Fix bankswitching: Difference between revisions
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The following info is based off | The following info is based off MAME's source and was not yet verified against real hardware. It can therefore be inaccurate/incomplete. | ||
The {{Chipname|NEO-CMC}} chips exploit the fact that the [[GPU]] continues to render the [[fix layer]] outside of the visible screen area. This allows them to use the normally unused $7500~$75FF [[VRAM]] region. | |||
Probably a hack found when someone complained the [[S ROM]] max. size was too small :) | |||
== NEO-CMC 42 == | == NEO-CMC 42 == |
Revision as of 18:55, 7 March 2016
The following info is based off MAME's source and was not yet verified against real hardware. It can therefore be inaccurate/incomplete.
The NEO-CMC chips exploit the fact that the GPU continues to render the fix layer outside of the visible screen area. This allows them to use the normally unused $7500~$75FF VRAM region.
Probably a hack found when someone complained the S ROM max. size was too small :)
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? ($0F works?) | Unused ? | Bank number |
Bank bits are inverted (00 = bank 3, 11 = bank 0).
MAME video/neogeo.c CMC42 handling:
if (banked && state->m_fixed_layer_bank_type == 1)
{
int garoubank = 0;
int k = 0;
int y = 0;
while (y < 32)
{
if (state->m_videoram[0x7500 + k] == 0x0200 && (state->m_videoram[0x7580 + k] & 0xff00) == 0xff00)
{
garoubank = state->m_videoram[0x7580 + k] & 3;
garouoffsets[y++] = garoubank;
}
garouoffsets[y++] = garoubank;
k += 2;
}
}
Mame driver increments fix line twice when a bank change is requested, which implies:
- It is only possible to bank lines 2 by 2.
- Actual vram offset position for each line varies with banking configurations
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