Dragon Warrior's battery save (nmikstas's disassembly and the US ROM)

Dragon Warrior saves the hero as a 32-byte record in battery-backed cartridge RAM; bytes 0-1 hold experience, 2-3 gold, 4-7 the inventory, 8 the magic keys (capped at 6), 9 the herbs, 10 the equipment, 11-13 spell, player, and story flags, 14-21 the name, 22 the message speed, 23 HP, 24 MP, 25 whether HP and MP are refilled at the start, 26-29 the filler byte $C8, and 30-31 a check value. dragon-warrior-save/record

About: Dragon Warrior Password and battery save

{
 "bytes": 32,
 "fields": [
  {
   "bytes": "0-1",
   "what": "experience"
  },
  {
   "bytes": "2-3",
   "what": "gold"
  },
  {
   "bytes": "4-7",
   "what": "inventory slots"
  },
  {
   "bytes": "8",
   "what": "magic keys, at most 6"
  },
  {
   "bytes": "9",
   "what": "herbs"
  },
  {
   "bytes": "10",
   "what": "weapon, armor, and shield"
  },
  {
   "bytes": "11",
   "what": "upper spells and flags"
  },
  {
   "bytes": "12",
   "what": "player flags"
  },
  {
   "bytes": "13",
   "what": "story flags"
  },
  {
   "bytes": "14-21",
   "what": "name"
  },
  {
   "bytes": "22",
   "what": "message speed"
  },
  {
   "bytes": "23",
   "what": "HP"
  },
  {
   "bytes": "24",
   "what": "MP"
  },
  {
   "bytes": "25",
   "what": "start status: $78 refill, $AB keep"
  },
  {
   "bytes": "26-29",
   "what": "filler, $C8"
  },
  {
   "bytes": "30-31",
   "what": "CRC"
  }
 ]
}

nmikstas counts the check value's bytes from 1, as bytes 31 and 32; from 0 they are 30 and 31.

Cited by: Password and battery save

Dragon Warrior's check value is a 16-bit CRC of the record's first 30 bytes, read from byte 29 down to byte 0, one bit at a time from the top, with the polynomial $1021 and both bytes seeded with $1D. dragon-warrior-save/crc

About: Dragon Warrior Password and battery save

{
 "width": 16,
 "polynomial": 4129,
 "seed": 7453,
 "bytes": "29 down to 0"
}

nmikstas's comment calls it "some kind of linear feedback shift register" that "appears" to make a CRC; the steps are those of the CRC-16 with polynomial $1021 (x^16 + x^12 + x^5 + 1), fed most significant bit first, with no final XOR. Calling it CRC-16-CCITT with a nonstandard seed is the atlas's reading.

Cited by: Dragon Warrior, Password and battery save

Dragon Warrior writes each save 10 times, one record after another, into its slot's 320 bytes; loading checks the copies in order, counts each one whose CRC fails, loads the first that passes, and rewrites all 10 from it, and if none passes the game marks the slot invalid (clearing its valid flag and mask bit, not its data) and says no deeds were recorded. dragon-warrior-save/ten-copies

About: Dragon Warrior Password and battery save

{
 "copies": 10,
 "recordBytes": 32,
 "slotBytes": 320,
 "slots": 3
}

Despite its label, Copy100Times checks at most 10 copies. Clearing the slot means clearing its valid byte ($C8 marks a slot in use, ValidSave1-3); the 320 bytes are left as they were. The check runs for every slot in use before the list of saved games is shown, and again when a game is continued.

Cited by: Dragon Warrior, Password and battery save

Dragon Warrior marks its battery RAM as initialized with the 10 ASCII bytes KEN MASUTA, written both before and after the three save slots; before showing the saved games it checks the copy after the slots and then the one before, and only if neither matches does it erase all three slots, then rewrites both copies. dragon-warrior-save/signature

About: Dragon Warrior Password and battery save

{
 "signature": "KEN MASUTA",
 "at": [
  24635,
  25640
 ]
}

The table's bytes are ASCII codes ($4B $45 $4E ...), not the game's own tile codes. nmikstas reads the string as a name; who Ken Masuta is isn't sourced here. Erasing also zeroes the valid bytes, the start statuses, and the CRC failure counts.

Cited by: Password and battery save