The buffer is ds 1300 and every real map plus its three-block border fits in it.
A header that says otherwise is one read mid-load, and decoding it would read
past the buffer into somebody else s WRAM, so it is a refusal rather than a
clamp.
A warp writes wCurMap before the map header and the block data: on Oak s lab
doormat wCurMap already reads PALLET_TOWN while the header still reads the lab s
ten-by-twelve. The decode agrees with the screen on such a frame -- both are the
old map -- so only the id is wrong, and a grid filed under it would stay wrong
for as long as it was cached. One byte answers it: does the cached grid still
agree with the screen about the tile the fly is standing on.
Measured on the cartridge, not derived: a map tile is 2x2 screen tiles and
CheckTilePassable matches one id, and the screen agrees with the lower-left tile
of the quadrant. Viridian Forest s (4, 32) reads 3, the second row of its
block, where the first row holds bash4. On open ground most quadrants hold one id
four times over, so the upper-left guess reads correctly on a town and falls
apart in a forest -- which is why the decode is cross-checked against the screen
before it is trusted.
route and frontier ask for the grid once per plan and fall back to the window
when there is none. With it: one plan crosses a whole map, GO WARP / GO OUT /
GO ROUTE route to their warp or connection tile rather than to the nearest tile
of an edge that reads Unknown, a tile-pair wall is planned around instead of
being learned by walking into it, and the frontier is the nearest unstood
walkable tile anywhere on the map instead of the nearest one on screen.
The walkable predicate answered for the ten-by-nine screen window and Unknown
everywhere else. MapGrid is the same rule over every tile of the loaded map:
block ids out of wOverworldMap, a blocks-to-tiles read of the tileset header s
blockset through the new bank-aware ROM read, the tileset s collision list as
before, and the TilePairCollisionsLand values as directed walls both ways.
The reader checks itself before it answers: the decode is compared against the
window predicate over the player s own neighbourhood, and a frame where the
window can answer for none of it -- a battle, a text box, a frame mid-warp -- is
refused, because wOverworldMap shares its bytes with the picture buffer. Every
refusal is named (GridRefusal) and leaves the window predicate in charge.
Cached per map id and size, so a map is decoded once on arrival rather than once
per question, and owned beside the session ledgers: never checkpointed.
MemoryReader::read8 reads the CPU bus, where banks 1 and up are whichever bank
the cartridge last switched to, so a table in another bank could only be read by
writing the mapper register -- and the joypad is the only write this workspace
makes into a running game. read_rom takes a bank and a CPU address and reads the
cartridge image the process already holds instead. Defaulted to None, so every
reader without a cartridge behind it narrows rather than guesses.
resolve_wram.py is a second reading of the disassembly beside gen_symbols.py:
it walks ram/wram.asm with a cursor that is only ever live while anchored on an
address symbols.rs already pins, re-derives 40 of the 63 it carries with no
disagreement, and emits an address only when a pinned one after it agrees too.
wOverworldMap, wCurMapTileset, wTilesetBank and wTilesetBlocksPtr come out of
that pass; no address is hand-written and nothing else in the table moves.
