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 Post subject: Mapper 153 questions
PostPosted: Mon Dec 31, 2018 5:51 am 
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Info from the Wiki below.
Code:
Outer PRG-ROM Bank Select ($8000-$8003 write)

Mask: $800F

7  bit  0
---- ----
xxxx xxxP
        |
        +-- Select 256 KiB outer PRG-ROM bank at CPU $8000-$FFFF

    The same value must be written to all four registers, or the outer PRG-ROM bank would be switched as the PPU is rendering.
    Because the ASIC's PA12 and PA13 inputs are grounded, only registers $8000-$8003 instead of $8000-$8007 are relevant.
    No CHR banking is available.

1. One bit selecting 256KiB !? Shouldn't it be 16KiB instead???
2. If a game is set as mapper 16 and has CHR RAM, should I assume mapper 153?

Code:
CPU $8000-$BFFF: 16 KiB switchable PRG-ROM bank
CPU $C000-$FFFF: 16 KiB PRG ROM bank, fixed to the last bank


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 Post subject: Re: Mapper 153 questions
PostPosted: Mon Dec 31, 2018 6:15 am 
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  1. No, one bit indeed selects the outer 256 KiB PRG-ROM bank, or PRG A18, for the entire CPU $8000-$FFFF range. The PRG-ROM bank at $8000-$BFFF within (hence "inner PRG-ROM bank") that 256 KiB "outer" PRG-ROM is selected normally via register $8008. This mechanism is not unique to Mapper 157: the Nintendo MMC1's SOROM/SUROM/SXROM variants are similar in repurposing a CHR register bit for switching an outer PRG-ROM bank, and requiring all CHR registers to have the bit set identically.
  2. No, because the Datach games (Mapper 157) also have CHR-RAM. If you must use heuristics, then assume that any Mapper 16 game with 512 KiB or more PRG-ROM and CHR-RAM is Mapper 153.


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 Post subject: Re: Mapper 153 questions
PostPosted: Mon Dec 31, 2018 12:56 pm 
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If one bit selects a 256KiB outer PRG ROM bank, then the game(s) will have 512KiB at maximum of PRG ROM. Am I wrong?

If this bit changes during the game, should the PRG inner pages be "updated" by using the last value written to?

Plus, it says $C000-$FFFF is fixed to the last bank... of the current PRG outer bank?


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 Post subject: Re: Mapper 153 questions
PostPosted: Mon Dec 31, 2018 1:21 pm 
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Zepper wrote:
If one bit selects a 256KiB outer PRG ROM bank, then the game(s) will have 512KiB at maximum of PRG ROM. Am I wrong?
Correct. If Bandai had ever made a game with one MiB PRG-ROM, they could have extended their setup by using further bits in $8000..$8003.
Zepper wrote:
If this bit changes during the game, should the PRG inner pages be "updated" by using the last value written to?
Written to $8008, OR'ed with the outer bank bit (SHL 4).
Zepper wrote:
Plus, it says $C000-$FFFF is fixed to the last bank... of the current PRG outer bank?
Yes. Basically, PRG-ROM bankswitching works like this:

Code:
SetPRG_ROM16(0x8, (reg8008 &0x0F) | ((reg8000 &1) <<4));
SetPRG_ROM16(0xC,           0x0F  | ((reg8000 &1) <<4));

The parameters to SetPRG_ROM16 being a 16 KiB PRG-ROM bank number. That pseudo-code is a simplification, of course, because it's not always "reg8000", but any one of reg8000..reg8003 depending on from which bank the PPU is rendering right now.


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 Post subject: Re: Mapper 153 questions
PostPosted: Mon Dec 31, 2018 2:36 pm 
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It really is just like SUROM. The differences are just: four CHR banking registers (instead of two) reused as PRG banking registers; and no ability to select a mode where you can only use one register.


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