NES 2.0 submappers/Proposals

This page collects proposals for NES 2.0 submappers that are not yet ready for implementation.

1. Explain what game or ROM is incompatible with existing submappers.
2. Explain how the proposed submapper should be implemented.
3. Allow one or more other members of the community to independently verify that both 1 and 2 are correct. (We'll perform peer review, commentary, and possible revision/iteration here.)
4. Allocate and document the new submapper, listing the relevant game/ROM.

When allocating new submappers, please consult kevtris' original proposal before choosing a number. If it is something he already assigned that we have just not adopted yet, use his existing assignment: submappers.txt

If there is no existing game or ROM that requires a submapper, it should not yet be allocated. There is no end to possible variations of hardware, and there is no need to speculate on the future. If you want to work on a project that will require a new mapper, engage the community and/or seek help from others. Do not pre-emptively add a new mapper to the spec until there is something we can run with it. The spec will still be here when you're finished your project.

021, 023, 025: VRC2 / VRC4

these submappers are fully specified, allocation is pending the existence of test ROMs

These three mappers collect various configurations of VRC2 and VRC4 boards.

VRC2 is mostly a subset of VRC4, with differences including:

• VRC2 has a single bit of memory mapped at $6000-6FFF that the VRC4 does not, though it is redundant if WRAM is used.
• VRC4 supports horizontal, vertical and one-screen nametable mirroring while VRC2 supports only horizontal and vertical mirroring
• VRC4 has an interrupt device that VRC2 does not.

Additionally, different boards connect the address lines for the registers in various arrangements. The three iNES mappers 21, 23, and 25 each combine multiple boards whose addresses overlap. This is enough for game compatibility, but it creates an ugly combination that does not accurately describe the original hardware of either board.

Submapper assignment

Konami's VRC2 and VRC4 mappers have several variations of how the board connects low CPU address lines among A7-A0 to the port select lines of the mapper. Mappers 21, 23 and 25 each combine two non-overlapping addressing variations, and neglect to specify VRC2 vs VRC4.

Mapper 22 is related, but only supports one variation (VRC2a) and has a different CHR banking arrangement than the others. It does not require submapper disambiguation.

There are three variations of the VRC2 boards:

Nickname	A2	A1	Registers	iNES mapper	NES 2.0 submapper
VRC2a	A1	A0	$x000, $x002, $x001, $x003	22	0
VRC2b	A0	A1	$x000, $x001, $x002, $x003	23	3
VRC2c	A1	A0	$x000, $x002, $x001, $x003	25	3

Nickname	A2	A1	Registers	iNES mapper	NES 2.0 submapper
VRC4a	A2	A1	$x000, $x002, $x004, $x006	21	1
VRC4b	A0	A1	$x000, $x002, $x001, $x003	25	1
VRC4c	A7	A6	$x000, $x040, $x080, $x0C0	21	2
VRC4d	A2	A3	$x000, $x008, $x004, $x00C	25	2
VRC4e	A3	A2	$x000, $x004, $x008, $x00C	23	2
VRC4f	A1	A0	$x000, $x001, $x002, $x003	23	1

This submapper assigment uses the following symmetrical arrangement:

• 0. Both addressing
• 1. Lower addressing, VRC4
• 2. Higher addressing, VRC4
• 3. Lower addressing, VRC2 (no known examples for mapper 21)
• 4. Higher addressing, VRC2 (no known examples)

021 / 023 / 025: 0

The default implementation acts as a VRC4 (mostly compatible superset of VRC2), and responds to register writes in one or more configurations simultaneously (supporting both boards at once).

021 / 023 / 025: 1, 2 VRC4

These allocations each request a single specific addressing scheme for VRC4, rather than the combined version used by submapper 0.

Test ROMs:

• 021: 1 VRC4a - please create a test ROM before allocating this submapper
• 025: 1 VRC4b - please create a test ROM before allocating this submapper
• 021: 2 VRC4c - please create a test ROM before allocating this submapper
• 025: 2 VRC4d - please create a test ROM before allocating this submapper
• 023: 2 VRC4e - please create a test ROM before allocating this submapper
• 023: 1 VRC4f - please create a test ROM before allocating this submapper

023: 1 VRC4f

Some second-source VRC4 clones used a memory layout that was identical to the VRC2b (the simplest contiguous in-order interpretation: 0,1,2,3). This was used in World Hero.

Test ROM:

• please create a test ROM before allocating this submapper

023: 3 VRC2b

Some games rely on a single bit of memory mapped in the region from $6000-$6FFF on VRC2. These games were previously supported with submapper 0 by implementing WRAM in this region (despite these games not having WRAM). Additionally, VRC2 does not implement VRC4's interrupts. This board was used in Contra (J) and Ganbare Goemon 2.

Test ROM:

• please create a test ROM before allocating this submapper

025: 3 VRC2c

Ganbare Goemon Gaiden: Keita Ougon Kiseru uses VRC2c. It is compatible with submapper 0, but VRC4 has an additional interrupt capability (unused by this game).

Test ROM:

• please create a test ROM before allocating this submapper

005: MMC5

Status: Wishlist

Vertical split mode:
0: SL (all known hardware)
1: CL

If only one kind (battery or non-battery) of PRG-RAM present:
0: PRG-RAM is contiguous (EKROM, EWROM)
2: PRG-RAM is not contiguous; is split in half across two chips

If both kinds of PRG-RAM present:
0: Chip 0 is battery-backed (ETROM (note: verify this))
2: Chip 1 is battery-backed

Pulse waves volume:
0: R1 is 6.8kΩ (as in all games that use expansion audio)
4: R1 is 15kΩ (the nominal value of this resistor)

It is safe to leave the submapper number at 0 for all known games.

016: Bandai FCG boards

KH assigned the following redundant submappers. All are deprecated in favor of using the actual mapper numbers and/or NVRAM size field.

0: iNES Mapper 153 (SRAM)
1: iNES Mapper 157 (Datach)
2: iNES Mapper 159 (128B EEPROM)
3: "Normal" iNES Mapper 016 behavior. (256B or no EEPROM)

019: Namco 129 and 163

Status: Problem outline

Mapper 19 designates the Namco 129 and 163, which supports expansion sound, IRQs, and ROM nametables.

Different 163-using PCBs used a different resistor to change the volume of the expansion audio relative to the internal 2A03 audio. It is unclear if this variation warrants a submapper.

KH allocated a submapper specifically for the N163-using game Mindseeker. It is not known what is different about this game.

Tentative suggestion:
Mapper 19:
1: KH assigned to Mindseeker. Specifics unknown. Deprecated until such time as sufficient documentation is made.
2: N163, expansion audio unused (mixing resistor: 0Ω)
3: N163, mixing resistor: 4.7kΩ
4: N163, mixing resistor: 10kΩ
5: N163, mixing resistor: 15kΩ
9: N129. Expansion audio is known buggy relative to N163, but other differences are not known.

Source: KH's submappers

083: Cony

There's actually three different versions of the hardware, all assigned to the same mapper.

Unfortunately, we have no idea which is which.

Here's Kevtris's assignments:

083: 0

"Bog-standard Cony mapper. 1K CHR ROM banks, no WRAM."

083: 1

"Same, but with 2K CHR ROM banks instead."

083: 2

"This is the standard Cony mapper with the following changes:

1. 1K CHR ROM banks (like 83.0)

1. a 4 bit 256K CHR/PRG bank select register:
   • B000h: bits 6 and 7 select the 256K superbank

1. 1 byte of RAM at 5103h (stores the last game played) Game will not start without this RAM byte.

1. WRAM at 6000-7FFFh. WRAM is banked with the PRG/CHR superbank. This gives a total of 32K. It is battery backed.

iNES Mapper 185

Status: Draft

A few NROM-like games were released on CNROM boards where all four bits of the latch were solely used as an anti-piracy measure. While a documented heuristic exists for which values were used, we tentatively suggest that the submapper here indicate the value to be written to the latch for normal operation (submapper = (latch&0x30)/4+(latch&3))

3210  
||||
|||+- Bit 0 (bank number)
||+-- Bit 1 (bank number)
|+--- Bit 4 (diode config)
+---- Bit 5 (diode config)

In the case that any of the bits are "don't care", use 0.

---

On second thought, lidnariq would be happier with

3210
xxCC
||||
|||+- Bit 0 (bank number)
||+-- Bit 1 (bank number)
++--- Always "01", so that submapper 0 (use heuristic) doesn't collide with "write 0 for correct operation"

References

• Atari Age forum post - Kevtris' Analogue NT Mini firmware notes including a slightly updated submapper list.