INES Mapper 028

iNES Mapper 028 is assigned to a multicart mapper developed by Tepples and Infiniteneslives in October 2012, as an extension of the Action 53 project to cover CNROM and UNROM games. It is used for STREEMERZ: Action 53 Function 16 Volume One.

Implemented in FCEUX as of r2739 (tracking bug) but appears to have been dropped sometime between 2.2.0 and 2.2.1. Implemented in Bizhawk as of r3680.

Disch

Aping Disch's style:

========================
=  Mapper 028          =
========================

aka
--------------------------
Multi-discrete


Example Games:
--------------------------
STREEMERZ: Action 53 Function 16 Volume One


Notes:
--------------------------
This mapper is designed for making multicarts of games that
use NROM, CNROM, UNROM, BNROM, or AOROM.  It looks like the
love child of MMC3, MMC1, and the Nintendo discrete mappers.
It has separate address and data ports like MMC3 (and FME-7),
its mode register is laid out in a way reminiscent of MMC1,
and it has the data port at $8000-$FFFF for compatibility with
the discrete mappers that it is designed to simulate.

Registers:
--------------------------

Range,Mask:   $5000-$5FFF, $F000

  $5000:  [A... ...A]   Address for use with $8000

Range,Mask:   $8000-$FFFF, $8000

  $8000:  [..DD DDDD]   Data port
    R:$00 ->  CHR Reg
    R:$01 ->  PRG Reg
    R:$80 ->  Mode
    R:$81 ->  Outer PRG Reg

These registers are internal, written through the data port:

  R:$00:  [...M ..CC]
    C = CHR Reg
    M = Mirroring
        This bit overwrites bit 0 of R:$80, but only if bit 1 of
        R:$80 is clear

  R:$01:  [...M PPPP]
    P = PRG Reg
    M = Mirroring
        This bit overwrites bit 0 of R:$80, but only if bit 1 of
        R:$80 is clear

  R:$80:  [..GG PSMM]
    G = Game Size (0=32K, 1=64K, 2=128K, 3=256K)
    P = PRG Size (0=32k mode, 1=16k mode)
    S = Slot select:
        0 = $C000 swappable, $8000 fixed to bottom of 32K outer bank
        1 = $8000 swappable, $C000 fixed to top of 32K outer bank
        This bit is ignored when 'P' is clear (32k mode)
    M = Mirroring control:
        %00 = 1ScA
        %01 = 1ScB
        %10 = Vert
        %11 = Horz

  R:$81:  [..BB BBBB]
    Outer PRG Reg


CHR Setup:
---------------------------

    $0000   $0400   $0800   $0C00   $1000   $1400   $1800   $1C00 
  +---------------------------------------------------------------+
  |                             R:$00                             |
  +---------------------------------------------------------------+


PRG Setup:
---------------------------

There is 1 PRG reg and 3 PRG modes, corresponding to mappers 34, 2,
and 180. When 32K PRG mode is used (P = 0), R:$01 specifies a 32K
bank number between 0 and 7 (i.e., the high bit is ignored). When
16K PRG mode is used (P = 1), R:$01 specifies a 16K bank number
between 0 and F.

                     $8000   $A000   $C000   $E000
                   +-------------------------------+
  P = 0:           |             R:$01             |
                   +-------------------------------+
  P = 1, S = 0:    |    "Fixed"    |     R:$01     |
                   +---------------+---------------+
  P = 1, S = 1:    |     R:$01     |    "Fixed"    |
                   +---------------+---------------+

The "fixed" banks aren't really fixed.  They represent the first and
second half of the currently selected outer bank, as if temporarily
forcing P=0 and G=0 for a single read.

The 4-bit output I (Inner) from this phase tells which 16K bank within a
single game is being read.  The value is fed into the multicart logic.
The game size G controls how many bits of I are kept, the rest being
replaced with bits of the outer bank number O = 2 * R:$81.  This can
be thought of as an inner-AND and outer-AND value controlled by G.
I is ANDed with inner-AND, O is ANDed with outer-AND, and the results
are ORed to form a 16K page number.

  'G'   Block size   Inner-AND   Outer-AND
  ----------------------------------------
   0        32K         $01        $7E
   1        64K         $03        $7C
   2       128K         $07        $78
   3       256K         $0F        $70

This 16K page number is concatenated with bits A13-A0 to form the
PRG ROM address.

Powerup and Reset:
---------------------------

The last 16K PRG bank is switched in on powerup.
Registers are unchanged on reset.

References

• Forum post announcing reservation of #28
• Forum post announcing implementation in NESICIDE