Retro1.org codex

READY                                                    CP Privilege Class:  G

Use the READY command to set a device-end interruption pending for a specified
virtual device.  The format of the READY command is:
+----------+------------------------------------------------------------------+
| READY    | vaddr                                                            |
+----------+------------------------------------------------------------------+
where:

vaddr    is a virtual device address (cuu).  If the virtual device is in a
         NOTREADY status, it is changed to a READY status.