There are a small fixed number of "wait objects". A wait object holds a value called "todw". A BWAIT key designates a wait object.

See (p2,waitc) for a way to get many keys to wait on.

BWAIT(0;==>c;) returns with c=0. It never returns much later than when the real clock exceeds todw of the wait object, and it seldom returns earlier. If there is already a domain waiting on the designated wait object, the return is immediate with c=1. {The waiting domain will not be paged out.}

When the kernel is restarted, the todw of all wait objects are set to 0. I think that this is the only time that the order code 0 returns sooner than the indicated todw value.

Programing notes:

BWAIT may be used to create a multiplexed timer facility. The earliest time of expiration in the queue is placed in the BWAIT key by the queue handling domain while another domain waits on the BWAIT key with order code zero. If an earlier time of expiration arrives the first domain can reset the time with BWAIT order code 1 without disturbing the domain that is waiting on BWAIT order code zero.

A watchdog timer facility may be constructed to check that, for example, some message arrives at least once every every 5 seconds. The watch dog waits on the BWAIT key and the processing domain for the signal resets the BWAIT key time to now + 5 seconds whenever a message is processed.

Since the kernel itself has need for bwait-like functions {migration, timeouts, etc} we found that it was as easy to provide several wait objects as just one. Heavy duty or primordial domains that are trusted not to lose their BWAIT key can thus have their own wait objects.