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//

// ram.v -- simulate external RAM

//          8M x 32 bit = 32 MB

//

`timescale 1ns/10ps

`default_nettype none

//

// use this set of parameters for minimal access times

//

//`define RD_CYCLES     4'd2    // # cycles for read, min = 2

//`define WR_CYCLES     4'd2    // # cycles for write, min = 2

//

// use this set of parameters for realistic access times

//

`define RD_CYCLES       4'd6    // # cycles for read, min = 2

`define WR_CYCLES       4'd4    // # cycles for write, min = 2

module ram(clk, rst,

           stb, we, addr,

           data_in, data_out, ack);

    input clk;

    input rst;

    input stb;

    input we;

    input [24:2] addr;

    input [31:0] data_in;

    output reg [31:0] data_out;

    output ack;

  reg [31:0] mem[0:8388607];

  reg [3:0] counter;

  always @(posedge clk) begin

    if (stb) begin

      if (we) begin

        // write cycle

        mem[addr] <= data_in;

      end else begin

        // read cycle

        data_out <= mem[addr];

      end

    end

  end

  always @(posedge clk) begin

    if (rst) begin

      counter[3:0] <= 4'h0;

    end else begin

      if (counter[3:0] == 4'h0) begin

        if (stb & ~we) begin

          // a read may need some clock cycles

          counter[3:0] <= `RD_CYCLES - 1;

        end

        if (stb & we) begin

          // a write may need some clock cycles

          counter[3:0] <= `WR_CYCLES - 1;

        end

      end else begin

        counter[3:0] <= counter[3:0] - 1;

      end

    end

  end

  assign ack = (counter[3:0] == 4'h1) ? 1 : 0;

endmodule