/*
 Gameboy MBC5/MBC1 Hybrid - 2MB ROM, 128KB SRAM
 Firmware: R1.1
 Author: Alex from insideGadgets (www.insidegadgets.com)
 Created: 7/11/2018
 Last Modified: 8/09/2018
 
 Checksum of pof file: D95E3
 
 Note: Having some MBC1 support may break a few MBC5 games.
 Our MBC5 implementation differs from the standard, switching to bank 0 gives bank 1 like in MBC1/3.
 MBC1 support now allows for ROM banking mode to 2MB but for RAM banking mode it's locked to 8KB RAM.

*/

module mbc5_2mb_v1 (
//clockin, // For ModelSim 
reset, 
inputAddress, inputData, inputCE, inputWR, inputRD,
highAddress, ramCE);

// Inputs
input reset;
input [3:0] inputAddress; // a15 - a12
input [6:0] inputData; // d6 - d0
input inputCE;
input inputRD;
input inputWR;

// Outputs / Registers
output [7:0] highAddress; // a20 - a13
reg [7:0] highAddress;

reg [6:0] romBank; // a20 - a14 
reg [3:0] ramBank; // a16 - a13 
reg ramEnabled;

output ramCE;
reg ramCE;

reg mbc1Detect303FOn;
reg mbc1Detected607F;
reg mbc3or5Locked;
reg mbc1RomRamSelect;

// For ModelSim
/*input clockin;
reg clockin;
reg reset;

reg [3:0] inputAddress; // a12 - a15
reg [6:0] inputData; // d0 - d7
reg inputCE;
reg inputRD;
reg inputWR;
reg audioIn;
*/

// ROM a0-a13 straight through, a14-a21 from CPLD highAddress [1:8]
// RAM a0-a12 straight through, a13-a16 from CPLD highAddress [0:3]
// input RD, WR, CS straight through to ROM
// input RD, WR, straight through to RAM, CE handled by CPLD
// romCE to A15

always @ (reset or inputCE or inputRD or inputWR) begin
	if (!reset) begin
		highAddress <= 8'b0;
		romBank <= 7'b1;
		ramBank <= 4'b0;
		ramEnabled <= 1'b0;
		ramCE <= 1'b1;
		mbc1Detect303FOn <= 1'b0;
		mbc1Detected607F <= 1'b0;
		mbc3or5Locked <= 1'b0;
		mbc1RomRamSelect <= 1'b0;
	end
	else begin
		// *** ROM Functions ***
		// Only pass through on 0x0000-7FFF if reading or writing flash
		if (inputAddress <= 4'd7 && (!inputRD || !inputWR)) begin
			if (inputAddress <= 4'd3) begin // 0x0000-3FFF, Bank 0 always
				highAddress <= 8'b0; // All set to 0
			end
			else begin
				highAddress <= (romBank << 1); // Start at a14 for ROM
			end
		end
		
		// 0x2000-3FFF - Low 7 bits of ROM Bank Number (Write Only) with little MBC1 detection hack
		if (((inputAddress == 4'd2) || inputAddress == 4'd3 && mbc1Detect303FOn) && !inputWR && inputRD && inputCE) begin
			if (inputData == 7'd0) begin // Detects MBC1 (most of the time)
				if (mbc1Detected607F && !mbc3or5Locked && !mbc1RomRamSelect) begin // Allow for more than 512KB ROM on MBC1
					romBank <= (romBank & 7'h60) | 7'b1;
				end
				else begin // For all others
					romBank <= 1'b1;
				end
				mbc1Detect303FOn <= 1'b1;
			end
			else begin
				if (inputData >= 7'd32) begin // Lock to MBC3/5 mode if bank switched to >= 0x20
					mbc3or5Locked <= 1'b1;
				end
				
				// MBC1 handling of ROM size
				if (mbc1Detected607F && !mbc3or5Locked) begin
					if (mbc1RomRamSelect) begin // 32KB RAM mode (not supported)
						romBank <= inputData[4:0];
					end
					else begin // Up to 2MB ROM Mode
						romBank <= (romBank & 7'h60) | inputData[4:0];
					end
				end
				else begin // MBC 2/3/5
					romBank <= inputData;
				end
			end
		end
		
		
		
		// *** RAM Functions ***
	   ramCE <= 1'b1;
		
		// Only pass through on 0xA000-BFFF if RAM is enabled
		if ((inputAddress == 4'hA || inputAddress == 4'hB) && ramEnabled && (!inputRD || !inputWR)) begin
			ramCE <= inputCE;
			highAddress <= ramBank;
		end
	
		// 0x0000-1FFF - RAM Enable (Write Only). 0x0A = Enable, 0x00 Disable
		if ((inputAddress == 4'd1 || inputAddress == 4'd0) && !inputWR && inputRD && inputCE) begin
			if (inputData == 7'hA) begin // Enable RAM
				ramEnabled <= 1'd1;
			end
			else begin // Disable RAM
				ramEnabled <= 1'd0;
			end
		end
  
		// 0x4000-5FFF - RAM Bank Number (Write Only)
		if ((inputAddress == 4'd4 || inputAddress == 4'd5) && !inputWR && inputRD && inputCE) begin
			if (mbc1Detected607F && !mbc3or5Locked) begin // MBC1 ROM size handling, ignores RAM banking request
				if (!mbc1RomRamSelect) begin
					romBank <= (romBank & 7'h1F) | (inputData[1:0] << 5); //inputData[1:0] << 5;
				end
			end
			else begin // Allow access to all RAM banks for regular MBC 3/5 carts
				ramBank <= inputData[3:0];
			end
		end
		
		// 0x6000-7FFF - MBC1 ROM/RAM Mode Detection
		if ((inputAddress == 4'd6 || inputAddress == 4'd7) && !inputWR && inputRD && inputCE) begin
			mbc1Detect303FOn <= 1'b1;
			mbc1Detected607F <= 1'b1;
			mbc1RomRamSelect <= inputData[0:0];
		end
	end
end


// For ModelSim
/*initial
begin
   reset = 1'b0;
   clockin = 1'b0;
   inputAddress = 1'b0;
   inputData = 1'b0;
   inputCE = 1'b1;
   inputRD = 1'b1;
   inputWR = 1'b1;
   audioIn = 1'b1;
   forever #5 clockin = ~clockin;
end*/

endmodule