Company | InfiniteNESLives |
Complexity | CPLD |
Boards | |
PRG ROM capacity | 8M (512K/256K inner) |
PRG ROM window | 32K or 16K/16K |
PRG RAM capacity | None |
CHR capacity | 32K |
CHR window | 8K |
Nametable mirroring | H, V, or 1-screen switchable |
Bus conflicts | No |
IRQ | No |
Audio | No |
iNES mappers | 028 |
The Action 53 mapper allows making a multicart of games that use multiple discrete mappers. It was assigned to iNES Mapper 028.
Examples:
There are no bus conflicts.
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. Games ported from SGROM or SNROM may rewrite register $80, usually to change mirroring. Games ported from SUROM may change register $81's value to access the first or second 256 KiB of the ROM.
7654 3210 M BB | ++- Set CHR RAM A14-A13 +------ Set mirroring mode bit 0 if H/V mirroring is disabled
7654 3210 M BBBB | ++++- Set current PRG ROM bank +------ Set mirroring mode bit 0 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.
7654 3210 SS PPMM || ||++- Nametable mirroring mode || ++--- PRG bank mode ++------ PRG outer bank size
Mode | Effect | A10 output | Effect of write to register $00 or $01 |
---|---|---|---|
0 | 1-screen lower bank | 0 | D4 changes bit 0 of the mirroring mode |
1 | 1-screen upper bank | 1 | D4 changes bit 0 of the mirroring mode |
2 | Vertical | PPU A10 | D4 is ignored |
3 | Horizontal | PPU A11 | D4 is ignored |
While the mirroring mode is 0 or 1 (1-screen), bit 0 can be written in three places: bit 0 of $80, bit 4 of $00, or bit 4 of $01.
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 Current bank in $C000-$FFFF |
3 | UNROM (#2) | Current bank in $8000-$BFFF 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.
Size | Effect |
---|---|
0 | A15 and up controlled by outer bank (32 KiB) |
1 | A16 and up controlled by outer bank (64 KiB) |
2 | A17 and up controlled by outer bank (128 KiB) |
3 | A18 and up 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 A22-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 | Bank in $C000 |
---|---|---|---|---|
$00-$07 | 32 KiB | 32 KiB | oooooooo0 |
oooooooo1
|
$08-$0B | Fixed $8000 | 32 KiB | oooooooo0 |
ooooooooi
|
$0C-$0F | Fixed $C000 | 32 KiB | ooooooooi |
oooooooo1
|
$10-$17 | 32 KiB | 64 KiB | oooooooi0 |
oooooooi1
|
$18-$1B | Fixed $8000 | 64 KiB | oooooooo0 |
oooooooii
|
$1C-$1F | Fixed $C000 | 64 KiB | oooooooii |
oooooooo1
|
$20-$27 | 32 KiB | 128 KiB | ooooooii0 |
ooooooii1
|
$28-$2B | Fixed $8000 | 128 KiB | oooooooo0 |
ooooooiii
|
$2C-$2F | Fixed $C000 | 128 KiB | ooooooiii |
oooooooo1
|
$30-$37 | 32 KiB | 256 KiB | oooooiii0 |
oooooiii1
|
$38-$3B | Fixed $8000 | 256 KiB | oooooooo0 |
oooooiiii
|
$3C-$3F | Fixed $C000 | 256 KiB | oooooiiii |
oooooooo1
|
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.
7654 3210 BBBB Bbbb ++++-++++- Set outer PRG ROM bank
When the outer bank size is set greater than 32K, the least significant bits are ignored.
Bits 7 through 3 always control PRG ROM A22 through A18. Bits 2-0 control A17-A15 only when the outer bank size is small enough to require them.
Many implementations recognize only the lower 4 or 6 bits for two reasons: memory cost and the practical limit of 2 MiB PRG ROM in an iNES ROM image. The PowerPak uses only the low 4 bits, as it has only 512 KiB of RAM for PRG ROM. And by the 2010s, 5-volt 8-bit parallel flash memories larger than 2Mx8 had become hard to find. An implementation supporting NES 2.0 large ROMs should recognize all bits for a maximum of 8 MiB.
One document about this mapper describes a register at $4444 with unknown purpose. The released hardware does not respond to this address.[1]
At power on, the last 16 KiB of the ROM is mapped into $C000-$FFFF. The rest of the state is unspecified. The mapper state is unchanged on reset.
Non-normative: Once a program boots, it may set reg $81 = $FF and reg $80 = $02 to get into oversize-BNROM mode in the last bank.
Non-normative: If desired, games in a multicart can be patched with an appropriate reset stub to allow returning to the menu. The Action 53 build tool does this semi-automatically for NROM games.
See Action 53 mapper/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.
Supported in the following emulators:
Two implementations in Verilog are designed for use on a CPLD. One has been tested on a PowerPak as MAP1C.MAP.
The MiSTer FPGA computer includes an implementation in SystemVerilog.
Input pins: 2 power, 16 signal
Output pins: 12 signal
A CPLD requires one macrocell per bit of state and one for each output pin controlled by a multiplexer, plus possibly a couple more for more complex operations. Depending on maximum PRG ROM size (512 KiB to 8 MiB), this mapper requires 18 to 22 bits of state and 7 multiplexed outputs, which fits comfortably in a 32- or 36-cell CPLD.
After synthesizing a 2 MiB design 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 1 MB by shaving off PRG ROM A20 would save 1 Macrocell and 1 pin, if desired.
Categories: Mappers using $4020-$5FFF, Mappers with single-screen mirroring, CPLD mappers, Multicart mappers