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.
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.
