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`timescale 1ns / 1ps
/*
 * File         : FIFO_NoFull_Count.v
 * Project      : University of Utah, XUM Project MIPS32 core
 * Creator(s)   : Grant Ayers (ayers@cs.utah.edu)
 *
 * Modification History:
 *   Rev   Date         Initials  Description of Change
 *   1.0   24-May-2010  GEA       Initial design.
 *
 * Standards/Formatting:
 *   Verilog 2001, 4 soft tab, wide column.
 *
 * Description:
 *   A synchronous FIFO of variable data width and depth. 'enQ' is ignored when
 *   the FIFO is full and 'deQ' is ignored when the FIFO is empty. If 'enQ' and
 *   'deQ' are asserted simultaneously, the FIFO is unchanged and the output data
 *   is the same as the input data.
 *
 *   This FIFO is "First word fall-through" meaning data can be read without
 *   asserting 'deQ' by merely supplying an address. However, when 'deQ' is
 *   asserted, the data is "removed" from the FIFO and one location is freed.
 *   If the FIFO is empty and 'enQ' and 'deQ' are not asserted simultaneously,
 *   the output data will be 0s.
 *
 * Variation:
 *   - There is no output to indicate the FIFO is full.
 *   - Output 'count' indicates how many elements are in the FIFO, from 0 to 256
 *     (for 8-bit ADDR_WIDTH).
 */
module FIFO_NoFull_Count(clock, reset, enQ, deQ, data_in, data_out, empty, count);
    parameter DATA_WIDTH = 8;
    parameter ADDR_WIDTH = 8;
    parameter RAM_DEPTH = 1 << ADDR_WIDTH;   
    input clock;
    input reset;
    input enQ;
    input deQ;
    input [(DATA_WIDTH-1):0] data_in;
    output [(DATA_WIDTH-1):0] data_out;
    output empty;
    output reg [(ADDR_WIDTH):0] count;       // How many elements are in the FIFO (0->256)
 
    reg [(ADDR_WIDTH-1):0] enQ_ptr, deQ_ptr; // Addresses for reading from and writing to internal memory
 
    assign empty = (count == 0);
    wire full = (count == (1 << ADDR_WIDTH));
 
    wire [(DATA_WIDTH-1):0] w_data_out;
    assign data_out = (empty) ? ((enQ & deQ) ? data_in : 0) : w_data_out;
 
    wire w_enQ = (full) ? 0 : enQ;   // Mask 'enQ' when the FIFO is full
    wire w_deQ = (empty) ? 0 : deQ;  // Mask 'deQ' when the FIFO is empty
 
    always @(posedge clock) begin
        if (reset) begin
            enQ_ptr <= 0;
            deQ_ptr <= 0;
            count <= 0;
        end
        else begin
            enQ_ptr <= (w_enQ) ? enQ_ptr +1 : enQ_ptr;
            deQ_ptr <= (w_deQ) ? deQ_ptr +1 : deQ_ptr;
            count <= (w_enQ ~^ w_deQ) ? count : ((w_enQ) ? count +1 : count -1);
        end
    end
 
    SRAM #(
        .DATA_WIDTH (DATA_WIDTH),
        .ADDR_WIDTH (ADDR_WIDTH),
        .RAM_DEPTH  (RAM_DEPTH))
        ram(
        .clock   (clock), 
        .wEn     (w_enQ), 
        .rAddr   (deQ_ptr), 
        .wAddr   (enQ_ptr), 
        .dIn     (data_in), 
        .dOut    (w_data_out)
    );
 
endmodule