (i + 1)) + rec;
      }
   }
   return (unsigned char)(checksum & 0xFF);
}

0x186 is a checksum of offsets 0x180-0x184 and is computed by procedure at 800B492C and 0x187 is computed by procedure at 800B49E4

eeprom[0x186] = (eeprom[0x180] + 1 + ((eeprom[0x181] + 1) * 2) + 1 + ((eeprom[0x182] + 1) * 3) + 2 + ((eeprom[0x183] + 1) * 4) + 3 + ((eeprom[0x184] + 1) * 5) + 4) & 0xFF
eeprom[0x187] = (eeprom[0x186] + 0x5A) & 0xFF

// eeprom: base of EEPROM data
// offset: offset in eeprom to checksum (either 0x188 or 0x1C0)
void Checksum1BEand1F6(unsigned char eeprom[], int offset)
{
   int checksum = 0;
   int blockOffset = offset;
   for (int block = 0; block < 3; block++) {
      int loopOffset = blockOffset + 1;
      checksum += (eeprom[blockOffset] + 1) * (block + 1);
      for (int byteOff = 1; byteOff < 0x11; byteOff += 0x4) {
         int mult2 = (eeprom[loopOffset] + 1) * (block + 1);
         checksum += mult2 + byteOff;
         int mult3 = (eeprom[loopOffset + 1] + 1) * (block + 1);
         checksum += mult3 + byteOff + 1;
         int mult4 = (eeprom[loopOffset + 2] + 1) * (block + 1);
         checksum += mult4 + byteOff + 2;
         int mult5 = (eeprom[loopOffset + 3] + 1) * (block + 1);
         checksum += mult5 + byteOff + 3;
         loopOffset += 4;
      }
      blockOffset += 0x11;
   }
   eeprom[offset + 0x36] = (unsigned char)(checksum & 0xFF);
   eeprom[offset + 0x37] = (unsigned char)((data[offset + 0x36] + 0x5A) & 0xFF);
}

• http://origami64.net/showthread.php?tid=47&pid=2936#pid2936