fceu 0.98.12-4 / usr / share / doc / fceu / Documentation / tech / exp / mmc5_bank_switch.txt

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MMC5 Bankswitching
by Kevin Horton
--------------------

5100:  Controls paging of RAM and ROM

Bits: ???? ??xx

For xx:

00: 32K bankswitching.  Only 5117 can be used to control banks. 5114 thru
     5116 have no effect.


01: 16K bankswitching. Only 5115 and 5117 can be used to control banks.
     5114 and 5116 have no effect.


10: 8K/16K bankswitching. 5115-5117 are used.  5114 has no effect.

11: 8K bankswitching. 5114-5117 are used.

(See below for detailed description)

--

5113:  RAM page 6000-7FFF bank.  Lower 3 bits are used, for a possible
        64K of WRAM. (Note more bits *may* be possible for more RAM.
        This has not been confirmed yet).

Bits: ???? ?xxx

WRAM follows a certain convention, based on the style of MMC5 board used.
8K and 32K carts are usually implemented with a single chip, while 16K
carts use two 8K'ers.  This is important since enabling changes, and hence
valid banks.

for xxx:

      8K         16K        32K        40K        64K

0: bank 0     bank 0     bank 0     bank 0     bank 0
1: bank 0     bank 0     bank 1     bank 1     bank 1
2: bank 0     bank 0     bank 2     bank 2     bank 2
3: bank 0     bank 0     bank 3     bank 3     bank 3
4: open bus   bank 1     open bus   bank 4     bank 4
5: open bus   bank 1     open bus   bank 4     bank 5
6: open bus   bank 1     open bus   bank 4     bank 6
7: open bus   bank 1     open bus   bank 4     bank 7

Note that the 40K and 64K examples are hypothetical.  The first three,
however *are* real and is what you find inside a real MMC5 cart.

Also note, that 5114-5116 follow this identical convention, if set up
to switch in RAM banks.

--

Bankswitching is a bit complicated.  This table should make things clearer.
The numbers at the top are what you write to 5100 to select mode.

Legend:

-   = this has no effect
--- = this register is not used, and writes to it are ignored
R   = PRG ROM/WRAM select.  0=WRAM, 1=PRG ROM
b   = bank bits



5100:    00          01          10          11

5114     ---         ---         ---      Rbbb bbbb
5115     ---      Rbbb bbb-   Rbbb bbb-   Rbbb bbbb
5116     ---         ---      Rbbb bbbb   Rbbb bbbb
5117  -bbb bb--   -bbb bbb-   -bbb bbbb   -bbb bbbb


Mode 00
-------

Only one 32K page can be selected.  The lower 2 bits of the desired bank
are ANDed out.  writing 084h, 085h, 086h, and 087h to 5117 in this mode
all result in selection of the same 32K.  No RAM is available in this mode.

Mode 01
-------

There are two selectable 16K pages.  Similar to above, the lowest bit written
is not used to select banks.  In this mode, writing to 5115 selects 16K
at 8000-BFFF, and 5117 selects 16K at C000-FFFF. RAM can be enabled in this
mode for 8000-BFFF.  If RAM is enabled for 8000-BFFF, remember that the
lowest bank select bit is not used.

Mode 10
-------

This is the oddest one.  There is one 16K selectable page, and two 8K
selectable pages.  5115 selects the 16K page at 8000-BFFF, 5116 selects
an 8K page at C000-DFFF, and 5117 selects an 8K page at E000-FFFF.
RAM can be enabled for 8000-DFFF. (16K of RAM at 8000-BFFF via bit 7 of
D115, and 8K of RAM at C000-DFFF via bit 7 of d116).  Note that RAM banking
works the same as mode 01, above for the 16K bank.


Mode 11
-------

There are 4 8K selectable pages. 5114 controls 8000-9FFF, etc. all the way
up to 5117 that controls E000-FFFF.  The first 3 pages can use RAM, while
the last page cannot.


--

WRAM write enable.

5102, 5103

To enable writing to RAM, 5102 must have 02h written to it, and 5103
must have 01h written to it. If this is not the case, you can still
*read* the RAM, but writes to it have no effect.  Supposidly only the
lower two bits of 5102 and 5103 are checked, but I didn't verify this.
I *did* however verify that setting the two registers to the above
values allows writing.  If voltage goes out of tolerance (Read: you
turn the power on/off) the RAM writing is disabled.  (To prevent
corruption of saved-games during power cycling)

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