`include "opcodes.sv"

module datapath( DataBus, Address, IR, ZReg, Function, UpdateZ,
		 EnableACC, LoadACC, EnableX, LoadX, EnableS, LoadS,
		 EnablePC, LoadPC, IncPC, SelPC, LoadIR, LoadMAR, EnableReg,
		 Clock, nReset );

inout  [15:0] DataBus;
output [15:0] Address, IR;
output ZReg;
input  [3:0] Function;
input  UpdateZ, EnableACC, LoadACC, EnableX, LoadX, EnableS, LoadS;
input  EnablePC, LoadPC, IncPC, SelPC, LoadIR, LoadMAR, EnableReg;
input  Clock, nReset;

reg  [15:0] IR, ACC, X, S, PC, MAR;
reg  ZReg;
wire [15:0] ALUoutBus, RegBus, Address;

//
// ALU Instance
//
   alu ALU ( ALU_Zflag, ALUoutBus, RegBus, DataBus, Function);

//
// Drive RegBus and DataBus as required
//
   assign RegBus = (EnableACC) ? ACC : 16'bz;
   assign RegBus = (EnableX)   ? X   : 16'bz;
   assign RegBus = (EnableS)   ? S   : 16'bz;
   assign RegBus = (EnablePC)  ? PC  : 16'bz;

   assign DataBus = ( EnableReg ) ? RegBus : 16'bz;

//
// Address multiplexor
//
   assign Address = (SelPC) ? PC : MAR;

//
// Update Registers as required
//
   always_ff @(posedge Clock, negedge nReset)
      if (!nReset)
         begin
            PC   <= 0;
            IR   <= 0;
            ACC  <= 0;
            X    <= 0;
            S    <= 0;
            MAR  <= 0;
            ZReg <= 0;
         end
      else
         begin
            if (LoadPC)  PC   <= #20ns ALUoutBus;
             else if (IncPC) PC <= #20ns PC + 1;
            if (LoadIR)  IR   <= #20ns DataBus;
            if (LoadACC) ACC  <= #20ns ALUoutBus;
            if (LoadX)   X    <= #20ns ALUoutBus;
            if (LoadS)   S    <= #20ns ALUoutBus;
            if (LoadMAR) MAR  <= #20ns ALUoutBus;
            if (UpdateZ) ZReg <= #20ns ALU_Zflag;
         end
endmodule