// CSE141L
import Definitions::*;
// control decoder (combinational, not clocked)
// inputs from instrROM, ALU flags
// outputs to program_counter (fetch unit)
module Ctrl (
  input[ 8:0]   Instruction,	   // machine code
  input[ 7:0]   DatMemAddr,
  output logic  Branch    ,
                BranchEn  ,
			    RegWrEn   ,	   // write to reg_file (common)
			    MemWrEn   ,	   // write to mem (store only)
			    LoadInst  ,	   // mem or ALU to reg_file ?
			    TapSel    ,
			    Ack		  ,      // "done w/ program"
  output logic[1:0] PCTarg,
//  output logic[2:0]  ALU_inst
  );

/* ***** All numerical values are completely arbitrary and for illustration only *****
*/

// alternative -- case format
always_comb	begin
// list the defaults here
   Branch    = 'b0;
   BranchEn  = 'b0;
   RegWrEn   = 'b1; 
   MemWrEn   = 'b0;
   LoadInst  = 'b0;
   TapSel    ' 'b0;     //
   PCTarg    = 'b0;     // branch "where to?"
   case(Instruction[8:6])  // list just the exceptions 
     3'b000:   begin
                  MemWrEn = 'b1;   // store, maybe
				  RegWrEn = 'b0;
			   end
     3'b001:   LoadInst = 'b1;  // load
     3'b010:   begin end
     3'b011:   begin end
     3'b100:   begin end
     3'b101:   begin end
     3'b110:   begin end
// no default case needed -- covered before "case"
   endcase
end

assign Ack = ProgCtr == 971;
// alternative Ack = Instruction == 'b111_000_111

// ALU commands
//assign ALU_inst = Instruction[2:0]; 

// STR commands only -- write to data_memory
assign MemWrEn = Instruction[8:6]==3'b110;

// all but STR and NOOP (or maybe CMP or TST) -- write to reg_file
assign RegWrEn = Instruction[8:7]!=2'b11;

// route data memory --> reg_file for loads
//   whenever instruction = 9'b110??????; 
assign LoadInst = Instruction[8:6]==3'b110;  // calls out load specially

assign tapSel = LoadInst &&	 DatMemAddr=='d62;
// jump enable command to program counter / instruction fetch module on right shift command
// equiv to simply: assign Jump = Instruction[2:0] == RSH;
always_comb
  if(Instruction[2:0] ==  RSH)
    Branch = 1;
  else
    Branch = 0;

// branch every time instruction = 9'b?????1111;
assign BranchEn = &Instruction[3:0];

// whenever branch or jump is taken, PC gets updated or incremented from "Target"
//  PCTarg = 2-bit address pointer into Target LUT  (PCTarg in --> Target out
assign PCTarg  = Instruction[3:2];

// reserve instruction = 9'b111111111; for Ack
assign Ack = &Instruction; // = ProgCtr == 385;

endmodule