L0 ROM



LO stands for "LOokup". It's a 64KiB ROM chip found in every NeoGeo systems, which contains byte values used by the GPU to shrink sprites vertically.

The dump is often called 000-lo.lo in BIOS sets.

Data
The data contained in this ROM is actually 256 tables of 256 bytes, each table corresponding to a value of the vertical shrinking for sprites.

For the first 256 lines (first half of a full sprite), the index in the table is the horizontal line number of the sprite currently being drawn (scanline - Y position).

Each byte is used as 2 nibbles:
 * The upper nibble is the tile number index to read in the tilemap table in VRAM SCB1 (0 to 15).
 * The lower nibble is the line number of that tile to fetch in the C ROMs.

For the last 256 lines (second half of a full sprite), the index in the table is 255-((scanline - Y position) & 255). The table is read backwards.

Each byte is used as 2 nibbles:
 * The upper nibble is the tile number index XOR $1F to read in the tilemap table in VRAM SCB1 (0 to 15).
 * The lower nibble is the line number XOR $F of that tile to fetch in the C ROMs.

Example of GPU processing
(Maybe place this part in the GPU page ?)

Sprite with Y zoom value = $1B and tile height = 2 (32 pixels).

... ...
 * Line 0 of sprite. GPU asks LO data at address $1B00. Data is $00: Tilemap index 0, line 0 of tile.
 * Line 1 of sprite. GPU asks LO data at address $1B01. Data is $08: Tilemap index 0, line 8 of tile.
 * Line 2 of sprite. GPU asks LO data at address $1B02. Data is $10: Tilemap index 1, line 0 of tile.
 * Line 26 of sprite. GPU asks LO data at address $1B1A. Data is $E8: Tilemap index is $E, line 8 of tile.
 * Line 27 of sprite. GPU asks LO data at address $1B1B. Data is $F8: Tilemap index is $F, line 8 of tile.
 * Line 28 of sprite. GPU asks LO data at address $1B1C. Data is $FF: Tilemap index is $F, line $F of tile.
 * Line 29 of sprite. GPU asks LO data at address $1B1D. Data is $FF: Tilemap index is $F, line $F of tile.

Datasheet
Official TC531001 datasheet: []