Castlevania II - Simon's Quest (USA), the password, read in the ROM

Simon's Quest's password packs 10 bytes: 7 of state, the byte FoxCunning labels Simon's level, the day count in BCD, the laurel and garlic counts in 4 bits each (the encoder writes a count above 8 as 8), the bytes labeled body parts, misc items 1, and misc items 2, and the whip level; then the 16-bit sum of those 7 as a check value; then a scramble byte. Hearts are not kept: accepting a password zeroes the bytes FoxCunning labels hearts and experience, sets HP to 48 and lives to 3, and zeroes $30 and $50-$57 (labeled the map type, scene, sub-room, and scroll position), $82 (night), $84, and $85 (the time wait counter and the minute). simons-quest-save/layout

{
 "bytes": 10,
 "fields": [
  {
   "byte": 0,
   "what": "Simon's level (CurrentLevel, $8B); the decoder takes 0-6"
  },
  {
   "byte": 1,
   "what": "day count, two BCD digits (CurrentDayBCD, $83)"
  },
  {
   "byte": 2,
   "bits": "0-3",
   "what": "laurels (InventoryNumLaurels, $4C), at most 8"
  },
  {
   "byte": 2,
   "bits": "4-7",
   "what": "garlics (InventoryNumGarlics, $4D), at most 8"
  },
  {
   "byte": 3,
   "what": "body parts (InventoryBodyParts1, $91); the decoder takes below $80"
  },
  {
   "byte": 4,
   "what": "misc items 1 (InventoryMiscItems1, $92); the decoder takes below $10"
  },
  {
   "byte": 5,
   "what": "misc items 2 (InventoryMiscItems2, $4A); the decoder takes below $80"
  },
  {
   "byte": 6,
   "what": "whip level (CurrentWhipLevel, $0434); the decoder takes 0-4"
  },
  {
   "byte": "7-8",
   "what": "check value, the 16-bit sum of bytes 0-6, high byte first"
  },
  {
   "byte": 9,
   "bits": "4-7",
   "what": "the add, 0-4"
  },
  {
   "byte": 9,
   "bits": "0-3",
   "what": "the XOR table's number, 0-15"
  }
 ]
}

The field names are FoxCunning's labels; the atlas hasn't read which bit of the item bytes is which item. Maximum HP is set from the level, 48 below level 2, 64 below 4, 80 below 6, and 96 at level 6 and above (computed from the same range), while HP starts at 48.

Cited by: Castlevania II: Simon's Quest, Password and battery save

Simon's Quest writes the 10 bytes, 80 bits, as 16 characters of 5 bits each, high bit first, so the scramble byte fills the last 8 bits; the screen draws a value n as tile n + 1 of CHR bank 16, where by the atlas's reading of the tiles 1-36 are A-Z and 0-9. Since the scramble adds up to 4 to characters 1-14, they can show all 36 (computed), and characters 15 and 16 only A-Z and 0-5. On the entry grid, pressing A on cells 0-35 enters that symbol, cells 36 and 37 step the cursor back and forward, and a later cell submits. simons-quest-save/characters

{
 "characters": 16,
 "bitsPerCharacter": 5,
 "alphabet": "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
}

Cells 36 and 37 draw tiles $25 and $26, a left and a right arrow.

Cited by: Password and battery save

To make a password, Simon's Quest sums the 7 state bytes into a 16-bit check value and takes a scramble from the frame counter: its low 4 bits number one of 16 tables of 14 XOR values, and its low 3 bits pick an add from 0, 1, 2, 3, 4, 1, 2, 3. After packing, characters 1-14 are XORed with the table and then have the add added; characters 15 and 16 carry the add and the table number unscrambled. Since the add follows from the table number, one state has at most 16 passwords (computed). simons-quest-save/scramble

{
 "check": "16-bit sum",
 "tables": 16,
 "scrambled": 14,
 "adds": [
  0,
  1,
  2,
  3,
  4,
  1,
  2,
  3
 ],
 "passwordsPerState": 16
}

The add table's 1, 2, and 3 appear twice, so the frame counter picks them twice as often as 0 or 4. The XOR values are 5-bit, so a scrambled character stays below 36.

Cited by: Castlevania II: Simon's Quest, Password and battery save

On entry Simon's Quest rejects a password with any of its 16 characters blank. Otherwise it subtracts the add from characters 1-14, XORs them, unpacks the 10 bytes, and accepts only when both bytes of the sum match the 7 state bytes again, the level is below 7, the body-parts and misc-items-2 bytes are below $80, misc items 1 is below $10, the whip level is below 5, both of the day's digits are 0-9, and laurels and garlics are each at most 8. A rejection writes MISS and ENTRY to the screen, plays sound $24, and after 128 frames brings back the ENTER YOUR PASSWORD prompt and returns to input, writing nothing to the entered characters on the way; an acceptance writes ENTRY COMPLETED. simons-quest-save/reject

The check reads only the decoded bytes: the decoder takes any add, 0-15, from characters 15 and 16, though the encoder writes only 0-4. A sum's high byte is at most 6, since 7 bytes sum to at most 1,785.

Cited by: Castlevania II: Simon's Quest, Password and battery save