Difference between revisions of "MMC2"

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(moving the CHR banking stuff to its own section)
 
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|complexity=ASIC
 
|complexity=ASIC
 
|boards=PNROM, PEEOROM
 
|boards=PNROM, PEEOROM
 +
|pinout=MMC2 pinout
 
|prgmax=128K
 
|prgmax=128K
 
|prgpage=8K + 24K fixed
 
|prgpage=8K + 24K fixed
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The [[iNES]] format assigns '''Mapper 009''' to '''PxROM'''.
 
The [[iNES]] format assigns '''Mapper 009''' to '''PxROM'''.
 
This chip appeared in November 1987.
 
This chip appeared in November 1987.
 +
[[Category:Mappers triggering on reads]]
  
 
== Banks ==
 
== Banks ==
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== CHR banking ==
 
== CHR banking ==
  
When the PPU reads from specific tiles ($FD or $FE) in the pattern table during rendering, the MMC2 sets a latch that tells it to use a different 4 KB bank number. On the background layer, this has the effect of setting a different bank for all tiles to the right of a given tile, virtually increasing the tile count limit from 256 to 512 without monopolizing the CPU.
+
The main interest of the MMC2 and [[MMC4]] mappers is that they allow switching two pairs of 4 KB CHR-ROM banks at the same time, automatically alternating between banks within one pair during rendering. When the PPU reads from specific tiles ($FD or $FE) in the pattern tables, the MMC2/4 sets a latch that switches between two different 4 KB banks. This allows the tile limit to increase from 256 to 512 with bank splits, without involving the CPU or an IRQ.
  
 
*PPU reads $0FD8: latch 0 is set to $FD for subsequent reads
 
*PPU reads $0FD8: latch 0 is set to $FD for subsequent reads
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* The right ($1000-1FFF) pattern table switches on ''every row'' of the 8x8 tile
 
* The right ($1000-1FFF) pattern table switches on ''every row'' of the 8x8 tile
  
Because latch 1 is expected to use the background nametable, the same tiles are going to be fetched for 8 scanlines, so extra circuitry to select only the first row was unnecessary. Latch 0 is intended for sprites, which allows more arbitrary arrangement, hence it is designed to operate only on the first row of the tile.
+
With this mapper, the left pattern table ($0000) is intended for use with sprites, and the right pattern ($1000) table for background. Backgrounds require a switch on every row. Because sprites aren't constrained to an 8x8 grid, triggering on only the first row allows switching sprites to be placed closer together if needed. This nuance is absent in the [[MMC4]], where both pattern table switches from the entire tiles in a symmetrical fashion.
  
Note that the latch is updated ''after'' either pattern table byte is fetched, so the tiles $FD and $FE are drawn using the old CHR-bank before the new latch value is set.
+
Note that the latch is updated ''after'' either pattern table byte is fetched, so the switching tiles $FD or $FE themselves are drawn using the old CHR-bank before the latch value is changed. As the PPU fetches 34 background tiles per scanline (and at most 33 are drawn), if vertical [[mirroring]] is used, background switching tiles can be placed past the edge of the screen where they will be unseen.
 
+
An additional trick is possible with the background: since the PPU fetches 34 background tiles per scanline (and at most 33 are drawn), you can use vertical [[mirroring]] to place a switching tile just past the edge of the screen, where it will be unseen.
+
  
 
== Hardware ==
 
== Hardware ==
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The PEEOROM board is used in the re-issue of ''Mike Tyson's Punch-Out!!''. Unlike PNROM, and unlike most other boards used in NES Game Paks sold to the public, it can be configured to support EPROM memory through jumpers on the board.
 
The PEEOROM board is used in the re-issue of ''Mike Tyson's Punch-Out!!''. Unlike PNROM, and unlike most other boards used in NES Game Paks sold to the public, it can be configured to support EPROM memory through jumpers on the board.
  
A pirate clone that exclusively uses discrete logic has been found and reverse-engineered. [http://forums.nesdev.com/viewtopic.php?f=9&t=10652]
+
A pirate clone that exclusively uses discrete logic has been found and reverse-engineered.<ref>[http://forums.nesdev.com/viewtopic.php?f=9&t=10652 nesdev forum: Punch Out Cartridge have only TTLs instead of MMC2!]</ref>
  
 
== Variants ==
 
== Variants ==
 
Nintendo's [[MMC4]], used in the [[FxROM]] board set, is a similar mapper with PRG RAM support and PRG bank sizes of 16kb instead of 8kb. It also suppresses the different banking behavior of the left pattern table.
 
Nintendo's [[MMC4]], used in the [[FxROM]] board set, is a similar mapper with PRG RAM support and PRG bank sizes of 16kb instead of 8kb. It also suppresses the different banking behavior of the left pattern table.
  
Because of the extreme similarity between both chips (MMC2 and MMC4), it is possible to make a circuit that simulates a MMC4 from a MMC2, although, it is doubtful whether Nintendo ever exploited this publicly.
+
Because of the extreme similarity between the MMC2 and MMC4, it is possible to make a circuit that simulates an MMC4 from an MMC2 with the help of a [[7402]] quad-NOR gate and a [[7420]] 4-input NAND gate to [[PRG RAM circuit|decode PRG RAM]].
 
+
For example, with the help of a [[7402]] quad-NOR gate and a [[7420]] 4-input NAND gate to [[PRG RAM circuit|decode PRG RAM]], one can make the MMC2 act like an MMC4.
+
 
The following circuit "tricks" the MMC2 into thinking the program is still in the $8000-$9fff range when reading from $A000-$BFFF, but doesn't affect mapper writes.
 
The following circuit "tricks" the MMC2 into thinking the program is still in the $8000-$9fff range when reading from $A000-$BFFF, but doesn't affect mapper writes.
 
It also shifts all addresses left one bit so that it switches 16kB instead of 8kB banks, and it shortcuts around the different behavior for pattern tables at $0000 and $1000.
 
It also shifts all addresses left one bit so that it switches 16kB instead of 8kB banks, and it shortcuts around the different behavior for pattern tables at $0000 and $1000.
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* [http://nesdev.com/mappers.zip Comprehensive NES Mapper Document] by \Firebug\, information about mapper's initial state and lates are inaccurate.
 
* [http://nesdev.com/mappers.zip Comprehensive NES Mapper Document] by \Firebug\, information about mapper's initial state and lates are inaccurate.
 
* [http://www.romhacking.net/documents/362/ NES Mapper list] by Disch.
 
* [http://www.romhacking.net/documents/362/ NES Mapper list] by Disch.
 +
 +
== References ==

Latest revision as of 16:09, 20 May 2020


MMC2
PxROM
Company Nintendo
Games 1 in NesCartDB
Complexity ASIC
Boards PNROM, PEEOROM
Pinout MMC2 pinout
PRG ROM capacity 128K
PRG ROM window 8K + 24K fixed
PRG RAM capacity 8K (PC10 ver.)
PRG RAM window Fixed
CHR capacity 128K
CHR window 4K + 4K (triggered)
Nametable mirroring H or V, switchable
Bus conflicts No
IRQ No
Audio No
iNES mappers 009

The Nintendo MMC2 is an ASIC mapper, used on the PNROM and PEEOROM Nintendo Game Pak boards for Mike Tyson's Punch Out!!. The iNES format assigns Mapper 009 to PxROM. This chip appeared in November 1987.

Banks

  • CPU $6000-$7FFF: 8 KB PRG RAM bank (PlayChoice version only; contains a 6264 and 74139)
  • CPU $8000-$9FFF: 8 KB switchable PRG ROM bank
  • CPU $A000-$FFFF: Three 8 KB PRG ROM banks, fixed to the last three banks
  • PPU $0000-$0FFF: Two 4 KB switchable CHR ROM banks
  • PPU $1000-$1FFF: Two 4 KB switchable CHR ROM banks

The two 4 KB PPU banks each have two 4 KB banks, which can be switched during rendering by using the special tiles $FD or $FE in either a sprite or the background. See CHR banking below.

Registers

PRG ROM bank select ($A000-$AFFF)

7  bit  0
---- ----
xxxx PPPP
     ||||
     ++++- Select 8 KB PRG ROM bank for CPU $8000-$9FFF

CHR ROM $FD/0000 bank select ($B000-$BFFF)

7  bit  0
---- ----
xxxC CCCC
   | ||||
   +-++++- Select 4 KB CHR ROM bank for PPU $0000-$0FFF
           used when latch 0 = $FD

CHR ROM $FE/0000 bank select ($C000-$CFFF)

7  bit  0
---- ----
xxxC CCCC
   | ||||
   +-++++- Select 4 KB CHR ROM bank for PPU $0000-$0FFF
           used when latch 0 = $FE

CHR ROM $FD/1000 bank select ($D000-$DFFF)

7  bit  0
---- ----
xxxC CCCC
   | ||||
   +-++++- Select 4 KB CHR ROM bank for PPU $1000-$1FFF
           used when latch 1 = $FD

CHR ROM $FE/1000 bank select ($E000-$EFFF)

7  bit  0
---- ----
xxxC CCCC
   | ||||
   +-++++- Select 4 KB CHR ROM bank for PPU $1000-$1FFF
           used when latch 1 = $FE

Mirroring ($F000-$FFFF)

7  bit  0
---- ----
xxxx xxxM
        |
        +- Select nametable mirroring (0: vertical; 1: horizontal)

CHR banking

The main interest of the MMC2 and MMC4 mappers is that they allow switching two pairs of 4 KB CHR-ROM banks at the same time, automatically alternating between banks within one pair during rendering. When the PPU reads from specific tiles ($FD or $FE) in the pattern tables, the MMC2/4 sets a latch that switches between two different 4 KB banks. This allows the tile limit to increase from 256 to 512 with bank splits, without involving the CPU or an IRQ.

  • PPU reads $0FD8: latch 0 is set to $FD for subsequent reads
  • PPU reads $0FE8: latch 0 is set to $FE for subsequent reads
  • PPU reads $1FD8 through $1FDF: latch 1 is set to $FD for subsequent reads
  • PPU reads $1FE8 through $1FEF: latch 1 is set to $FE for subsequent reads

Notice that latch 0 only responds to one address, but latch 1 responds to a range of addresses. This means that:

  • The left ($0000-0FFF) pattern table only switches on the top row of the 8x8 tile
  • The right ($1000-1FFF) pattern table switches on every row of the 8x8 tile

With this mapper, the left pattern table ($0000) is intended for use with sprites, and the right pattern ($1000) table for background. Backgrounds require a switch on every row. Because sprites aren't constrained to an 8x8 grid, triggering on only the first row allows switching sprites to be placed closer together if needed. This nuance is absent in the MMC4, where both pattern table switches from the entire tiles in a symmetrical fashion.

Note that the latch is updated after either pattern table byte is fetched, so the switching tiles $FD or $FE themselves are drawn using the old CHR-bank before the latch value is changed. As the PPU fetches 34 background tiles per scanline (and at most 33 are drawn), if vertical mirroring is used, background switching tiles can be placed past the edge of the screen where they will be unseen.

Hardware

The MMC2 is implemented in a 40-pin shrink-DIP package. At least two revisions are known to exist, the MMC2 and the MMC2-L.

The PEEOROM board is used in the re-issue of Mike Tyson's Punch-Out!!. Unlike PNROM, and unlike most other boards used in NES Game Paks sold to the public, it can be configured to support EPROM memory through jumpers on the board.

A pirate clone that exclusively uses discrete logic has been found and reverse-engineered.[1]

Variants

Nintendo's MMC4, used in the FxROM board set, is a similar mapper with PRG RAM support and PRG bank sizes of 16kb instead of 8kb. It also suppresses the different banking behavior of the left pattern table.

Because of the extreme similarity between the MMC2 and MMC4, it is possible to make a circuit that simulates an MMC4 from an MMC2 with the help of a 7402 quad-NOR gate and a 7420 4-input NAND gate to decode PRG RAM. The following circuit "tricks" the MMC2 into thinking the program is still in the $8000-$9fff range when reading from $A000-$BFFF, but doesn't affect mapper writes. It also shifts all addresses left one bit so that it switches 16kB instead of 8kB banks, and it shortcuts around the different behavior for pattern tables at $0000 and $1000.

MMC2 A16  ----------------------------------  PRG A17

MMC2 A15  ----------------------------------  PRG A16

MMC2 A14  ----------------------------------  PRG A15
                ____              ___
MMC2 A13  -----\    `.       ,---\   `.
                )     )o-----+    )    )o---  PRG A14
CPU A14   -----/____,'       `---/___,'  

CPU A13   ---+------------------------------  PRG A13
             |    ___
             +---\   `.         ___
             |    )    )o------\   `.
             `---/___,'         )    )o-----  MMC2 A13
                          ,----/___,'
R/W       ----------------'

GND       --------------------+-------------  MMC2 PA2
                              |
                              +-------------  MMC2 PA1
                              |
                              `-------------  MMC2 PA0

See also

References

  1. nesdev forum: Punch Out Cartridge have only TTLs instead of MMC2!