A valid schedule key. A domain without a valid schedule key simply does not fetch instructions. A valid keeper key. The absence of a keeper key is only an issue when the domain takes a fault. If no keeper can be notified, the domain simply becomes unable to execute instructions.
All implementations impose the following usage constraints on the slots in a domain root.
DR10 Architecture Defined DR3 Reserved for Symbol table space key. DR12 Architecture Defined DR5 Number key providing trap code and associated information DR13 Reserved for alternate message buffer key. DR6 Register slot Message receive descriptor. The message send and receive information share a common structure for nibbles 0 through 15. Note that the send and receive information blocks correspond to specific registers in the implementation architecture.