/*
 * servile_rf_mem_if.v : Arbiter to allow a shared SRAM for RF and memory accesses. RF is mapped to the highest 128 bytes of the memory. Requires 8-bit RF accesses.
 *
 * SPDX-FileCopyrightText: 2024 Olof Kindgren <olof.kindgren@gmail.com>
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none
module servile_rf_mem_if
  #(//Memory parameters
    parameter depth = 256,
    //RF parameters
    parameter rf_regs = 32,
    //Internally calculated. Do not touch
    parameter rf_depth = $clog2(rf_regs*4),
    parameter aw = $clog2(depth))
   (
    input wire		      i_clk,
    input wire		      i_rst,
    input wire [rf_depth-1:0] i_waddr,
    input wire [7:0]	      i_wdata,
    input wire		      i_wen,
    input wire [rf_depth-1:0] i_raddr,
    output wire [7:0]	      o_rdata,
    input wire		      i_ren,

    output wire [aw-1:0]      o_sram_waddr,
    output wire [7:0]	      o_sram_wdata,
    output wire		      o_sram_wen,
    output wire [aw-1:0]      o_sram_raddr,
    input wire [7:0]	      i_sram_rdata,
    output wire		      o_sram_ren,

    input wire [aw-1:2]	      i_wb_adr,
    input wire [31:0]	      i_wb_dat,
    input wire [3:0]	      i_wb_sel,
    input wire		      i_wb_we,
    input wire		      i_wb_stb,
    output wire [31:0]	      o_wb_rdt,
    output reg		      o_wb_ack);

   reg [1:0] 		bsel;

   wire 		wb_en = i_wb_stb & !i_wen & !o_wb_ack;

   wire 		wb_we = i_wb_we & i_wb_sel[bsel];

   wire [aw-1:0] rf_waddr = ~{{aw-rf_depth{1'b0}},i_waddr};
   wire [aw-1:0] rf_raddr = ~{{aw-rf_depth{1'b0}},i_raddr};

   assign o_sram_waddr = wb_en ? {i_wb_adr[aw-1:2],bsel} : rf_waddr;
   assign o_sram_wdata = wb_en ? i_wb_dat[bsel*8+:8]     : i_wdata;
   assign o_sram_wen   = wb_en ? wb_we : i_wen;
   assign o_sram_raddr = wb_en ? {i_wb_adr[aw-1:2],bsel} : rf_raddr;
   assign o_sram_ren   = wb_en ? !i_wb_we : i_ren;

   reg [23:0] 		wb_rdt;
   assign o_wb_rdt = {i_sram_rdata, wb_rdt};

   reg 			regzero;
   always @(posedge i_clk) begin

      if (wb_en) bsel <= bsel + 2'd1;
      o_wb_ack <= wb_en & &bsel;
      if (bsel == 2'b01) wb_rdt[7:0]   <= i_sram_rdata;
      if (bsel == 2'b10) wb_rdt[15:8]  <= i_sram_rdata;
      if (bsel == 2'b11) wb_rdt[23:16] <= i_sram_rdata;
      if (i_rst) begin
	 bsel <= 2'd0;
	 o_wb_ack <= 1'b0;
      end
      regzero <= !(|i_raddr[rf_depth-1:2]);
   end

   assign o_rdata = regzero ? 8'd0 : i_sram_rdata;

endmodule