module alu (a,b,op,alu_go,result,flags);

input  [15:0] a, b;
input  [3:0]  op;
input         alu_go;

output [15:0]   result;
output [3:0]    flags;

reg    [15:0]   result,a_reg,  b_reg, result_reg;
reg    [3:0]    op_reg, flags,  newflags;

parameter ALUOP_NOP =  4'h0;
parameter ALUOP_AND  = 4'h1;
parameter ALUOP_OR   = 4'h2;
parameter ALUOP_NOT  = 4'h3;
parameter ALUOP_XOR  = 4'h4;

parameter ALUOP_ADD  = 4'h5;
parameter ALUOP_SUB  = 4'h6;
parameter ALUOP_MULH = 4'h7;
parameter ALUOP_MULL = 4'h8;
parameter ALUOP_DIV  = 4'h9;
parameter ALUOP_MOD  = 4'hA;
parameter ALUOP_NEG  = 4'hB;
parameter ALUOP_COMP = 4'hC;
parameter ALUOP_INC_A= 4'hD;
parameter ALUOP_B    = 4'hE;
parameter ALUOP_DEC_A= 4'hF;


parameter TALUOP = 50;

initial begin
  op_reg = 0; newflags = 0;flags = 0;result_reg = 0;
end

always @(posedge alu_go) begin

  if (~(op == ALUOP_NOP)) begin
    op_reg = op;a_reg = a;b_reg = b;newflags = 0;

#1
  $display("%t %m Befehl %dd(%hh,%hh) wird ausgeführt.",$time, op, a, b);
  case (op)
// flags reihenfolge: ocnz (overflow, carry, negativ, zero)
                ALUOP_AND:  #TALUOP result_reg  = a & b;  // oberere flags bleiben null
                ALUOP_OR:   #TALUOP result_reg  = a | b;  // "
                ALUOP_NOT:  #TALUOP result_reg  = ~a;     // "
                ALUOP_XOR:  #TALUOP result_reg  = a ^ b;  // "
                ALUOP_INC_A:#TALUOP result_reg  = a+1;    // "
                ALUOP_DEC_A:#TALUOP result_reg  = a-1;    // "
                ALUOP_B:    #TALUOP result_reg  = b;      // "


                ALUOP_ADD:  begin
                  #TALUOP result_reg  = a + b;
                  newflags[2] = (a+b > 'hFFFF);
                  // ab~result + ~a~by
                  newflags[3] = a[15] && b[15] && ~result_reg[15] || ~a[15] && ~b[15] && result_reg[15];
                end
                ALUOP_SUB:  begin
                  #TALUOP result_reg  = a - b;
                  newflags[2] = (a+b > 'hFFFF);
                  // a~b~result + ~aby  andersherum als im Motorola!!
                  newflags[3] = a[15] && ~b[15] && ~result_reg[15] || ~a[15] && b[15] && result_reg[15];
                end
                ALUOP_COMP:  begin
                  #TALUOP result_reg  = a - b;
                  newflags[2] = (a+b > 'hFFFF);
                  // a~b~result + ~aby  andersherum als im Motorola!!
                  newflags[3] = a[15] && ~b[15] && ~result_reg[15] || ~a[15] && b[15] && result_reg[15];
                end

                ALUOP_MULH: begin
                  #TALUOP result_reg  = ((a * b) / 'h10000);
                  newflags[3] =   ~((a * b) % 'h10000 == 0);// unteres wort <> 0
                end
                ALUOP_MULL: begin
                  #TALUOP result_reg  = a * b;
                  newflags[3] = ~((a * b) / 'h10000 == 0);  // oberes wort <> 0
                end
                ALUOP_DIV:  begin
                  #TALUOP result_reg  = a / b;
                  newflags[3] = ~((a % b) == 0);         // rest <> 0
                end
                ALUOP_MOD:  begin
                  #TALUOP result_reg  = a % b;
                  newflags[3] = ~((a / b) == 0);         // quotient <> 0
                end
                ALUOP_NEG:  begin
                  newflags[3] = (b == 'h8000); // 32768 gibt es nicht
                  #TALUOP result_reg  = - a;
                end
                default:    #TALUOP result_reg  = 1234;
              endcase
              

              newflags[1] = result_reg[15];     // Negativ-bit
              newflags[0] = (result_reg == 0);  // Zero-bit
              flags = newflags; newflags = 0;

              $display("%t %m Ergebnis (%h, %h) ist %hh (%bb)",$time,a,b, result_reg, flags);
              if (op != ALUOP_COMP) result=result_reg; else result=a;

  end
end
endmodule