INES Mapper 176

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iNES Mapper 176 denotes an enhanced MMC3 chipset used by many multicarts as well as Chinese single-game and educational computer cartridges. Curiously, it is also used by Techno Source's Intellivision X2 Plug-and-Play console. At least four incompatible variations exist that are denoted via NES 2.0 Submapper. UNIF board names do not correspond exactly to them.

Submappers
# MMC3 PRG limit PRG A21+ CHR A21+ $46/47 CNROM latch Extra WRAM 1-Screen Mirroring UNIF
0 512 KiB CHR A20 bit in $5xx2 - normal not present not supported not supported BMC-FK23C/BMC-FK23CA/BMC-Super24in1SC03
1 2 MiB $5xx5 $5xx6 normal not present not supported not supported BMC-FK23C/BMC-FK23CA
2 512 KiB various bits in $5xx0/$5xx2 - swapped not present supported via $A001 supported WAIXING-FS005
3 512 KiB - - - present not supported not supported BMC-FK23CA
  • The Waixing variant swaps the meaning of MMC3's $8000 values $46 and $47.
  • Variants with no CHR-RAM and only CHR-RAM that repurpose higher CHR address lines as PRG-ROM address lines do not have their own submappers.
  • MMC3 PRG limit of 2 MiB means that bit 6 and 7 of $8000.6/7 are applied, and the fixed banks are $FE/$FF rather than $3E/$3F. This implies that the multicart's reset vector lies at address PRG-ROM address $1FFFFC rather than $7FFFC.

Registers

Submapper 2 can disable the registers in the $5000-$5FFF range using the RAM Configuration Register ($A001). Writes to $5xx8-$5xxF have no effect.

Mode Register ($5xx0)

Mask: $5xxF, x determined by solder pad setting 

7654 3210
---- ----
PCTm PMMM
|||| ||||
|||| |+++- Select PRG Banking Mode (ignored in Extended MMC3 Mode)
|||| |      0: MMC3 PRG Mode, 2 MiB (Submapper 1)/512 KiB (all others) Outer PRG Bank Size
|||| |      1: MMC3 PRG Mode, 256 KiB Outer PRG Bank Size
|||| |      2: MMC3 PRG Mode, 128 KiB Outer PRG Bank Size
|||| |      3: NROM-128 PRG Mode, 16 KiB PRG at $8000-$BFFF mirrored at $C000-$FFFF
|||| |      4: NROM-256 PRG Mode, 32 KiB PRG at $8000-$FFFF
|||| |      5: UNROM PRG Mode, 16 KiB PRG at $8000-$BFFF, inner bank #7 at $C000-$FFFF
|||| |      6-7: Never used
|||| +---- PRG Base A21 (Submapper 2 only)
|||+------ Select Outer CHR Bank Size
|||         0: In MMC3 CHR Mode: 256 KiB
|||            In CNROM CHR Mode: 32 KiB (submapper 3 only)
|||         1: In MMC3 CHR Mode: 128 KiB
|||            In CNROM CHR Mode: 16 KiB (submapper 3 only)
||+------- Select CHR Memory Type
||          0: CHR-ROM
||          1: CHR-RAM
|+-------- CHR Mode
|           0: MMC3 CHR Mode
|           1: CNROM (Submapper 3)/NROM (all others) CHR Mode
+--------- PRG Base A22 (Submapper 2 only)

Power-on value: $00
  • It is possible to use NROM mode for PRG banking and MMC3 mode for CHR banking.
  • Bit 5 applies to CHR Memory in its entirety. Submapper 2 provides for mixed CHR_ROM/RAM mode using Bit 2 of its RAM Configuration Register ($A001) selects mixed CHR-ROM/RAM mode.
  • The inner and outer bank numbers are combined ...
    • ... in MMC3 PRG/CHR modes: by masking the MMC3 bank register content according to the specified size (128 or 256 KiB) and OR'ing with the opposite-masked content of the PRG ($5xx1)/CHR ($5xx2) Base;
    • ... in NROM PRG/CHR mode: by using the PRG ($5xx1)/CHR Base ($5xx2) directly.
    • ... in CNROM CHR mode (Submapper 3 only): by masking the CNROM Latch according to the selected Outer CHR Bank Size, and OR'ing with the unmasked content of the CHR Base ($5xx2);
    • ... in Extended MMC3 mode by OR'ing the unmasked (extended) bank register content with the unmasked content of the PRG ($5xx1)/CHR ($5xx2) Base.

PRG Base Register LSB ($5xx1)

Mask: $5xxF, x determined by solder pad setting 

7654 3210
---- ----
.PPP PPPP
 ||| ||||
 +++-++++- PRG Base A20..A14

Power-on value: $00

PRG Base Register MSB ($5xx5), Submapper 1 only

Mask: $5xxF, x determined by solder pad setting 

7654 3210
---- ----
.... PPPP
     ||||
     ++++- PRG Base A24..A21

Power-on value: $00

CHR Base Register LSB ($5xx2)

Mask: $5xxF, x determined by solder pad setting 

7654 3210
---- ----
ccdC CCCC
|||| ||||
++++-++++- CHR Base A20..A13
||+------- PRG Base A25 (submapper 2 only)
++-------- PRG Base A24..A23 (submapper 2 only)
+--------- PRG Base A21 (submapper 0 without CHR-ROM only)

Power-on value: $00
  • Writing to the CHR Base Register also resets the CNROM latch in submapper 3.
  • Submapper 0 and 2 boards that only use CHR-RAM reuse some of the higher CHR address lines as PRG address lines.

CHR Base Register MSB ($5xx6), Submapper 1 only

Mask: $5xxF, x determined by solder pad setting 

7654 3210
---- ----
.... PPPP
     ||||
     ++++- CHR Base A24..A21

Power-on value: $00

Extended Mode Register ($5xx3)

Mask: $5xxF, x determined by solder pad setting 

7654 3210
---- ----
.... ..E.
       | 
       +- Extended MMC3 Mode
           0: disable
           1: enable

Power-on value: $00

Mirroring Register ($A000)

Mask: $E001

7654 3210
---- ----
.... ..MM
       ++- Select nametable mirroring 
           0: Vertical
           1: Horizontal
           2: Single-screen, page 0 (Submapper 2 only)
           3: Single-screen, page 1 (Submapper 2 only)
Power-on value: $00

Single-screen mirroring is only available when the RAM Configuration Register is enabled ($A001.5).

RAM Configuration Register ($A001), Submapper 2 only

Mask: $E001

This register functions like MMC3 register $A001 until bit 5 is set, which turns it into the RAM Configuration Register. It is only present on later chipset revisions. 

7654 3210
---- ----
RFE. SCWW
|||  ||||
|||  ||++- Select 8 KiB PRG-RAM bank at $6000-$7FFF. Ignored if Bit 5 is clear.
|||  |+--- Select the memory type in the first 8 KiB of CHR space. Ignored if Bit 5 is clear.
|||  |      0: First 8 KiB are CHR-ROM
|||  |      1: First 8 KiB are CHR-RAM
|||  +---- Unknown
||+------- RAM Configuration Register Enable
||          0: RAM Configuration Register disabled, $A001 functions as on MMC3, 8 KiB of WRAM
||          1: RAM Configuration Register enabled, 32 KiB of WRAM
|+-------- FK23C Registers Enable. Ignored if Bit 5 is clear.
|           0: FK23C Registers disabled, $5000-$5FFF maps to the second 4 KiB of the 8 KiB WRAM bank 2
|           1: FK23C Registers enabled in the $5000-$5FFF range
+--------- PRG RAM enable (0: disable, 1: enable)

Power-on value: $00

UNROM latch ($8000-$FFFF)

In UNROM Mode (PRG Mode 5), writing to this address range changes the 16 KiB inner PRG bank at $8000-$BFFF.

CNROM latch ($8000-$FFFF), Submapper 3 only

In CNROM Mode, writing to this address range changes the inner CHR bank.

MMC3-compatible registers, Extended MMC3 Mode ($8000/$8001, $C000/$C001, $E000/$E001)

If the "Extended MMC3 Mode" bit in register $5xx3 is clear, then these registers function identically to the MMC3. If the "Extended MMC3 Mode" bit is set, four more bank registers become available at $8000/$8001, so that the original two 2 KiB CHR banks become four 1 KiB CHR banks, and the two fixed 8 KiB PRG banks become selectable, similar to the RAMBO-1. Register $8000 if $5xx3 bit 1 is set (Mask: $E001): 

7  bit  0
---- ----
CP.. RRRR
||   ||||
||   ++++- Specify which bank register to update on next write to Bank Data register
||         $0: Select 1 KB CHR bank at PPU $0000-$03FF (or $1000-$13FF)
||         $1: Select 1 KB CHR bank at PPU $0800-$0BFF (or $1800-$1BFF)
||         $2: Select 1 KB CHR bank at PPU $1000-$13FF (or $0000-$03FF)
||         $3: Select 1 KB CHR bank at PPU $1400-$17FF (or $0400-$07FF)
||         $4: Select 1 KB CHR bank at PPU $1800-$1BFF (or $0800-$0BFF)
||         $5: Select 1 KB CHR bank at PPU $1C00-$1FFF (or $0C00-$0FFF)
||         $6: Select 8 KB PRG ROM bank at $8000-$9FFF (or $C000-$DFFF)
||         $7: Select 8 KB PRG ROM bank at $A000-$BFFF
||         $8: Select 8 KB PRG ROM bank at $C000-$DFFF (or $8000-$9FFF)
||         $9: Select 8 KB PRG ROM bank at $E000-$FFFF
||         $A: Select 1 KB CHR bank at PPU $0400-$07FF (or $1400-$17FF)
||         $B: Select 1 KB CHR bank at PPU $0C00-$0FFF (or $1C00-$1FFF)
|+-------- Invert PRG A14
+--------- Invert CHR A12

Power-on values:
* Standard MMC3 Registers $0-$7: $00, $02, $04, $05, $06, $07, $00, $01
* Extended MMC3 Registers $8-$B: $FE, $FF, $FF, $FF

Solder Pad

The address mask in the $5000-$5FFF range is determined by the solder pad setting: 

Pad setting  Address mask
-----------  ------------
0            $501F
1            $502F
2            $504F
3            $508F
4            $510F
5            $520F
6            $540F
7            $580F
  • A solder pad setting of zero (address mask $501F) will produce a usable result for any ROM image.
  • Some multicarts only display their menu at settings other than 0.

Protection (Submapper 2 only)

Later Waixing games (and re-releases of earlier games) use the RAM Configuration Register for copy-protection purposes:

  • Write $A1 to $A001: Address range $5000-$5FFF to second half of 8 KiB WRAM bank 2, mapper registers there are disabled.
  • Write three values to $5000, $5010 and $5013.
  • Do further initialization.
  • Write $E2 to $A001. Mapper registers in address range $5000-$5FFF; WRAM at CPU $6000-$7FFF points to 8 KiB WRAM bank 2.
  • Copy 20 bytes from $7000 to $6000.
  • Copy and XOR bytes from $6000, $6010 and $6013 to $0100-$0102.
  • Execute code at CPU $0100.

Hacked ROMs can be detected by them writing to $5000/$5010/$5013 but then no longer jumping to $0100.

See also

  • NES 2.0 Mapper 523 is a variant of this mapper with hard-wired mirroring that connects CHR-ROM differently to produce 4/2 KiB instead of 2/1 KiB banks.
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