A compiler directive remains in effect (possibly spanning multipel files) until a different compiler directive specifies otherwise.
| `define | `undef |
| `ifdef | `else | `endif |
| `default_nettype |
| `include |
| `resetall |
| `timescale |
| `unconnected_drive | `nounconnected_drive |
| `celldefine | `endcelldefine |