`default_nettype none
module serv_mem_if
  #(parameter WITH_CSR = 1)
  (
   input wire 	      i_clk,
   //State
   input wire 	      i_en,
   input wire 	      i_init,
   input wire 	      i_cnt_done,
   input wire [1:0]   i_bytecnt,
   input wire [1:0]   i_lsb,
   output wire 	      o_misalign,
   output wire 	      o_sh_done,
   output wire 	      o_sh_done_r,
   //Control
   input wire 	      i_mem_op,
   input wire 	      i_shift_op,
   input wire 	      i_signed,
   input wire 	      i_word,
   input wire 	      i_half,
   //Data
   input wire 	      i_op_b,
   output wire 	      o_rd,
   //External interface
   output wire [31:0] o_wb_dat,
   output wire [3:0]  o_wb_sel,
   input wire [31:0]  i_wb_rdt,
   input wire 	      i_wb_ack);

   reg           signbit;
   reg [31:0] 	 dat;

   wire [2:0] 	 tmp = {1'b0,i_bytecnt}+{1'b0,i_lsb};

   wire 	 dat_en = i_shift_op | (i_en & !tmp[2]);

   wire 	 dat_cur =
		 ((i_lsb == 2'd3) & dat[24]) |
		 ((i_lsb == 2'd2) & dat[16]) |
		 ((i_lsb == 2'd1) & dat[8]) |
		 ((i_lsb == 2'd0) & dat[0]);

   wire dat_valid =
	i_word |
	(i_bytecnt == 2'b00) |
	(i_half & !i_bytecnt[1]);

   assign o_rd = i_mem_op & (dat_valid ? dat_cur : signbit & i_signed);

   assign o_wb_sel[3] = (i_lsb == 2'b11) | i_word | (i_half & i_lsb[1]);
   assign o_wb_sel[2] = (i_lsb == 2'b10) | i_word;
   assign o_wb_sel[1] = (i_lsb == 2'b01) | i_word | (i_half & !i_lsb[1]);
   assign o_wb_sel[0] = (i_lsb == 2'b00);

   assign o_wb_dat = dat;

   /* The dat register has three different use cases for store, load and
    shift operations.
    store : Data to be written is shifted to the correct position in dat during
            init by dat_en and is presented on the data bus as o_wb_dat
    load  : Data from the bus gets latched into dat during i_wb_ack and is then
            shifted out at the appropriate time to end up in the correct
            position in rd
    shift : Data is shifted in during init. After that, the six LSB are used as
            a downcounter (with bit 5 initially set to 0) that triggers
            o_sh_done and o_sh_done_r when they wrap around to indicate that
            the requested number of shifts have been performed
    */
   wire [5:0] dat_shamt = (i_shift_op & !i_init) ?
	      //Down counter mode
	      dat[5:0]-1 :
	      //Shift reg mode with optional clearing of bit 5
	      {dat[6] & !(i_shift_op & i_cnt_done),dat[5:1]};

   assign o_sh_done = dat_shamt[5];
   assign o_sh_done_r = dat[5];

   always @(posedge i_clk) begin
      if (dat_en | i_wb_ack)
	dat <= i_wb_ack ? i_wb_rdt : {i_op_b, dat[31:7], dat_shamt};

      if (dat_valid)
        signbit <= dat_cur;
   end

   /*
    mem_misalign is checked after the init stage to decide whether to do a data
    bus transaction or go to the trap state. It is only guaranteed to be correct
    at this time
    */
   assign o_misalign = WITH_CSR & ((i_lsb[0] & (i_word | i_half)) | (i_lsb[1] & i_word));

endmodule