Metroid's password (nmikstas's disassembly)

Metroid's password holds 18 bytes; bytes 0-7 carry one bit for each of 59 items, doors, and Tourian objects collected, byte 8 the starting area (bits 0-5) and whether Samus wears her suit (bit 7), byte 9 her equipment, byte 10 her missile count, bytes 11-13 her age, byte 15 the Kraid and Ridley statue and defeat bits, byte 14 a copy of the RAM byte after the age, which nmikstas labels unused, byte 16 a scramble count, and byte 17 a check byte. metroid-save/layout

About: Metroid Password and battery save

{
 "bytes": 18,
 "fields": [
  {
   "bytes": "0-7",
   "what": "item history, one bit per unique item (59)"
  },
  {
   "bytes": "8",
   "bits": "0-5",
   "what": "starting area (InArea)"
  },
  {
   "bytes": "8",
   "bits": "7",
   "what": "suitless Samus (JustInBailey)"
  },
  {
   "bytes": "9",
   "what": "SamusGear"
  },
  {
   "bytes": "10",
   "what": "MissileCount"
  },
  {
   "bytes": "11-13",
   "what": "SamusAge"
  },
  {
   "bytes": "14",
   "what": "SamusStat01 ($6880), a RAM byte labeled unused"
  },
  {
   "bytes": "15",
   "bits": "4-7",
   "what": "Ridley defeated, Ridley statue up, Kraid defeated, Kraid statue up"
  },
  {
   "bytes": "16",
   "what": "scramble count, 1-15"
  },
  {
   "bytes": "17",
   "what": "check byte, the sum of bytes 0-16"
  }
 ]
}

nmikstas's comment on PasswordByte0E says it stores no data; the loop writes SamusStat01 there, and LoadPasswordData's matching loop copies it back. The item bits follow ItemData's order (Bank00.asm, from L9029), which mixes items, red doors, Mother Brain, and the zebetites. Byte 16's range of 1-15 is the encoder's; the decoder accepts any value there.

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Metroid writes the 18 bytes as 24 characters of 6 bits each, three bytes to every four characters, from a 64-symbol set of 0-9, A-Z, a-z, ?, and -; the entry grid's 65th symbol, a space, enters as $FF, all 8 bits set. metroid-save/characters

About: Metroid Password and battery save

{
 "characters": 24,
 "bitsPerCharacter": 6,
 "alphabet": "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz?-",
 "space": 255
}

Symbols 0-63 run 0-9, A-Z, a-z, ?, - in the grid's order. The screen shows the password as two lines of 12 characters, each split into two groups of 6 (CursorPosTbl).

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To make a password, Metroid sums bytes 0-16 into byte 17, then rotates bytes 0-15 right as one 128-bit number, one bit at a time, as many times as byte 16 says; byte 16 is the low 4 bits of a random byte, redrawn until it is not 0. metroid-save/checksum

About: Metroid Password and battery save

{
 "check": "sum of bytes 0-16, mod 256",
 "scramble": "rotate bytes 0-15 right by n bits",
 "n": "1-15"
}

The check byte and the count are not scrambled, so the count is readable in characters 21 and 22 of every password.

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When the player presses Start, Metroid rebuilds the 18 bytes from the 24 characters, rotates bytes 0-15 back left by byte 16's count, and compares their sum with byte 17; on a mismatch it plays the wrong-password sound and shows ERROR TRY AGAIN for 120 frames, and on a match it starts the game; nothing else in the password is checked. metroid-save/reject

About: Metroid Password and battery save

Timer3 counts down once every 10 frames (Metroid_Defines.asm, line 58), so its 12 is 120 frames. The decoder takes byte 16 as the count whatever it holds; at 0 the loop runs 256 times, a full turn of 128 bits twice, which leaves the bytes as they were.

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Metroid compares the first 16 characters entered with NARPASSWORD00000 before anything else and, on a match, sets a flag and ignores the rest; while that flag is set, the game engine writes $03 to Samus's health high byte, $FF to her equipment byte, and 5 to her missile count, every frame. metroid-save/narpassword

About: Metroid Password and battery save

HealthHi's high nibble is the energy tank count and its low nibble the tens digit (metroid/health-format), so the $03 written each frame is no full tanks and a tens digit of 3; nmikstas's comment calls $FF in SamusGear every power-up. The path that sets the flag returns with the carry still set from the last comparison, so the first Start may report ERROR TRY AGAIN; that hasn't been run in the game.

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If Mother Brain has been destroyed when Metroid makes a password, it clears her bit and every Tourian bit in bytes 6 and 7, so the password brings back Mother Brain, the zebetites, and Tourian's missile doors. metroid-save/mother-brain-reset

About: Metroid Password and battery save

The game keeps bits 0 and 1 of byte 6, items 48 and 49, and clears the rest of byte 6 and all of byte 7.

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Computed from the decoder's lines, the password JUSTIN BAILEY ------ ------ passes; after 255 rotations, its bytes 0-16 sum to $FF mod 256, which equals its check byte, byte 17, and byte 8's bit 7 is set, which the game loads as Samus without her suit. metroid-save/justin-bailey

About: Metroid Password and battery save

{
 "bytes": [
  166,
  243,
  142,
  164,
  185,
  101,
  36,
  169,
  209,
  127,
  255,
  255,
  255,
  255,
  255,
  255,
  255,
  255
 ],
 "sum": 255
}

The decoded bytes also give a starting area of $11 (byte 8's low 6 bits), SamusGear $7F, 255 missiles, and all four statue bits. The 255 rotations are 127 more than a full turn of 128 bits.

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