The Legend of Zelda's battery save (Aldo Núñez's disassembly and the US ROM)

The Legend of Zelda keeps three save files in battery-backed cartridge RAM; each holds an 8-byte name, a 40-byte item block, 384 bytes of world flags, and four single bytes (whether the slot is in use, one unlabeled byte, the death count, and the quest), with a 16-bit checksum that is the plain sum of those 436 bytes, and, kept apart and outside the sum, two marker bytes, $5A and $A5. zelda-save/file

About: The Legend of Zelda Password and battery save

{
 "files": 3,
 "fields": [
  {
   "what": "name",
   "bytes": 8
  },
  {
   "what": "items",
   "bytes": 40
  },
  {
   "what": "world flags",
   "bytes": 384
  },
  {
   "what": "slot in use",
   "bytes": 1
  },
  {
   "what": "unlabeled",
   "bytes": 1
  },
  {
   "what": "death count",
   "bytes": 1
  },
  {
   "what": "quest number",
   "bytes": 1
  }
 ],
 "check": "16-bit sum of the 436 bytes",
 "markers": [
  90,
  165
 ]
}

The sum adds each byte into a 16-bit total with carry, so it catches a changed byte but not two bytes swapped. Aldo Núñez leaves the fourth single byte unnamed (a TODO). CalculateFileAChecksum reads only the name, items, world flags, and the four single bytes; the markers ($651E, $6521) and the sum itself are checked on their own.

Cited by: Password and battery save

The Legend of Zelda saves in two steps; it first writes the whole file to a second copy, file B, sums it, stores that sum, and marks B uncommitted, and on the next step it sums B again and, if the sums agree, clears file A's markers, copies B over A, writes A's markers and sum, and marks B committed. zelda-save/two-step-save

About: The Legend of Zelda Password and battery save

File B's pointers start at $6898 (items), $6910 (world flags), and $6880 (name), in battery RAM from $6880 to $6D9B; the disassembly also labels that range as level data (LevelBlockAttrsA from $687E). The commit flag, IsSaveFileBCommitted, is itself in battery RAM at $652A.

Cited by: Password and battery save

When its title demo or its file select screen starts, The Legend of Zelda checks each of the three slots; a file B still marked uncommitted whose sum matches is copied over file A, finishing a save that was cut off, and a file A whose markers are not $5A and $A5 or whose sum doesn't match is formatted, its name set to spaces and every other byte to 0. zelda-save/file-check

About: The Legend of Zelda Password and battery save

The routine is submode 1 of mode 0, the demo, and the init of mode 1, the file select screen. FormatFileA then writes a fresh sum and the markers, so an erased slot reads as an empty, valid file.

Cited by: Password and battery save