Fix bankswitching

NEO-CMC 42

The 042 version of NEO-CMC is able to change the tile bank (4 maximum) for each line of fix tiles (8 pixels), allowing to use more than 4096 tiles. For this, the $7500~$75BF area of VRAM is used.

• The $7500~$753F area even words contains a flag to indicate if the bank has to be changed for the corresponding tile line.
• The $7580~$75BF area even words contains bank numbers for each of the tile lines.

If for a given line, the flag is set to $0200, the bank is read and changed and will remain the same until it's changed again (doesn't return to 0 if the next line hasn't the flag set).

Bank number:

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Def	Has to be $FF								Unused ?						Bank number

NEO-CMC 50

The 050 version of NEO-CMC is able to add 2 bits to the fix tile numbers, allowing to use 16384 tiles at most (512KiB S ROM) at any time. For this, the $7500~$75DF area of VRAM is used.

Each word in this area gives the bank numbers for 6 horizontally consecutive fix tiles:

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Def	Unused ?				a		b		c		d		e		f

Those bank bits are inverted (00 = bank 3, 11 = bank 0).

The words are organized as in the fix map for a PAL screen (first and last lines aren't used): from top to bottom and left to right. The "e" and "f" group of bits are unused in the last column ($75C0+).

Examples:

• Tile at X=0, Y=1 has its bank number in bits a of $7500
• Tile at X=0, Y=5 has its bank number in bits a of $7504
• Tile at X=3, Y=2 has its bank number in bits d of $7501
• Tile at X=7, Y=9 has its bank number in bits b of $7528