collectable_items_bitmap
Zelda 2 tracks collectable items throughout the game with a bitmap which
represents every screen in the game. The initial state of this bitmap
is stored in bank 5 at $bb15.
When a new save file is created, this bitmap is copied into the player's
battery backed RAM. When a game is started, the player's copy is copied
to address $600 in NES RAM.
It is a limitation of the game that each individual screen may have only 1 collectable item. Recall that every "world" in Zelda2 can have 63 side-scroll areas, and each side-scroll area can be up to 4 screens wide.
Therefore, each "world" bitmap is 32 bytes long.
| ROM Bank | Overworld | World | Description | Base Address |
|---|---|---|---|---|
| 1 | 0 | 0 | West Hyrule | $600 |
| 1 & 2 | 1 | 0 | Death Mt. / Maze Is. | $620 |
| 2 | 2 | 0 | East Hyrule | $640 |
| 3 | x | 1, 2 | Towns | $660 |
| 4 | x | 3 | Palaces 1, 2, 5 | $680 |
| 4 | x | 4 | Palaces 3, 4, 6 | $6a0 |
| 5 | x | 5 | Great Palace | $6c0 |
To calculate which bit represents which screen:
addr = lookup_base_address(overworld, world) + (sidescroll_area / 2);
bit = 7 - (screen_num + 4 * (sidescroll_area & 1));
The candle is in Palace 1 room 8, directly next to a crumble bridge with a 50-point bag in room 9. Note that the map for room 9 actually puts two P-bags on the bridge.
| Room 8 | Room 9 |
|---|---|
 |
 |
The data for these two rooms is in $684 (calculated as $680 + 8 / 2).
The default (new-game) value for $684 is $fd, which has a bit pattern of
1111 1101. Because of the default value of the bitmap for room 9, only the
leftmost P-bag is present in room 9.