Mega Man 2's password (lsmmega's disassembly)

Mega Man 2's password is 9 dots on a 5 by 5 grid; the entry screen counts down from 9 as the player places dots with A and takes them back with B, and reads the password when the ninth is placed. mega-man-2-save/grid

About: Mega Man 2 Password and battery save

{
 "cells": 25,
 "rows": 5,
 "columns": 5,
 "dots": 9
}

The cursor tables give cell n the row n / 5 and the column n mod 5, so cells 0-4 are the top row. The lsmmega/mm2 build is the atlas's Mega Man 2 (USA) ROM with an iNES header.

Cited by: Mega Man 2, Password and battery save

Mega Man 2's password holds the E-tank count and, for each of the 8 Robot Masters, whether he is beaten; a dot in the top row's first five cells gives 0-4 E-tanks, and each Robot Master has one "beaten" cell and one "not beaten" cell among the other 20, which also include 4 cells that mean nothing. mega-man-2-save/layout

About: Mega Man 2 Password and battery save

{
 "etankRow": [
  0,
  1,
  2,
  3,
  4
 ],
 "slots": [
  {
   "i": 0,
   "boss": "none"
  },
  {
   "i": 1,
   "boss": "Heat Man",
   "beaten": true
  },
  {
   "i": 2,
   "boss": "none"
  },
  {
   "i": 3,
   "boss": "Quick Man",
   "beaten": true
  },
  {
   "i": 4,
   "boss": "Wood Man",
   "beaten": false
  },
  {
   "i": 5,
   "boss": "Flash Man",
   "beaten": true
  },
  {
   "i": 6,
   "boss": "none"
  },
  {
   "i": 7,
   "boss": "Bubble Man",
   "beaten": false
  },
  {
   "i": 8,
   "boss": "Quick Man",
   "beaten": false
  },
  {
   "i": 9,
   "boss": "Crash Man",
   "beaten": true
  },
  {
   "i": 10,
   "boss": "Bubble Man",
   "beaten": true
  },
  {
   "i": 11,
   "boss": "Air Man",
   "beaten": false
  },
  {
   "i": 12,
   "boss": "Wood Man",
   "beaten": true
  },
  {
   "i": 13,
   "boss": "none"
  },
  {
   "i": 14,
   "boss": "Heat Man",
   "beaten": false
  },
  {
   "i": 15,
   "boss": "Metal Man",
   "beaten": false
  },
  {
   "i": 16,
   "boss": "Crash Man",
   "beaten": false
  },
  {
   "i": 17,
   "boss": "Air Man",
   "beaten": true
  },
  {
   "i": 18,
   "boss": "Flash Man",
   "beaten": false
  },
  {
   "i": 19,
   "boss": "Metal Man",
   "beaten": true
  }
 ],
 "cell": "5 + (etanks + i) mod 20"
}

_show_password sets the cell numbered by the E-tank count, then walks the 20 table entries, setting a dot where a boss's flag in the beaten byte (z00) or in its complement (z01) is set. The value's i is the table index; the cells 5-24 it lands on depend on the E-tank count.

Cited by: Mega Man 2, Password and battery save

Mega Man 2 shifts the 20 boss cells by the E-tank count; table entry i goes to cell 5 + (E-tanks + i) mod 20, wrapping from the last cell back to the first cell of the second row, so the same bosses beaten give a different grid for each E-tank count. mega-man-2-save/shift

About: Mega Man 2 Password and battery save

Value: 5 + (etanks + i) mod 20

The encoder and the decoder use the same start (E-tanks + 5) and the same wrap (25 to 5).

Cited by: Mega Man 2, Password and battery save

Mega Man 2 reads the E-tank count as the first dot in the top row's first four cells, or 4 if none of them holds one, without looking at the fifth; it then ORs each dotted cell's boss into a beaten byte or a not-beaten byte and accepts the password only if the two together cover all 8 bosses, else it writes PASSWORD ERROR ! and, after two waits of 125 frames, returns to the menu of START and PASSWORD. mega-man-2-save/reject

About: Mega Man 2 Password and battery save

A boss with dots in both its cells counts as beaten, since the beaten byte is what the game loads. The check needs at least one cell per boss, not exactly one; with 9 dots and an E-tank dot in the top row, 8 dots are left for 8 bosses, so each gets exactly one. A dot in a cell that means nothing leaves a boss uncovered.

Cited by: Mega Man 2, Password and battery save

A correct Mega Man 2 password gives Mega Man the weapon of every boss in its beaten byte, Item 1 if Heat Man is beaten, Item 2 if Air Man is, and Item 3 if Flash Man is, plus its E-tank count; the password holds nothing else. mega-man-2-save/items

About: Mega Man 2 Password and battery save

Bits 0, 1, and 5 of the beaten byte (Heat, Air, Flash) become item bits 0, 1, and 2. Which item each bit is comes from _show_password's sprite list (item 1, item 2, item 3).

Cited by: Mega Man 2, Password and battery save