A word about pseudo 16x16 tiles:
  Several games (including 'Mario World', 'Metroid', 'Zelda'...) use a little
  trick to make levels appear more detailed than they really are. Although the
  physical playfield (where your character, any items, and the enemies all
  interact) is actually made of large 16x16 tiles, the visible tilemap is
  composed of four smaller 8x8 tiles. The reason for this is to have a large
  variety of blocks without using up too much memory; only so many tile
  graphics can fit into the small 64k of VRAM. So, larger tiles are broken
  down into smaller parts based on what is redundant. For example, a simple
  block with a shadow on the bottom is symmetrical horizontally, so the same
  two tiles can be used for both sides if they are simply flipped. A perfect
  16x16 sphere could be made up of just a single 8x8 tile if it was flipped
  all four directions. Another advantage of subtiling is that more colors can
  be used per tile. Rather than only fifteen colors (assuming it was using
  video mode 1) for the entire tile, each of the four subtiles can have its
  own fifteen color set from the palette.

  'Mario World' may be the best example, since the blocks in its levels fit
  into a distinctly visible grid, unlike some later games which do much better
  of blending their levels together, sometimes well enough to not be able to
  tell just where the tile boundaries are. Even the old NES games used this
  same trick to build their levels and make them look better, so it is nothing
  new.

  The games use a large lookup table in the ROM to turn simple blocky levels
  into something worth looking at. Each unit in the tilemap is actually a
  pointer multiplied by eight to index into to the table. Just how large it is
  depends on how many tiles can be addressed by the unit. If it is a byte as
  in 'Mario World', the maximum number of tiles of 256; so the table could
  not be larger than 1024 bytes (256*8). 'Zelda' uses words in its Overworld
  map. So theoretically up to 65536 different tiles could be used, but in
  reality, it never uses more than 4096 tiles. Of course, a mere 4096 is more
  than plenty! It uses 12 bits per word, so the size of its table would be
  32768 bytes (4096*8). That is just right, as anything larger than 32k would
  not be addressable in a single bank (while using the Low ROM memory mode).

  The eight bytes for the four subtiles are always grouped together in the
  table, but just what order they come in seems to be a matter of choice.
  For example, 'Zelda' stores its four tiles in the order, top-left,
  top-right, bottom-left, and bottom-right. Otoh, 'Mario World' stores its
  vertically first, then horizontally, in the order top-left, bottom-left,
  top-right, bottom-right.

Tilemap unit:  (x)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿        ll³mm mm³nn nn³oo oo³pp
                                      ³        --Å-----ÚÄÄÄÄÄ¿-----Å--
ROM table:     abcdefghijklmnopqrstuvwxyz      qq³rr rr³ab cd³tt tt³uu
                                ÚÄÄÄÄÄÁÄÄÄÄÄ¿  qq³rr rr³ef gh³tt tt³uu
Four 16bit words:               ³ab cd ef ghÃÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÙ-----Å--
                                ÀÄÄÄÄÄÄÄÄÄÄÄÙ  vv³ww ww³xx xx³yy yy³zz

Frank Dwayne (FDwR@hotmail.com) Feb 23,1999
members.tripod.com/FDwR/snes.htm