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).

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

$80: Mode

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

Mirroring modes

Mode	Effect
0	1-screen lower bank
1	1-screen upper bank
2	Vertical (and ignore writes to bit 4 of registers $00 and $01)
3	Horizontal (and ignore writes to bit 4 of registers $00 and $01)

PRG bank mode

Mode	Effect
0, 1	Current 32 KiB bank in $8000-$FFFF
2	Bottom half of outer bank in $8000-$BFFF and current bank in $C000-$FFFF
3	Current bank in $8000-$BFFF and top half of outer bank in $C000-$FFFF

PRG outer bank size

Mode	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)

Here are some examples of how bank modes work, assuming the outer bank is set to $12 and inner bank is $07, along with which bits from the inner bank are used:

Mode value	PRG bank mode	Outer bank size	Bank in $8000-$BFFF	Bank in $C000-$FFFF
$00-$07	32 KiB	32 KiB	$12 bottom (xxxx)	$12 top (xxxx)
$08-$0B	Fixed $8000	32 KiB	$12 bottom (xxxx)	$12 top (xxx1)
$0C-$0F	Fixed $C000	32 KiB	$12 top (xxx1)	$12 top (xxxx)
$10-$17	32 KiB	64 KiB	$13 bottom (xxx1)	$13 top (xxx1)
$18-$1B	Fixed $8000	64 KiB	$12 bottom (xxxx)	$13 top (xx11)
$1C-$1F	Fixed $C000	64 KiB	$13 top (xx11)	$12 top (xxxx)
$20-$27	32 KiB	128 KiB	$13 bottom (xx11)	$13 top (xx11)
$28-$2B	Fixed $8000	128 KiB	$12 bottom (xxxx)	$13 top (x111)
$2C-$2F	Fixed $C000	128 KiB	$13 top (x111)	$12 top (xxxx)
$30-$37	32 KiB	256 KiB	$17 bottom (x111)	$17 top (x111)
$38-$3B	Fixed $8000	256 KiB	$12 bottom (xxxx)	$13 top (0111)
$3C-$3F	Fixed $C000	256 KiB	$13 top (0111)	$12 top (xxxx)

$81: Outer bank

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

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

Implementation notes

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: 6 bits (D4 is directed to register 0)
• Register $01: 4 bits (D4 is directed to register 0)
• Register $80: 6 bits
• Register $81: 2 bits (D4 is directed to register 0)