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

Verilog Code

//--------------------------------------------------------------------------------
//	Tepples' Multi-discrete mapper
//	Designed for use with a multicart that supports many discrete mappers
//	such as NROM, AOROM, BNROM, CNROM, UxROM, UxROM variant #180
//	written for Xilinx 9500XL series CPLD XC9536XL
//
//	verilog code by: infiniteneslives (paul@InfiniteNesLives.com)
//	Created: 6 Sept 2012
//	Modified: 6 Sept 2012 
//
//	Description:  Designed to support 32KB of CHR-RAM, and 2MB PRG-ROM
//	see nesdev wiki for details:
//	http://wiki.nesdev.org/w/index.php/User:Tepples/Multi-discrete_mapper
//
//      NOTE:  BEWARE, This implementation is UNTESETED!
//--------------------------------------------------------------------------------


module mmc3 (
	input 			m2,		//clock input negedge triggered
	input			prg_rw,		//PRG R/W signal from NES
	input			prg_ce,		//PRG /CE signal from NES
	input	[7:0]		data,		//PRG Data bus input
	input	[14:12]		prg_addr,	//PRG A14-13 input
	input	[11:10]		chr_addr,	//CHR A12-10 inputj
	output	reg[20:14] 	pa,		//PRG ROM A18-13 outputs
	output	[14:13] 	ca,		//CHR ROM/RAM A17-10 outputs
	output	reg		p_ce,		//PRG-ROM /CE signal output
	output	reg		ciram_a10	//CIRAM A10 output
//	output	reg		debug,		//debug pin output
);


wire write_reg_sel, write_reg_val;
//active high when writing to $5000->reg_sel,  $8000->reg_val
assign write_reg_sel = (prg_addr[14] & ~prg_addr[13] & prg_addr[12] & m2 & prg_ce & ~prg_rw);
assign write_reg_val = (~prg_ce & prg_rw);

reg [1:0] reg_sel;	//register select S: supervisor=1/user=0 (D7),  R: register select (D0)
//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:	used for CNROM, and one screen mirroring
reg [1:0] chr_bank;	//D1-0 set CHR RAM A14-13
reg M;			//set CIRAM A10 if H/V disabled via mirroring mode below	

//$01 Inner bank: prg bank for A/B/UxROM etc.
reg [3:0] prg_inner_bank;	//D4-0 set PRG ROM A14/15 and up for single game in game use

//$80 Mode: used by multicart supervisor software
reg [1:0] prg_size;	//D5-4	0: 32KB, 1: 64KB, 2:128KB, 3:256KB
reg [1:0] prg_mode;	//D3-2	0,1: current 32KB @ $8000, 2: UNROM standard, 3: UNROM #180
reg [1:0] mirror_mode;	//D1-0	0: 1scn lower, 1: 1scn upper, 2: vert, 3: horiz

//$81 Outer bank: sets the upper PRG ROM bits to select current game
reg [5:0] prg_outer_bank;	//sets PRG ROM A15-20

//REGISTER SELECT:  chooses which of the above registers is written to
always @ (negedge write_reg_sel)
begin
	reg_sel[1] <= data[7];
	reg_sel[0] <= data[0];
end

//REGISTER VALUE WRITING 
always @ (negedge write_reg_val)
begin
	if (reg_sel[1] == 0)
		M <= data[4];	//this is written to for both $0x registers

	case (reg_sel)
		0:  chr_bank  <= data[1:0];
		1:  prg_inner_bank <= data[3:0];
	       	2: {prg_size, prg_mode, mirror_mode} <= data[5:0];  	
		3:  prg_outer_bank <= data[5:0];
	endcase
end


//chr bankswitching
assign ca = chr_bank;

//allow for simple handling of both UNROM versions
wire pa14_eff;
assign pa14_eff = prg_addr[14] ^ prg_mode[0];  	//only used in modes 2 and 3 so don't care about 1 & 0
//now using pa14_eff allows us to ignore the mapper #180 special case

//OUTPUTS combinational logic
always @ (pa14_eff, prg_ce, prg_rw, M, prg_inner_bank, prg_size, prg_mode, mirror_mode, prg_outer_bank, chr_addr, prg_addr)
begin
	
	//prevent bus conflicts for writes to $8000-FFFF
	if (prg_ce == 0 & prg_rw == 1)
		p_ce = 0;	//enabled
	else
		p_ce = 1;	//disabled

	//mirroring
	case (mirror_mode)
		0: ciram_a10 = M;
		1: ciram_a10 = M;
		2: ciram_a10 = chr_addr[10]; //vert
		3: ciram_a10 = chr_addr[11]; //horiz
	endcase

	pa[20:18] = prg_outer_bank[5:4];  //always controlled directly (independent of prg_mode)


	//prg bank selecting must control PRG ROM A17-14
	if (prg_mode[1] == 0)	//modes 0 and 1 NROM, A/BNROM 32KB banks
	begin
		pa[17:15] = prg_outer_bank[3:0];	//select bank
		pa[14] = prg_addr[14];			//32KB banks
	end

	else // modes 2 & 3 standard UNROM 16KB banks last fixed
	begin
		//64KB and useless 32KB
		if (prg_size[1] == 0) 
		begin
			pa[17:16] = prg_outer_bank[3:2];	//set to 64KB

			if (pa14_eff == 0)	//$8000-BFFF
				pa[15:14] = prg_inner_bank[1:0];
			else
				pa[15:14] = 2'b11;	//last bank fixed
		end

		//128KB
		else if (prg_size == 2) 
		begin
			pa[17] = prg_outer_bank[3];	//set to 128KB

			if (pa14_eff == 0)	//$8000-BFFF
				pa[16:14] = prg_inner_bank[2:0];
			else
				pa[16:14] = 3'b111;	//last bank fixed
		end

		//256KB
		else 
		begin
			if (pa14_eff == 0)	//$8000-BFFF
				pa[17:14] = prg_inner_bank[3:0];
			else
				pa[17:14] = 4'b1111;	//last bank fixed
		end
	end

end	
endmodule

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)

After synthesizing and laying fitting within a XC9536XL CPLD 28/36 Macrocells were consumed (78%). Additionally this design requires 25/34 available pins on the XC9536XL.