Implement CPU "cycle" decoder

Decoder.sv

@@ -0,0 +1,47 @@
+`default_nettype none
+
+module Decoder(
+  input var logic clock,
+  input var logic reset,
+  output var logic fetch,
+  output var logic decode,
+  output var logic execute,
+  output var logic writeback
+);
+
+  var logic was_reset;
+  var logic setvalue;
+  var logic[1:0] reset_val;
+
+  var logic[1:0] mode;
+  Counter #(.WIDTH(2)) mode_cnt(
+    .clock(clock),
+    .reset(0),
+    .decrement(0),
+    .setvalue(setvalue),
+    .valuein(reset_val),
+    .valueout(mode)
+  );
+
+  always @(posedge clock or posedge reset) if (reset) begin
+    // Set each output to 0 by setting was_reset.
+    was_reset = 1'b1;
+
+    // Also we already prepare the counter reset for the next clock, as the clock already
+    // happened when this is "executed".
+    setvalue = 1'b1;
+    reset_val = 2'b00;
+  end else if (was_reset == 1'b1) begin
+    // The counter uses the same clock as this, so the reset is performed "now".
+    // We can now "reset the reset" and continue counting regularly.
+    was_reset = 1'b0;
+    setvalue = 1'b0;
+    reset_val = 2'b00;
+  end
+
+  assign fetch = ~was_reset && (mode == 2'b00);
+  assign decode = ~was_reset && (mode == 2'b01);
+  assign execute = ~was_reset && (mode == 2'b10);
+  assign writeback = ~was_reset && (mode == 2'b11);
+
+endmodule

Decoder_TestBench.sv

@@ -0,0 +1,84 @@
+`default_nettype none
+
+module Decoder_TestBench;
+
+  var logic clock, reset;
+  tri fetch, decode, execute, writeback;
+  
+  Decoder dec(
+    .clock(clock),
+    .reset(reset),
+    .fetch(fetch),
+    .decode(decode),
+    .execute(execute),
+    .writeback(writeback)
+  );
+
+
+// synthesis translate_off
+  initial begin
+    $timeformat(-9, 2, " ns", 20);
+
+    $display("%0t Initial Reset", $time);
+    clock = 1'b0;
+    #20 reset = 1'b1;
+    #20 reset = 1'b0;
+    assert(fetch == 1'b0);
+    assert(decode == 1'b0);
+    assert(execute == 1'b0);
+    assert(writeback == 1'b0);
+
+    $display("%0t Fetch", $time);
+    #20 clock = 1'b1;
+    #20 clock = 1'b0;
+    assert(fetch == 1'b1);
+    assert(decode == 1'b0);
+    assert(execute == 1'b0);
+    assert(writeback == 1'b0);
+
+    $display("%0t Decode", $time);
+    #20 clock = 1'b1;
+    #20 clock = 1'b0;
+    assert(fetch == 1'b0);
+    assert(decode == 1'b1);
+    assert(execute == 1'b0);
+    assert(writeback == 1'b0);
+
+    $display("%0t Execute", $time);
+    #20 clock = 1'b1;
+    #20 clock = 1'b0;
+    assert(fetch == 1'b0);
+    assert(decode == 1'b0);
+    assert(execute == 1'b1);
+    assert(writeback == 1'b0);
+
+    $display("%0t Writeback", $time);
+    #20 clock = 1'b1;
+    #20 clock = 1'b0;
+    assert(fetch == 1'b0);
+    assert(decode == 1'b0);
+    assert(execute == 1'b0);
+    assert(writeback == 1'b1);
+
+    $display("%0t Fetch", $time);
+    #20 clock = 1'b1;
+    #20 clock = 1'b0;
+    assert(fetch == 1'b1);
+    assert(decode == 1'b0);
+    assert(execute == 1'b0);
+    assert(writeback == 1'b0);
+
+    $display("%0t Reset", $time);
+    clock = 1'b0;
+    #20 reset = 1'b1;
+    #20 reset = 1'b0;
+    assert(fetch == 1'b0);
+    assert(decode == 1'b0);
+    assert(execute == 1'b0);
+    assert(writeback == 1'b0);
+
+    $display("Success!");
+  end
+// synthesis translate_on
+
+endmodule