// Instruction decoder of Nandgame
// corresponds to levels
// - control selector
// - control unit

`timescale 1us/1us

`include "alu.sv"
`include "cond_check.sv"

module instruction_decode  #(
  parameter DATA_WIDTH = 16
) (
  input [15:0] instruction,
  
  input [(DATA_WIDTH-1):0] A_i,
  input [(DATA_WIDTH-1):0] D_i,
  input [(DATA_WIDTH-1):0] pA_i,
  
  output [(DATA_WIDTH-1):0] RES,
  output do_jump,

  output dst_a,
  output dst_d,
  output dst_pa
);

wire is_immediate;
// bit 15 set = ALU instruction
// bit 15 unset = immediate
assign is_immediate = !instruction[15];

assign dst_a = is_immediate || instruction[5];
assign dst_d = !is_immediate && instruction[4];
assign dst_pa = !is_immediate && instruction[3];

wire use_pointer_a_for_alu;
wire [(DATA_WIDTH-1):0] alu_y;
assign use_pointer_a_for_alu = instruction[12];
assign alu_y = use_pointer_a_for_alu ? pA_i : A_i;

wire [(DATA_WIDTH-1):0] alu_res;

alu #(
  .DATA_WIDTH(DATA_WIDTH)
) my_alu (
  .X(D_i),
  .Y(alu_y),
  .u(instruction[10]),
  .opcode(instruction[9:8]),
  .zx(instruction[7]),
  .sw(instruction[6]),
  .RES(alu_res)
);

wire result_jump;

cond_check #(
  .DATA_WIDTH(DATA_WIDTH)
) my_cond (
  .X(alu_res),
  .ltz(instruction[2]),
  .eqz(instruction[1]),
  .gtz(instruction[0]),
  .res(result_jump)
);

assign do_jump = is_immediate ? 0 : result_jump; 

assign RES = is_immediate ? instruction : alu_res;

endmodule