module RAM4GS(PHI2, MAin, CROW, Din, Dout,
			  nCCAS, nCRAS, nFWE,
			  RBA, RA, RD, nRCS, RCLK, RCKE,
			  nRWE, nRRAS, nRCAS, RDQMH, RDQML,
			  nUFMCS, UFMCLK, UFMSDI, UFMSDO);

	/* 65816 Phase 2 Clock */
	input PHI2;

	/* Async. DRAM Control Inputs */
	input nCCAS, nCRAS;

	/* Synchronized PHI2 and DRAM signals */
	reg PHI2r, PHI2r2, PHI2r3;
	reg RASr, RASr2, RASr3;
	reg CASr, CASr2, CASr3;
	reg FWEr;
	reg CBR;

	/* 65816 Data */
	input [7:0] Din;
	output [7:0] Dout = RD[7:0];

	/* Latched 65816 Bank Address */
	reg [7:0] Bank;

	/* Async. DRAM Address Bus */
	input [1:0] CROW;
	input [9:0] MAin;
	input nFWE;
	reg n8MEGEN = 0;
	reg XOR8MEG = 0;
	
	/* SDRAM Clock */
	input RCLK;
	
	/* SDRAM */
	reg RCKEEN;
	output reg RCKE = 0;
	output reg nRCS = 1, nRRAS = 1, nRCAS = 1, nRWE = 1;
	output reg [1:0] RBA;
	reg nRowColSel;
	reg RA11;
	reg RA10;
	reg [9:0] RowA;
	output [11:0] RA;
	assign RA[11] = RA11;
	assign RA[10] = RA10;
	assign RA[9:0] = ~nRowColSel ? RowA[9:0] : MAin[9:0];
	output RDQML = ~nRowColSel ? 1'b1 : ~MAin[9];
	output RDQMH = ~nRowColSel ? 1'b1 : MAin[9];
	reg [7:0] WRD;
	inout [7:0] RD = (~nCCAS & ~nFWE) ? WRD[7:0] : 8'bZ;

	/* UFM Interface */
	output reg nUFMCS = 1;
	output reg UFMCLK = 0;
	output reg UFMSDI = 0;
	input UFMSDO;

	/* UFM Command Interface */
	reg C1Submitted = 0;
	reg ADSubmitted = 0;
	reg CmdEnable = 0;
	reg CmdSubmitted = 0;
	reg Cmdn8MEGEN = 0;
	reg CmdUFMCLK = 0;
	reg CmdUFMSDI = 0;
	reg CmdUFMCS = 0;
	wire ADWR = Bank[7:0]==8'hFB & MAin[7:0]==8'hFF & ~nFWE;
	wire C1WR = Bank[7:0]==8'hFB & MAin[7:0]==8'hFE & ~nFWE;
	wire CMDWR = Bank[7:0]==8'hFB & MAin[7:0]==8'hFD & ~nFWE;
	
	/* State Counters */
	reg InitReady = 0; // 1 if ready for init sequence
	reg Ready = 0; // 1 if done with init sequence
	reg [1:0] S = 0; // post-RAS State counter
	reg [17:0] FS = 0; // Fast init state counter
	reg [3:0] IS = 0; // Init state counter
	reg WriteDone;
	
	/* Synchronize PHI2, RAS, CAS */
	always @(posedge RCLK) begin
		PHI2r <= PHI2; PHI2r2 <= PHI2r; PHI2r3 <= PHI2r2;
		RASr <= ~nCRAS; RASr2 <= RASr; RASr3 <= RASr2;
		CASr <= ~nCCAS; CASr2 <= CASr; CASr3 <= CASr2;
	end
	
	/* Latch 65816 bank when PHI2 rises */
	always @(posedge PHI2) begin
		if (Ready) RA11 <= (Din[6] & ~n8MEGEN) ^ XOR8MEG; // Set RA11
		else RA11 <= 1'b0; // Reserved in mode register
		Bank[7:0] <= Din[7:0]; // Latch bank
	end
	
	/* Latch bank address, row address, WE, and CAS when RAS falls */
	always @(negedge nCRAS) begin
		if (Ready) begin
			RBA[1:0] <= CROW[1:0];
			RowA[9:0] <= MAin[9:0];
		end else begin
			RBA[1:0] <= 2'b00;		// Reserved in mode register
			RowA[9] <= 1'b1;		// "1" for single write mode
			RowA[8] <= 1'b0;		// Reserved
			RowA[7] <= 1'b0;		// "0" for not test mode
			RowA[6:4] <= 3'b010;	// "2" for CAS latency 2
			RowA[3] <= 1'b0;		// "0" for sequential burst (not used)
			RowA[2:0] <= 3'b000;	// "0" for burst length 1 (no burst)
		end
		FWEr <= ~nFWE;
		CBR <= ~nCCAS;
	end

	/* Latch write data when CAS falls */
	always @(negedge nCCAS) begin
		WRD[7:0] <= Din[7:0];
	end

	/* State counter from RAS */
	always @(posedge RCLK) begin
		if (~RASr2) S <= 0;
		else if (S==2'h3) S <= 2'h3;
		else S <= S+1;
	end
	/* Init state counter */
	always @(posedge RCLK) begin
		// Wait ~4.178ms (at 62.5 MHz) before starting init sequence
		FS <= FS+1;
		if (FS[17:10] == 8'hFF) InitReady <= 1'b1;
	end

	/* SDRAM CKE */
	always @(posedge RCLK) begin
		// Only 1 LUT4 allowed for this function!
		RCKE <= ((RASr | RASr2) & RCKEEN) | (~RASr2 & RASr3);
	end

	/* SDRAM command */
	always @(posedge RCLK) begin
		if (Ready) begin
			if (S==0) begin
				if (RASr2) begin
					if (CBR) begin
						// AREF
						nRCS <= 1'b0;
						nRRAS <= 1'b0;
						nRCAS <= 1'b0;
						nRWE <= 1'b1;
						RA10 <= 1'b1; // RA10 is don't care
					end else begin
						// ACT
						nRCS <= 1'b0;
						nRRAS <= 1'b0;
						nRCAS <= 1'b1;
						nRWE <= 1'b1;
						RA10 <= 1'b1; // Bank RA10 consistently "1"
					end
					// Enable clock only for reads
					RCKEEN <= ~CBR & ~FWEr;
				end else if (RCKE) begin
					// PCall
					nRCS <= 1'b0;
					nRRAS <= 1'b0;
					nRCAS <= 1'b1;
					nRWE <= 1'b0;
					RA10 <= 1'b1; // "all"
					RCKEEN <= 1'b1;
				end else begin
					// NOP
					nRCS <= 1'b1;
					nRRAS <= 1'b1;
					nRCAS <= 1'b1;
					nRWE <= 1'b1;
					RA10 <= 1'b1; // RA10 is don't care
					RCKEEN <= 1'b1;
				end
				nRowColSel <= 1'b0; // Select registered row addres
			end else if (S==1) begin
				// NOP
				nRCS <= 1'b1;
				nRRAS <= 1'b1;
				nRCAS <= 1'b1;
				nRWE <= 1'b1;
				RA10 <= 1'b1; // RA10 is don't care
				nRowColSel <= 1'b1; // Select asynchronous column address
				RCKEEN <= ~CBR; // Disable clock if refresh cycle
			end else if (S==2) begin
				if (~FWEr & ~CBR) begin
					// RD
					nRCS <= 1'b0;
					nRRAS <= 1'b1;
					nRCAS <= 1'b0;
					nRWE <= 1'b1;
					RA10 <= 1'b1; // Auto-precharge
				end else begin
					// NOP
					nRCS <= 1'b1;
					nRRAS <= 1'b1;
					nRCAS <= 1'b1;
					nRWE <= 1'b1;
					RA10 <= 1'b1; // RA10 is don't care
				end
				nRowColSel <= 1'b1; // Select asynchronous column address
				RCKEEN <= ~CBR & FWEr; // Enable clock only for writes
			end else if (S==3) begin
				if (CASr2 & ~CASr3 & ~CBR & FWEr) begin
					// WR
					nRCS <= 1'b0;
					nRRAS <= 1'b1;
					nRCAS <= 1'b0;
					nRWE <= 1'b0;
					RA10 <= 1'b1; // Auto-precharge
				end else begin
					// NOP
					nRCS <= 1'b1;
					nRRAS <= 1'b1;
					nRCAS <= 1'b1;
					nRWE <= 1'b1;
					RA10 <= 1'b1; // RA10 is don't care
				end
				nRowColSel <= ~(~FWEr | CASr3 | CBR);
				RCKEEN <= ~(~FWEr | CASr2 | CBR);
			end
		end else if (InitReady) begin
			if (S==0 & RASr2) begin
				if (IS==0) begin
					// NOP
					nRCS <= 1'b1;
					nRRAS <= 1'b1;
					nRCAS <= 1'b1;
					nRWE <= 1'b1;
					RA10 <= 1'b1; // RA10 is don't care
				end else if (IS==1) begin
					// PC all
					nRCS <= 1'b0;
					nRRAS <= 1'b0;
					nRCAS <= 1'b1;
					nRWE <= 1'b0;
					RA10 <= 1'b1; // "all"
				end else if (IS==9) begin
					// Load mode register
					nRCS <= 1'b0;
					nRRAS <= 1'b0;
					nRCAS <= 1'b0;
					nRWE <= 1'b0;
					RA10 <= 1'b0; // Reserved in mode register
				end else begin
					// AREF
					nRCS <= 1'b0;
					nRRAS <= 1'b0;
					nRCAS <= 1'b0;
					nRWE <= 1'b1;
					RA10 <= 1'b1; // RA10 is don't care
				end
				IS <= IS+1;
			end else begin
				// NOP
				nRCS <= 1'b1;
				nRRAS <= 1'b1;
				nRCAS <= 1'b1;
				nRWE <= 1'b1;
				RA10 <= 1'b1; // RA10 is don't care
			end
			if (S==3 & ~RASr2 & IS==15) Ready <= 1'b1;
			nRowColSel <= 1'b0; // Select registered row address
			RCKEEN <= 1'b1;
		end else begin
			// NOP
			nRCS <= 1'b1;
			nRRAS <= 1'b1;
			nRCAS <= 1'b1;
			nRWE <= 1'b1;
			RA10 <= 1'b1; // RA10 is don't care
			nRowColSel <= 1'b0; // Select registered row address
			RCKEEN <= 1'b0;
		end
	end
	
	/* Submit command when PHI2 falls */
	always @(negedge PHI2) begin
		// Magic number check
		if (C1WR & Din[7:0]==8'hC1) begin // "C1" magic number
			if (ADSubmitted) begin
				CmdEnable <= 1'b1;
			end
			C1Submitted <= 1'b1;
			ADSubmitted <= 1'b0;
		end else if (ADWR & Din[7:0]==8'hAD) begin // "AD" magic number
			if (C1Submitted) begin
				CmdEnable <= 1'b1;
			end
			ADSubmitted <= 1'b1;
			C1Submitted <= 1'b0;
		end else if (C1WR | ADWR) begin // wrong magic number submitted
			CmdEnable <= 1'b0;
			C1Submitted <= 1'b0;
			ADSubmitted <= 1'b0;
		end else if (CMDWR) CmdEnable <= 1'b0;

		// Submit command
		if (CMDWR & CmdEnable) begin
			if (Din[7:4]==4'h0) begin
				XOR8MEG <= Din[0];
			end else if (Din[7:4]==4'h1) begin
				Cmdn8MEGEN <= ~Din[0];
				CmdSubmitted <= 1'b1;
			end else if (Din[7:4]==4'h3) begin
				Cmdn8MEGEN <= n8MEGEN;
				CmdUFMCS <= Din[2];
				CmdUFMCLK <= Din[1];
				CmdUFMSDI <= Din[0];
				CmdSubmitted <= 1'b1;
			end
		end
	end

	/* UFM Control */
	always @(posedge RCLK) begin
		if (~InitReady && FS[17:10]==8'h00) begin
			nUFMCS <= 1'b1;
			UFMCLK <= 1'b0;
			UFMSDI <= 1'b0;
		end else if (~InitReady && FS[17:10]==8'h01) begin
			nUFMCS <= 1'b0;
			UFMCLK <= 1'b0;
			UFMSDI <= 1'b0;
		end else if (~InitReady && FS[17:10]==8'h02) begin
			nUFMCS <= 1'b0;
			UFMCLK <= FS[4];
			case (FS[9:5]) // Shift out read data command (0x03)
				5'h00: UFMSDI <= 1'b0; // command bit 7 (0)
				5'h01: UFMSDI <= 1'b0; // command bit 6 (0)
				5'h02: UFMSDI <= 1'b0; // command bit 5 (0)
				5'h03: UFMSDI <= 1'b0; // command bit 4 (0)
				5'h04: UFMSDI <= 1'b0; // command bit 3 (0)
				5'h05: UFMSDI <= 1'b0; // command bit 2 (0)
				5'h06: UFMSDI <= 1'b1; // command bit 1 (1)
				5'h07: UFMSDI <= 1'b1; // command bit 0 (1)
				5'h08: UFMSDI <= 1'b0; // address bit 23 (0)
				5'h09: UFMSDI <= 1'b0; // address bit 22 (0)
				5'h0A: UFMSDI <= 1'b0; // address bit 21 (0)
				5'h0B: UFMSDI <= 1'b0; // address bit 20 (0)
				5'h0C: UFMSDI <= 1'b0; // address bit 19 (0)
				5'h0D: UFMSDI <= 1'b0; // address bit 18 (0)
				5'h0E: UFMSDI <= 1'b0; // address bit 17 (0)
				5'h0F: UFMSDI <= 1'b0; // address bit 16 (0)
				5'h10: UFMSDI <= 1'b0; // address bit 15 (0)
				5'h11: UFMSDI <= 1'b0; // address bit 14 (0)
				5'h12: UFMSDI <= 1'b0; // address bit 13 (0)
				5'h13: UFMSDI <= 1'b1; // address bit 12 (0)
				5'h14: UFMSDI <= 1'b0; // address bit 11 (0)
				5'h15: UFMSDI <= 1'b0; // address bit 10 (0)
				5'h16: UFMSDI <= 1'b0; // address bit 09 (0)
				5'h17: UFMSDI <= 1'b0; // address bit 08 (0)
				5'h18: UFMSDI <= 1'b0; // address bit 07 (0)
				5'h19: UFMSDI <= 1'b0; // address bit 06 (0)
				5'h1A: UFMSDI <= 1'b0; // address bit 05 (0)
				5'h1B: UFMSDI <= 1'b0; // address bit 04 (0)
				5'h1C: UFMSDI <= 1'b0; // address bit 03 (0)
				5'h1D: UFMSDI <= 1'b0; // address bit 02 (0)
				5'h1E: UFMSDI <= 1'b0; // address bit 01 (0)
				5'h1F: UFMSDI <= 1'b0; // address bit 00 (0)
			endcase
		end else if (~InitReady && FS[17:10]==8'h03) begin
			nUFMCS <= 1'b0;
			UFMCLK <= 1'b0;
			UFMSDI <= 1'b0;
			// Latch n8MEGEN
			if (FS[9:4]==6'h00 && FS[3:0]==4'hF) n8MEGEN <= ~UFMSDO;
		end else if (~InitReady && FS[17:10]!=8'hFE && FS[17:10]!=8'hFF) begin
			nUFMCS <= 1'b0;
			UFMCLK <= FS[1];
			UFMSDI <= 1'b0;
		end else if (~InitReady) begin
			nUFMCS <= 1'b1;
			UFMCLK <= 1'b0;
			UFMSDI <= 1'b0;
		end else if (~PHI2r2 & PHI2r3 & CmdSubmitted) begin
			// Set user command signals after PHI2 falls
			// Cmdn8MEGEN, CmdUFMCS, CmdUFMCLK, CmdUFMSDI
			n8MEGEN <= Cmdn8MEGEN;
			nUFMCS <= ~CmdUFMCS;
			UFMCLK <= CmdUFMCLK;
			UFMSDI <= CmdUFMSDI;
		end
	end
endmodule