.MP — map files

AMER2.MP, 12,534 bytes, ships with both the DOS and the Windows game, byte for byte identical. The map previews of win-tools and dos-tools draw it as a PNG.

WORD  width          58
WORD  height         72
WORD  version        4
BYTE  terrain[w*h]   plane 0
BYTE  plane1[w*h]    plane 1
BYTE  plane2[w*h]    plane 2

Every byte is accounted for: 6 + 3 × 58 × 72 = 12,534, exactly the file size.

The loader #

The DOS game's load_map_file (VICEROY.EXE file 0x71106) reads the size, then the version word, which it refuses unless it is 4, then exactly three planes into the game's planes 0, 1 and 2. A map of more than 12,000 squares is refused (map_check). The Windows loader has not been traced, but it reads the same file.

Plane 3 — which nations have seen each square — is not in a map file; it exists only in a save, which carries all four planes.

Planar, not three bytes per tile #

The evidence is plane 1: it is uniformly zero across all 4,176 tiles. Reading the data as interleaved gives three lanes that all look alike (71, 65 and 76 distinct values, similar histograms); reading it as planes gives one plane with 87 values, one with exactly one value, and one with 15. No interleaving of three per-tile fields produces a lane that is entirely zero.

The planes #

Plane 0 — terrain, and the byte is a bitfield rather than an id: the low five bits are a terrain id when bit 0x20 is clear, bit 0x20 means hills and with bit 0x80 mountains, bit 0x40 a river and with 0x80 a major one. That is terrain_from_map_byte (1040:42b5), and it is what turns the 87 distinct values into the 29 terrains of the game's own TERRAIN0..TERRAIN28 table. The two commonest values are 25 (2,109 tiles) and 26 (810) — ocean and sea lane — together just under half the map. Drawn through the game's own tile art the plane is the Americas: North, Central and South America, the Caribbean chain, Florida, the Gulf of Mexico, Baja California, with the Rockies and the Andes where they belong.

Plane 1 — all zero. The loader reads it into the game's plane 1, which in a running game is the bitfield of what is on a square: units, settlements, roads, plowing (see the save's planes). All zero is what a map nobody has played on would hold; whether every .MP must carry it zero is not established, as only one file ships.

Plane 2 — correlated with land and water, not established as either.

1 tracks water and 2 tracks land, but roughly 250 tiles disagree and the values 3, 5, 6 and 7 are unexplained. Every value is below 16, so all of it lives in the low nibble of the game's plane 2 — the nibble a save's plane 2 leaves unidentified, beside the owning nation in the high one.

Cross-check #

Every known save carries 3a 00 48 00 — 58 × 72 — at offset 0x0c: the same dimensions, in a different file, written by a different routine.