Hi,

This message is to report the mapper hardware used by a "52 Games" NES multi-
game cartridge. Thanks to Richard Garey for the loan of this cartridge.

The 52 Games mapper hardware is functionally identical to the 58-in-1/110-in-1
mapper which I described before.

NOTE: I am am using mapper 225 = $E1 to denote the 58-in-1/110-in-1/52 Games
mapper.

I have extracted the selection menu code and included that in .nes format
in this archive, so you can see what the selection menu looks like without
having an emulator that supports this mapper.



Hardware details
----------------

The 52 Games cartridge is made by SuperVision, who also made the 110-in-1
cartridge.

The cartridge uses three 74-series ICs, HC273, LS670 and LS174, and a PAL16L8
for the mapping.

Total PRG size is 8mbit, CHR size 4mbit. PRG data is contained in two ROMs,
marked as follows:
        KM23C4000                      KM23C4000
        6000-881		       6000-882

The CHR ROM is marked as follows:
        KM23C4000
        6000-880


The PRG setup is quite interesting. Every pin of each PRG ROM is connected to
the corresponding pin of the other PRG ROM. This must mean that the pinout
differs slightly; specifically, in one ROM pin 22 is /CS, in the other it is
CS.



Mapper description
------------------

- Like most other mappers, switching is achieved by writing to addresses $8000-
  $FFFF.

- However, unlike most other mappers the value written is irrelevant. It is the
  address written to which is relevant. It looks something like this:

      A15 A14|A13|A12|A11 A10  A9  A8  A7| A6| A5  A4  A3  A2  A1  A0
      -------+---+---+-------------------+---+-----------------------
       1   - | m | s | p   p   p   p   p |h/l| c   c   c   c   c   c

  A14 is unused in the 52 Game cartridge (it is used in the 110-in-1, which is
      double the size)

  A13 sets mirroring.
      0: PA10 connected to VRAM A10. Horizontal scrolling, "vertical mirroring"
      1: PA11 connected to VRAM A10. Vertical scrolling, "horizontal mirroring"
      (I was unable to conclusively confirm this for the 52 Games cartridge.)

  A12 sets PRG page size.
      0: 32Kb
      1: 16Kb

  A11-A7 select the 32Kb PRG-ROM page. A11 is the most significant bit.

  A6  is used if the page size is 16Kb. It determines whether the upper or
      lower half of the chosen 32Kb PRG page is used (appearing at both $8000
      and $C000). 0: lower, 1: upper. It is unused if game PRG size is 32Kb
      (i.e., if A12 is 0).

  A5-A0 set the 8Kb CHR-ROM page. A5 is the most significant bit.

At power-on time, the mapper functions as if a write to $8000 has just
occurred. This means: 32K page size, PRG page 0, CHR page 0, vertical
mirroring. (Well, this seems to be true for the PRG and CHR pages, probably
true for mirroring. Not that it matters, since the first instruction executed
sets the mirroring.)


In addition, this mapper implements four 4-bit registers. These are accessed by
reading/writing in the region $5800-$5FFF. A0 & A1 select which register to
access, and D0-D3 is the value to write or read. For example:
  - writing $F6 to address $5800 sets register 0 to 6.
  - writing $A9 to address $5801 sets register 1 to 9.
  - writing $0C to address $5DCB sets register 3 to $C.
(That is, register 0 is accessed at $5800, $5804, $5808, ..., $5FFC, for
example.)

Reading from the registers, the register value is returned in D0-D3 (i.e. the
low 4 bits of the data bus). What is on the upper bits is undefined; in
practice it may be whatever value was last there (the last instruction byte
read perhaps). At power-on time, all four registers seem to contain $F.

When the user presses Reset, nothing out of the ordinary should happen; just
jump to the address that the reset vector points to as normal.

The selection menu must use the 4-bit registers to store the last-selected
game. Pressing reset returns to the menu, with the last-played game
highlighted.


-- Mark