Difference between revisions of "Animal Crossing (NES game)"

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(RAM: I forgot about letters at the PO)
(RAM: reorganize to put a breakdown of adme.sav at the top)
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The editor's source code reveals that the game stores the state of the town in the first 0x15FE0 = 90080 bytes of a 256 KiB serial flash chip.
 
The editor's source code reveals that the game stores the state of the town in the first 0x15FE0 = 90080 bytes of a 256 KiB serial flash chip.
 
(The rest of the save chip contains a backup copy of the last known good state of the town and numerous letters that the players have saved.)
 
(The rest of the save chip contains a backup copy of the last known good state of the town and numerous letters that the players have saved.)
Most NES mappers, on the other hand, use a battery-backed 6264 SRAM chip, which provides 8192 bytes of nonvolatile memory.
+
It is split up as follows:
I did find a few easy cuts that one could make, but it would still need to get five times smaller to fit in a typical NES save chip:
+
* 51 patterns that the players can design at 560 bytes each (28560 bytes)
 +
** Each pattern consists of a 48-byte header and a 32x32x4-bit bitmap.
 +
** 8 patterns for each of 4 players
 +
** 8 patterns for Able Sisters
 +
** 1 pattern for each of 9 neighbors
 +
** 1 pattern for town flag
 +
** 1 pattern for Blanca's face
 +
* 16 acre maps at 544 bytes each (8704 bytes)
 +
** Each is a 16x16-cell array of 16-bit entries, with a special value indicating an empty cell, followed by a bit array to state whether the item is buried.
 +
* 14 room maps at 1024 bytes each (14336 bytes)
 +
** Two 16x16-cell arrays of 16-bit entries, denoting the items on top and on bottom.
 +
* 59 letters at 244 bytes each (14396 bytes)
 +
** Each letter has an addressee, a salutation, four lines of text, a closing, an optional item, and a state (outgoing, incoming unopened, or incoming opened).
 +
** 10 letters in mailbox
 +
** 10 letters for each of four players
 +
** 1 letter for each of 9 neighbors (incl. neighbor in transit)
 +
* Other data that I haven't broken down yet, including data about neighbors' relationships with players (24084 bytes)
 +
 
 +
Most NES boards with PRG RAM use a battery-backed 6264 SRAM chip, which provides 8192 bytes of nonvolatile memory.
 +
I did find a few easy cuts that one could make, but it would still need to get much smaller to fit in a typical NES save chip:
  
 
We already know from PPU above that we can get rid of the 50 patterns that ''ACWW'' lets players design:
 
We already know from PPU above that we can get rid of the 50 patterns that ''ACWW'' lets players design:
 
32 player patterns (8 for each of 4 players), 8 for neighbors' clothing (one for each player), 8 on display at Able Sisters, and one each for Blanca's face and the town flag.
 
32 player patterns (8 for each of 4 players), 8 for neighbors' clothing (one for each player), 8 on display at Able Sisters, and one each for Blanca's face and the town flag.
''ACWW'' stores each pattern in 560 bytes: a 48-byte header and a 32x32x4-bit bitmap.
 
 
There goes 28000 bytes, leaving 61280 bytes.
 
There goes 28000 bytes, leaving 61280 bytes.
  

Revision as of 20:12, 9 December 2009

Nintendo's Animal Crossing series was originally developed with the Nintendo 64 in mind. Some members of the nesdev.com board claim that technical limitations of a target platform do not limit gameplay possibilities. But here I show how a homebrew clone of AC for the Nintendo Entertainment System just won't happen.

PPU

The NES PPU (Picture Processing Unit) has a display list into which the CPU can load 64 instructions to draw "sprites", or small animated images, at any point on the screen. Sprites that appear earlier in this display list are drawn "in front" of later sprites. But only eight sprites can appear on each scanline at once; if there are more, the ones in back are hidden. The "flicker" that one sees in NES games comes from the game rapidly switching which sprites are in front so that important sprites don't disappear completely. Most NES games' characters are 16 pixels wide, consisting of two sprites.

NES has a backdrop color, four 3-color palettes for background tiles, and four 3-color palettes for sprites. A few games (such as Mega Man series) overlay multiple sprites on top of each other to squeeze more colors into a small space, but that increases the flicker problem, and fewer colors are available for different objects. Already, once the player walks up next to three animal neighbors, all four palettes would be in use, leaving nothing for the tools.

For a player character, the three colors might be the outline, clothes, and skin. This doesn't allow for clothes with patterns on them, unless patterns are reduced to a 1-bit pattern of clothes and outline. And even then, we'd need to build real-time texture-mapping software render the pattern onto the clothes as the player moves. That's not going to happen on an NES. The first Nintendo console designed for a fully texture-mapped environment was Nintendo 64, on which the Japanese beta version of Animal Crossing was released. So the Able Sisters would be reduced to selling solid colors.

The Game Boy Color has Harvest Moon GBC. Its sprite palettes follow the general scheme of outline, clothes, and skin, and there appears to be an overlay sprite for additional clothes. But the GBC has a narrower screen and more sprites on each scanline (ten on GBC vs. eight on NES), making flicker less of a problem. It also has twice as many palettes (eight for backgrounds and eight for sprites).

RAM

AnimalMap is an editor for Animal Crossing: Wild World saved games. The editor's source code reveals that the game stores the state of the town in the first 0x15FE0 = 90080 bytes of a 256 KiB serial flash chip. (The rest of the save chip contains a backup copy of the last known good state of the town and numerous letters that the players have saved.) It is split up as follows:

  • 51 patterns that the players can design at 560 bytes each (28560 bytes)
    • Each pattern consists of a 48-byte header and a 32x32x4-bit bitmap.
    • 8 patterns for each of 4 players
    • 8 patterns for Able Sisters
    • 1 pattern for each of 9 neighbors
    • 1 pattern for town flag
    • 1 pattern for Blanca's face
  • 16 acre maps at 544 bytes each (8704 bytes)
    • Each is a 16x16-cell array of 16-bit entries, with a special value indicating an empty cell, followed by a bit array to state whether the item is buried.
  • 14 room maps at 1024 bytes each (14336 bytes)
    • Two 16x16-cell arrays of 16-bit entries, denoting the items on top and on bottom.
  • 59 letters at 244 bytes each (14396 bytes)
    • Each letter has an addressee, a salutation, four lines of text, a closing, an optional item, and a state (outgoing, incoming unopened, or incoming opened).
    • 10 letters in mailbox
    • 10 letters for each of four players
    • 1 letter for each of 9 neighbors (incl. neighbor in transit)
  • Other data that I haven't broken down yet, including data about neighbors' relationships with players (24084 bytes)

Most NES boards with PRG RAM use a battery-backed 6264 SRAM chip, which provides 8192 bytes of nonvolatile memory. I did find a few easy cuts that one could make, but it would still need to get much smaller to fit in a typical NES save chip:

We already know from PPU above that we can get rid of the 50 patterns that ACWW lets players design: 32 player patterns (8 for each of 4 players), 8 for neighbors' clothing (one for each player), 8 on display at Able Sisters, and one each for Blanca's face and the town flag. There goes 28000 bytes, leaving 61280 bytes.

ACWW stores 512 bytes for each "acre" (there are 16) and 1024 bytes for each "room" (there are 5 in the player's house and 8 in neighbors' houses). That's 16x16 cells by 16 bits in each cell, with two "layers" for cells in rooms. That makes up 21504 bytes. We could impose a restriction that there be 32 or fewer items in each acre or room, which would sort of fit in with the spirit of the limitation on "floor strength" used by ACWW to limit texture memory and geometry length. Then we'd wind up with a 96-byte "acre" or "room". Each record would have 4 bits for x, 4 bits for y, and 16 bits for what item. There would be no need for "layers" because two objects could be put at the same (x, y) location. The 16 acres and 13 rooms would then take up 2784 bytes in all, cutting acre/room storage by 18720 bytes. This leaves 42560 bytes.

Nintendo promotes ACWW as a "communication" game. Players can send 244-byte letters to one another and to their animal neighbors. There are 58 letters carried at any moment, totaling 14152 bytes:

  • each of four players can carry ten letters
  • each of eight animal neighbors can carry one to show to another player
  • the post office appears to hold ten

We could cut it down to 10 letters at the post office and 5 letters per human, for a total of 38. And we could probably compress letters to 128 bytes per player with a static Huffman table (adjusting "ink" accordingly) for 4864 bytes. This saves 9288 bytes. Now we're down to 33,272 bytes, dangerously close to the size of the SRAM in EWROM.

With the low-hanging fruit shaken off, the rest of cutting will have to be done from the bottom up: finding specific things that we want to keep.

  • 16 acres at 96 bytes each: 1536 bytes
  • 13 rooms at 96 bytes each: 1248 bytes
  • Post office: 10 letters at 128 bytes each, for a total of 1280 bytes
  • Player basic data: let's say 1024 bytes per, including a 30-byte inventory and five 128-byte letters, for a total of 4096 bytes
  • NPC basic data: let's say 256 bytes per, including a 128-byte letter, for a total of 2048 bytes

Total: 10,208 bytes, which is still over the size in common boards.

Further research:

  • Determine what I'm not looking at in these byte counts
  • Figure out how to keep the bottom-up figure under 8,192 bytes