Test register.
| SCPQ | SHORTCUT PAUSE QUANTA. This bit reduces the effective PAUSE quanta from 64 byte-times to 1 byte-time. |
| TESTPAUSE | This bit causes the MAC Control sublayer to inhibit transmissions, just as if a PAUSE Receive Control frame with a nonzero pause time parameter was received. |
| TESTBP | TEST BACKPRESSURE. Setting this bit will cause the MAC to assert backpressure on the link. Backpressure causes preamble to be transmitted, raising carrier sense. A transmit packet from the system will be sent during backpressure. |
| RESERVED | Unused |