ghdl，一款開源的vhdl模擬器以及gtkwave，一款開源的波形觀察器，一個二位元全加法器的實作

參考來源：https://ghdl.readthedocs.io/en/latest/using/QuickStartGuide.html

先說載點：
ghdl:
https://ghdl.readthedocs.io/en/latest/getting/Releases.html
gtkwave:
https://sourceforge.net/projects/gtkwave/files/

然後根據ghdl的說明檔，先來實作一份加法器
ghdl quick start guide:
https://github.com/ghdl/ghdl/blob/master/doc/using/QuickStartGuide.rst#a-full-adder

步驟：
用文字編輯器撰寫vhdl
存檔並且副檔名為.vhd或是.vhdl
vhdl內的entity名稱不一定要與檔名一樣
先用ghdl -a [vhdl檔]

再用ghdl -e [entity名稱] ，elaborate這個entity但我不知道什麼是elaborate

最後ghdl -r [entity名稱]，跑這個vhdl，但是如果直接跑會沒有結果產生

因此需要製作testbench，testbench也是一段vhdl程式，他與一班vhdl差別在沒有port，是用來產生輸入信號以及檢查輸出是否為預期

重點：
vhdl檔案必須依照階層關係依序編譯，否則會造成頂端vhdl無法正確解譯

先用文字編輯器把下列程式存檔，檔名為「adder.vhdl」

entity adder is
  -- `i0`, `i1`, and the carry-in `ci` are inputs of the adder.
  -- `s` is the sum output, `co` is the carry-out.
  port (i0, i1 : in bit; ci : in bit; s : out bit; co : out bit);
end adder;

architecture rtl of adder is
begin
  --  This full-adder architecture contains two concurrent assignments.
  --  Compute the sum.
  s <= i0 xor i1 xor ci;
  --  Compute the carry.
  co <= (i0 and i1) or (i0 and ci) or (i1 and ci);
end rtl;

接著把下列測試檔存檔，檔名為「adder_tb.vhdl」

--  A testbench has no ports.
entity adder_tb is
end adder_tb;

architecture behav of adder_tb is
  --  Declaration of the component that will be instantiated.
  component adder
    port (i0, i1 : in bit; ci : in bit; s : out bit; co : out bit);
  end component;

  --  Specifies which entity is bound with the component.
  for adder_0: adder use entity work.adder;
  signal i0, i1, ci, s, co : bit;
begin
  --  Component instantiation.
  adder_0: adder port map (i0 => i0, i1 => i1, ci => ci,
                           s => s, co => co);

  --  This process does the real job.
  process
    type pattern_type is record
      --  The inputs of the adder.
      i0, i1, ci : bit;
      --  The expected outputs of the adder.
      s, co : bit;
    end record;
    --  The patterns to apply.
    type pattern_array is array (natural range <>) of pattern_type;
    constant patterns : pattern_array :=
      (('0', '0', '0', '0', '0'),
       ('0', '0', '1', '1', '0'),
       ('0', '1', '0', '1', '0'),
       ('0', '1', '1', '0', '1'),
       ('1', '0', '0', '1', '0'),
       ('1', '0', '1', '0', '1'),
       ('1', '1', '0', '0', '1'),
       ('1', '1', '1', '1', '1'));
  begin
    --  Check each pattern.
    for i in patterns'range loop
      --  Set the inputs.
      i0 <= patterns(i).i0;
      i1 <= patterns(i).i1;
      ci <= patterns(i).ci;
      --  Wait for the results.
      wait for 1 ns;
      --  Check the outputs.
      assert s = patterns(i).s
        report "bad sum value" severity error;
      assert co = patterns(i).co
        report "bad carry out value" severity error;
    end loop;
    assert false report "end of test" severity note;
    --  Wait forever; this will finish the simulation.
    wait;
  end process;
end behav;

先用

ghdl -a adder.vhdl

再用

ghdl -a adder_tb.vhdl
ghdl -e adder_tb

最後

ghdl -r adder_tb --vcd=adder.vcd

結果會在檔案所在資料夾中看到一個名為adder.vcd的檔案，點開來會自動啟動gtkwave
看到以下畫面：

先在左上的adder_0點一下，接著到左下幾個reg ...都點兩下，結果會在右側波形視窗出現綠線



接著用放大鏡-，一直按一直按，隨著時間刻度慢慢放大，大概到ns間距時，波形開始出現