//------------------------------------------------------------------------------
// lifo.sv
// Konstantin Pavlov, pavlovconst@gmail.com
//------------------------------------------------------------------------------

// INFO ------------------------------------------------------------------------
//  Single-clock LIFO buffer implementation, also known as "stack"
//
//  Features:
//  - single clock operation
//  - configurable depth and data width
//  - one write- and one read- port in "FWFT" or "normal" mode
//  - protected against overflow and underflow
//  - simultaneous read and write operations supported if not full and not empty
//  - only read operation is performed when (full && r_req && w_req)
//  - only write operation is performed when (empty && r_req && w_req)
//
//  See also "fifo_Single_clock_reg_*.sv" modules for similar FIFO buffer implementation


/* --- INSTANTIATION TEMPLATE BEGIN ---

lifo #(
  .FWFT_MODE( "TRUE" ),
  .DEPTH( 8 ),
  .DATA_W( 32 )
) LF1 (
  .clk( clk ),
  .nrst( 1'b1 ),

  .w_req(  ),
  .w_data(  ),

  .r_req(  ),
  .r_data(  ),

  .cnt(  ),
  .empty(  ),
  .full(  )
);

--- INSTANTIATION TEMPLATE END ---*/

module lifo #( parameter

  FWFT_MODE = "TRUE",             // "TRUE"  - first word fall-trrough" mode
                                  // "FALSE" - normal fifo mode

  DEPTH = 8,                      // max elements count == DEPTH, DEPTH MUST be power of 2

  DATA_W = 32                     // data field width
)(

  input clk,
  input nrst,                      // inverted reset

  // input port
  input w_req,
  input [DATA_W-1:0] w_data,

  // output port
  input r_req,
  output logic [DATA_W-1:0] r_data,

  // helper ports
  output logic [DEPTH_W-1:0] cnt = '0,
  output logic empty,
  output logic full,

  output logic fail
);

  // elements counter width, extra bit to store
  // "fifo full" state, see cnt[] variable comments
  localparam DEPTH_W = $clog2(DEPTH+1);


  // lifo data
  logic [DEPTH-1:0][DATA_W-1:0] data = '0;

  // data output buffer for normal fifo mode
  logic [DATA_W-1:0] data_buf = '0;

  // cnt[] vector always holds lifo elements count
  // data[cnt[]] points to the first empty lifo slot
  // when lifo is full data[cnt[]] points "outside" of data[]

  // filtered requests
  logic w_req_f;
  assign w_req_f = w_req && ~full;

  logic r_req_f;
  assign r_req_f = r_req && ~empty;


  integer i;
  always_ff @(posedge clk) begin
    if ( ~nrst ) begin
      data <= '0;
      cnt[DEPTH_W-1:0] <= '0;
      data_buf[DATA_W-1:0] <= '0;
    end else begin
      unique case ({w_req_f, r_req_f})
        2'b00: ; // nothing

        2'b01  : begin  // reading out
          for ( i = (DEPTH-1); i > 0; i-- ) begin
            data[i-1] <= data[i];
          end
          cnt[DEPTH_W-1:0] <= cnt[DEPTH_W-1:0] - 1'b1;
        end

        2'b10  : begin  // writing in
          data[cnt[DEPTH_W-1:0]] <= w_data[DATA_W-1:0];
          cnt[DEPTH_W-1:0] <= cnt[DEPTH_W-1:0] + 1'b1;
        end

        2'b11  : begin  // simultaneously reading and writing
          data[cnt[DEPTH_W-1:0]-1] <= w_data[DATA_W-1:0];
          // data counter does not change here
        end
      endcase

      // data buffer works only for normal lifo mode
      if( r_req_f ) begin
        data_buf[DATA_W-1:0] <= data[0];
      end
    end
  end


  always_comb begin
    empty = ( cnt[DEPTH_W-1:0] == '0 );
    full = ( cnt[DEPTH_W-1:0] == DEPTH );

    if( FWFT_MODE == "TRUE" ) begin
      if (~empty) begin
        r_data[DATA_W-1:0] = data[0]; // first-word fall-through mode
      end else begin
        r_data[DATA_W-1:0] = '0;
      end
    end else begin
      r_data[DATA_W-1:0] = data_buf[DATA_W-1:0];   // normal mode
    end

    fail = ( empty && r_req ) ||
           ( full && w_req );
  end

endmodule