Action 53 mapper

This is a sketch of a mapper that allows making a multicart of games that use multiple discrete mappers. As a random sample, it'd support 1943: The Battle of Midway/Valhalla (UNROM), 3-D Battles of Worldrunner (UNROM), Battle City (NROM-128), Battle Kid: Fortress of Peril (UOROM), Battle Tank (CNROM), Battleship (CNROM), Battletoads (AOROM), and Gekitotsu Yonku Battle (UNROM). iNES Mapper 028 has been preliminarily assigned.

Registers

$5000-$5FFF

Register select

$8000-$FFFF

Register value

$5000: Register select

7654 3210
S       R
|       +- Select register
+--------- 0: User registers; 1: Supervisor registers

In a multicart, registers $00 and $01 change the bank within a game, and registers $80 and $81 remain constant throughout a given game's execution.

$00: CHR bank

7654 3210
   M   BB
   |   ++- Set CHR RAM A14-A13
   +------ Set CIRAM A10 if H/V mirroring is disabled

$01: Inner bank

7654 3210
   M BBBB
   | ++++- Set current PRG ROM bank
   +------ Set CIRAM A10 if H/V mirroring is disabled

If the current mirroring mode is one of the 1-screen modes (0 or 1), writes to registers $00 and $01 change bit 0 of the mirroring mode to D4 of the written value. (This simulates the mirroring control of AxROM.) If the current mirroring mode is vertical or horizontal (2 or 3), D4 is ignored.

$80: Mode

7654 3210
  SS PPMM
  || ||++- Nametable mirroring mode
  || ++--- PRG bank mode
  ++------ PRG outer bank size

Mirroring modes

Mode	Effect	Effect of write to register $00 or $01
0	1-screen lower bank	D4 changes bit 0 of the mirroring mode
1	1-screen upper bank	D4 changes bit 0 of the mirroring mode
2	Vertical	D4 is ignored
3	Horizontal	D4 is ignored

While the mirroring mode is 0 or 1 (1-screen), bit 0 is accessible in three places: bit 0 of $80, bit 4 of $00, or bit 4 of $01.

PRG bank mode

Mode	Simulates	Effect
0, 1	BNROM/AOROM	Current 32 KiB bank in $8000-$FFFF
2	UNROM (#180)	Fixed bottom half of outer bank in $8000-$BFFF and current bank in $C000-$FFFF
3	UNROM (#2)	Current bank in $8000-$BFFF and fixed top half of outer bank in $C000-$FFFF

When the fixed bank ($8000-$BFFF in mode 2 or $C000-$FFFF in mode 3) is accessed, it treats accesses to the fixed bank the same way as accesses in mode 0 with 32K: the outer bank bits are passed straight through. For example, this would allow the fixed $C000 bank in mode 3 128K to be set to 16K bank 7 (as in mapper 2) or 1, 3, or 5. In mode 2 128K, the fixed $8000 bank could be configured as 16K bank 0 (as in mapper 180) or 2, 4, or 6.

PRG outer bank size

Size	Effect
0	A20-A15 controlled by outer bank (32 KiB)
1	A20-A16 controlled by outer bank (64 KiB)
2	A20-A17 controlled by outer bank (128 KiB)
3	A20-A18 controlled by outer bank (256 KiB)

Again, when a fixed bank is being accessed, this is temporarily forced to 32K, allowing all outer bank bits to come through.

These are the outputs on A20-A14 in each of the 12 combinations of mode and size, with o used for outer bank bits and i used for inner bank bits:

Mode value	PRG bank mode	Outer bank size	Bank in $8000-$BFFF	Bank in $C000-$FFFF
$00-$07	32 KiB	32 KiB	oooooo0	oooooo1
$08-$0B	Fixed $8000	32 KiB	oooooo0	ooooooi
$0C-$0F	Fixed $C000	32 KiB	ooooooi	oooooo1
$10-$17	32 KiB	64 KiB	oooooi0	oooooi1
$18-$1B	Fixed $8000	64 KiB	oooooo0	oooooii
$1C-$1F	Fixed $C000	64 KiB	oooooii	oooooo1
$20-$27	32 KiB	128 KiB	ooooii0	ooooii1
$28-$2B	Fixed $8000	128 KiB	oooooo0	ooooiii
$2C-$2F	Fixed $C000	128 KiB	ooooiii	oooooo1
$30-$37	32 KiB	256 KiB	oooiii0	oooiii1
$38-$3B	Fixed $8000	256 KiB	oooooo0	oooiiii
$3C-$3F	Fixed $C000	256 KiB	oooiiii	oooooo1

For all of these cases, the "o"s come from the topmost outer bank bits and the "i"s come from the bottommost inner bank bits.

$81: Outer bank

7654 3210
  BB BBBB
  ++-++++- Set outer PRG ROM bank

Only 6 bits are defined currently for two reasons: parallel flash memory pricing and the practical limit of 2 MiB in an iNES ROM image. In fact, the PowerPak implementation is expected to implement only the low 4 bits, as it has only 512 KiB of RAM for PRG ROM. Once emulators support NES 2.0 large ROMs, however, an implementation MAY add bits 6 and 7 to this register, resulting in an oversize extension to 8 MiB.

When the outer bank size is set greater than 32K, the bank number is not barrel shifted. Instead, the least significant bits are ignored. Bits 5 through 3, for example, always control PRG ROM A20 through A18.

Power up state

At power on, the last 16 KiB of the ROM is mapped into $C000-$FFFF. The rest of the state is unspecified. One possible init sequence involves setting reg $81 = $FF and reg $80 = $02 to get into oversize-BNROM mode.

Currently, the mapper state is unchanged on reset.

Configurations

NROM-128 (#0)

Outer bank size 0, PRG mode 2 or 3, mirroring H or V, select $01

NROM-256 (#0)

Outer bank size 0, PRG mode 0, mirroring H or V, select $01

CNROM (#3)

Outer bank size 0, PRG mode 0, mirroring H or V, select $00

BNROM (#34)

Outer bank size 1-3, PRG mode 0, mirroring H or V, select $01

BNROM oversize (#34 as emulated)

Outer bank size 0, PRG mode 0, mirroring H or V, select $81, and modify bus-conflict-avoidance table for position within multicart

UNROM (common) (#2)

Outer bank size 1-3, PRG mode 3, mirroring H or V, select $01

UNROM (Crazy Climber and MGC 2011) (#180)

Outer bank size 1-3, PRG mode 2, mirroring H or V, select $01

AOROM (#7)

Outer bank size 1-3, PRG mode 0, mirroring 1-screen, select $01

Reference implementations

See /Reference implementations for functions in Python and 6502 assembly language that calculate the bank number output to PRG ROM A20-A14 as described above. These may be used to verify emulator or hardware implementations.

Verilog Code

See /Verilog for an implementation designed for a CPLD. This implementation has been tested on a PowerPak as MAP1C.MAP.

Implementation notes

Input pins: 2 power, 16 signal

• Power and ground
• CPU D7, D5-D0 (D6 not currently used but may be used in future oversize versions)
• CPU A14-A12, /PRGSEL, M2, R/W
• PPU A12-A10

Output pins: 12 signal

• CHR RAM A14-A13
• CIRAM A10
• PRG ROM A20-A14, /CE
• Optional PRG RAM enable ($6000-$7FFF)

A CPLD requires at least one macrocell per bit of state, plus more macrocells for more complex operations. This mapper requires 20 bits of state, which leaves plenty of breathing room in a 36-cell CPLD for mapper logic.

• Register select: 2 bits
• Register $00: 2 bits (D4 is directed to register $80)
• Register $01: 4 bits (D4 is directed to register $80)
• Register $80: 6 bits
• Register $81: 6 bits

After synthesizing and laying fitting within a XC9536XL CPLD 27/36 Macrocells were consumed (75%). Additionally this design requires 25/34 available pins on the XC9536XL.

Adding WRAM control requires 2 Macrocells and 2 pins.

Lowering to 1MB by shaving off PRG ROM A20 would save 1 Macrocell and 1 pin, if desired.