VRC4: Difference between revisions

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[[Category:ASIC mappers]][[Category:Mappers with cycle IRQs]]
#REDIRECT [[VRC2 and VRC4]]
The Konami VRC4 is an [[:Category:ASIC mappers|ASIC]] [[MMC|mapper]].
 
 
__TOC__
 
 
== Overview ==
* PRG ROM size: Up to 256 KiB
* PRG ROM bank size: 8 KiB at $A000, and $8000 OR $C000
* PRG RAM: none, 2 KiB, or 8 KiB
* CHR ROM size: Up to 512 KiB
* CHR bank size: 1 KiB
* Nametable [[mirroring]]: Controlled by mapper
* Subject to [[bus conflict]]s: No
 
The Konami VRC4 is almost identical to the [[VRC2|VRC2]], but with a bit more capabilities, such as the option to swap PRG at $C000 instead of $8000 and an IRQ counter.
 
See [[VRC4 pinout]] for chip pinout.
 
== Banks ==
* CPU $6000-$6FFF: optional 2 KiB PRG RAM bank (mirrored once), or
* CPU $6000-$7FFF: optional 8 KiB PRG RAM bank
* CPU $8000-$9FFF (or $C000-$DFFF): 8 KiB switchable PRG ROM bank
* CPU $A000-$BFFF: 8 KiB switchable PRG ROM bank
* CPU $C000-$DFFF (or $8000-$9FFF): 8 KiB PRG ROM bank, fixed to the second-last bank
* CPU $E000-$FFFF: 8 KiB PRG ROM bank, fixed to the last bank
* PPU $0000-$03FF: 1 KiB switchable CHR bank
* PPU $0400-$07FF: 1 KiB switchable CHR bank
* PPU $0800-$0BFF: 1 KiB switchable CHR bank
* PPU $0C00-$0FFF: 1 KiB switchable CHR bank
* PPU $1000-$13FF: 1 KiB switchable CHR bank
* PPU $1400-$17FF: 1 KiB switchable CHR bank
* PPU $1800-$1BFF: 1 KiB switchable CHR bank
* PPU $1C00-$1FFF: 1 KiB switchable CHR bank
 
== Revisions ==
 
This mapper has been used with several board revisions, each of which uses different address lines.
All boards use A15-A12 the same way, along with two lines from A7-A0.
Here is a listing of known revisions, which address lines are used, and which [[iNES]] mapper number is used to represent it:
 
variant  lines    registers                      iNES Mapper Number
----------------------------------------------------------------------
VRC4a    A1, A2    $x000, $x002, $x004, $x006      [[INES Mapper 021|021]]
VRC4b    A1, A0    $x000, $x002, $x001, $x003      025
VRC4c    A6, A7    $x000, $x040, $x080, $x0C0      021
VRC4d    A3, A2    $x000, $x008, $x004, $x00C      025
VRC4e    A2, A3    $x000, $x004, $x008, $x00C      023
VRC4 #27  A0, A1    $x000, $x001, $x002, $x003      [[iNES Mapper 027|027]]
 
Some variants that use the same address lines use them in different ways.  For example, VRC4d and VRC4e both use A2 and A3, however VRC4d has them reversed.
 
For historical reasons, some variants share a mapper number.
These may be detected with a hash of the PRG ROM, or the address lines used may be expressed in NES 2.0 with [[NES 2.0 submappers#VRC4|submappers]].
 
The RAM decoding circuit that's part of the VRC4 itself only decodes RAM from $6000-$6FFF. For the [http://bootgod.dyndns.org:7777/profile.php?id=1573 one game] with 8 KiB of RAM, an external [[PRG RAM circuit|circuit was added]].
 
== Registers ==
This page lists registers as they are in the VRC4a variant.  For other variants, you can use the diagram above for determining which registers are used.
 
=== PRG Swap Mode control ($9004, $9006) ===
 
7  bit  0
---------
.... ..M.
        |
        +-- Swap Mode
 
When 'M' is clear:
* the 8 KiB page at $8000 is controlled by the $800x register
* the 8 KiB page at $C000 is fixed to the second last 8 KiB in the ROM
 
When 'M' is set:
* the 8 KiB page at $8000 is fixed to the second last 8 KiB in the ROM
* the 8 KiB page at $C000 is controlled by the $800x register
 
 
=== PRG Select 0 ($8000, $8002, $8004, $8006) ===
 
7  bit  0
---------
...P PPPP
    | ||||
    +-++++- Select 8 KiB PRG bank at $8000 or $C000 depending on Swap Mode
 
=== PRG Select 1 ($A000, $A002, $A004, $A006) ===
 
7  bit  0
---------
...P PPPP
    | ||||
    +-++++- Select 8 KiB PRG bank at $A000
 
 
=== Mirroring Control ($9000, $9002) ===
 
7  bit  0
---------
.... ..MM
        ||
        ++- [[Mirroring]] (0: vertical; 1: horizontal; 2: one-screen, lower bank; 3: one-screen, upper bank)
 
=== CHR Select 0 ($B000, $B002) ===
 
  $B000        $B002
7  bit  0    7  bit  0
---------    ---------
.... LLLL    ...H HHHH
      ||||      | ||||
      ||||      +-++++- High 5 bits of 1 KiB CHR bank at PPU $0000
      ++++-------------- Low 4 bits
 
 
=== CHR Select 1 ($B004, $B006) ===
 
  $B004        $B006
7  bit  0    7  bit  0
---------    ---------
.... LLLL    ...H HHHH
      ||||      | ||||
      ||||      +-++++- High 5 bits of 1 KiB CHR bank at PPU $0400
      ++++-------------- Low 4 bits
 
 
=== CHR Select 2…7 ($C000-$EFFF) ===
The other six CHR bank selects continue the pattern:
{| class="wikitable"
! colspan=2|Write to CPU address
|-
! (low 4 bits) !! (high 5 bits) !! 1KB CHR bank affected
|-
| $C000 || $C002 || $0800-$0BFF
|-
| $C004 || $C006 || $0C00-$0FFF
|-
| $D000 || $D002 || $1000-$13FF
|-
| $D004 || $D006 || $1400-$17FF
|-
| $E000 || $E002 || $1800-$1BFF
|-
| $E004 || $E006 || $1C00-$1FFF
|}
 
=== IRQ Control ($F00x) ===
$F000:  IRQ Latch, low 4 bits
$F002:  IRQ Latch, high 4 bits
$F004:  IRQ Control
$F006:  IRQ Acknowledge
 
Many VRC mappers use the same IRQ system.  For details on IRQ operation, see [[VRC IRQ]]s.
 
== References ==
*[http://www.romhacking.net/documents/362/ NES Mapper List] by Disch
*[http://nesdev.org/konami-e.txt Konami Mapper] by goroh, translated by Sgt. Bowhack
*[http://nesdev.org/mappers.zip Comprehensive NES Mapper Document] by \Firebug\, information about mapper's initial state is inaccurate.
* The CHR registers really are 9 bits: http://forums.nesdev.org/viewtopic.php?t=8569

Latest revision as of 20:11, 28 April 2017

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