Sample: your first entity (2-to-1 MUX)

A multiplexer is the simplest combinational device that has a "choice" in it. The 2-to-1 version selects between two inputs based on one sel bit:

  sel = 0  →  y = a
  sel = 1  →  y = b

The VHDL for that is one line:

y <= a when sel = '0' else b;

What the sample does

Open the sample and you'll get a project named Your first entity with:

• mux21.vhd (the design above)
• A companion simulation config that drives sel, a, and b over 40 microseconds

When you Run, the Waveforms tab shows four rows — the three inputs and the output. Watch y flip between a and b as sel toggles.

Things to notice

The assignment y <= a when sel = '0' else b; is a concurrent statement: it lives outside of any process, and it describes a permanent wiring. There's no "when does this line execute?" — it just is.

The condition inside a when/else can be any boolean expression:

y <= a when sel = '0' else
     b when enable = '1' else
     'Z';

Chains like that synthesize to a priority multiplexer (top case wins ties).

Your turn: extend it

Once you've seen the sample work:

1. Change the entity to a 4-to-1 MUX — sel becomes 2 bits, and you choose between a, b, c, d.
2. Add a fifth port output_enable. When it's '0', drive y <= 'Z' (high impedance) regardless of sel.
3. Compare when/else against the with/select version you'll meet in the next module.

Kmila's editor highlights the VHDL keywords and knows about std_logic — hover over when to see the built-in documentation.