hp-saturn/hp48_01_io_ram.v

`include "bus_commands.v"

`ifndef _HP48_IO_RAM
`define _HP48_IO_RAM

/**************************************************************************************************
 *
 * I/O ram 
 * length: 64 nibbles
 *
 *
 */


module hp48_io_ram (
	input				strobe,
	input				reset,
	input		[19:0]	address,
	input		[3:0]	command,
	input		[3:0]	nibble_in,
	output	reg [3:0]	nibble_out,
	output				active,
	input				daisy_in,
	output  			daisy_out,
	output	reg			error
);

localparam IO_RAM_LEN		= 64;

// localparam BUSCMD_DP_WRITE	= C_BUSCMD_DP_WRITE;
// localparam BUSCMD_CONFIGURE = C_BUSCMD_CONFIGURE;


reg	[0:0]	configured;
reg [19:0]	base_addr;
reg [19:0]	pc_ptr;
reg [19:0]	dp_ptr;
reg [3:0]	mmio_ram [0:IO_RAM_LEN-1];

assign daisy_out = configured;

/*
 *
 *
 */

initial
	begin
`ifdef SIM
		$display("io_ram: set unconfigured");
`endif
		configured = 0;
`ifdef SIM
		$display("io_ram: reset error flag");
`endif
		error = 1'b0;	
`ifdef SIM
		$display("io_ram: setting data pointer to 0");
`endif
		dp_ptr = 0;
`ifdef SIM
		// initialization only for simulation
		$display("io_ram: initializing to 0");
		for (base_addr = 0; base_addr < IO_RAM_LEN; base_addr++)
			begin
				$write(".");
				mmio_ram[base_addr] = 0; 
			end
		$display("io_ram: setting base address to 0");
`endif
		base_addr = 0;

`ifdef SIM	
		$write("\n");
		$display("io_ram: initialized");
`endif

		// $monitor("MMIO MON | PC %h | DP %h | P %h | A %h | B %h | L %h | CNF %b | RD %b | WR %b | ACT %b",
		// 	pc_ptr, dp_ptr, ptr_value, access_addr,	base_addr, last_addr,
		// 	configured, can_read, can_write, active);
	end

/*
 *
 *
 */
wire            cmd_bus_pc;
wire            cmd_bus_dp;
wire            cmd_read;
wire            cmd_write;

assign cmd_bus_pc = (command == `BUSCMD_PC_READ) | (command == `BUSCMD_PC_WRITE);
assign cmd_bus_dp = (command == `BUSCMD_DP_READ) | (command == `BUSCMD_DP_WRITE);
assign cmd_read = (command == `BUSCMD_PC_READ) | (command == `BUSCMD_DP_READ);
assign cmd_write = (command == `BUSCMD_PC_WRITE) | (command == `BUSCMD_DP_WRITE);

wire [19:0]		last_addr;

assign last_addr = base_addr + IO_RAM_LEN - 1;

wire 			pc_lower;
wire			pc_higher;
wire			use_pc;
wire			dp_lower;
wire			dp_higher;
wire			use_dp;

assign pc_lower  = pc_ptr < base_addr;
assign pc_higher = pc_ptr > last_addr;
assign use_pc = !(pc_lower | pc_higher);
assign dp_lower  = dp_ptr < base_addr;
assign dp_higher = dp_ptr > last_addr;
assign use_dp = !(dp_lower | dp_higher);


wire [19:0] ptr_value;
wire [19:0] access_addr;
assign ptr_value = (cmd_bus_dp?dp_ptr:pc_ptr);
assign access_addr = ptr_value - base_addr;

wire			read_pc;
wire			write_pc;
wire			read_dp;
wire			write_dp;

assign read_pc  = use_pc & cmd_bus_pc & cmd_read;
assign write_pc = use_pc & cmd_bus_pc & cmd_write;
assign read_dp  = use_dp & cmd_bus_dp & cmd_read;
assign write_dp = use_dp & cmd_bus_dp & cmd_write;

wire			can_read;
wire			can_write;

assign can_read  = configured & (read_pc | read_dp);
assign can_write = configured & (write_pc | write_dp); 

assign active = can_read | can_write;

always @(posedge strobe) begin
    // read from ram
    if (can_read) begin
        nibble_out = mmio_ram[access_addr];
	end
end    

always @(posedge strobe) begin
    // write to ram
    if (can_write) begin
        mmio_ram[access_addr] <= nibble_in;
	end
end

always @(posedge strobe) begin
	case (command)
	`BUSCMD_PC_READ: begin
		pc_ptr <= pc_ptr + 1;
		$display("MMIO (%b - %5h) ACT %b - %s_PC %5h (%5h) -> %h", 
			configured, base_addr, active,
			cmd_read?"READ":"WRITE", ptr_value, access_addr,
			cmd_read?nibble_out:nibble_in);
	end
	`BUSCMD_DP_READ, `BUSCMD_DP_WRITE: begin
		dp_ptr <= dp_ptr + 1;
		$display("MMIO (%b - %5h) ACT %b - %s_DP %5h (%5h) -> %h", 
			configured, base_addr, active,
			cmd_read?"READ":"WRITE", ptr_value, access_addr,
			cmd_read?nibble_out:nibble_in);
	end
	`BUSCMD_LOAD_PC: begin
		// $display("MMIO (%b - %5h) - LOAD_PC %5h", configured, base_addr, address);
		pc_ptr <= address;
	end
	`BUSCMD_LOAD_DP: begin
		dp_ptr <= address;
		// $display("MMIO (%b - %5h) - LOAD_DP %5h", configured, base_addr, address);
	end
	`BUSCMD_CONFIGURE: begin
		if (!configured) begin
			if (daisy_in) begin
				base_addr <= address;
				configured <= 1;
				// $display("MMIO (%b - %5h) - CONFIGURE %5h", configured, base_addr, address);
			end else begin
    			$display("MMIO (%b - %5h) - CAN'T CONFIGURE - DAISY_IN NOT SET %5h", configured, base_addr, address);
			end
		end else begin
			// $display("MMIO (%b - %5h) - ALREADY CONFIGURED %5h", configured, base_addr, address);
		end
	end
	`BUSCMD_RESET: begin
		base_addr <= 0;
		configured <= 0;
		$display("MMIO (%b - %5h) - RESET", configured, base_addr);
	end
	default: begin
		$display("MMIO (%b - %5h) - UNIMPLEMENTED COMMAND %d %5h", configured, base_addr, command, address);
		error <= 1;
	end
	endcase      
end

endmodule

`endif