Primary keys that do not always return immediately, namely BWAIT, TAPE, TBA and GTT, can be distinguished from entries. Jumping to such a key may cause some or all of the side effects of the key to occur even though the jump appears not to have occurred. In effect, when the key senses that a jump is about to occur, it performs its side effects {which may include waiting for some event} and enters a special state. A jump to the key can only proceed in the special state; when it does, the return {of parameters saved when the side effects were performed} occurs immediately. {To use such a key intelligently, only one domain should be trying to jump to it at any one time and it should not change its mind about what it is doing. This is one reason that such keys are restricted.}

This problem can be avoided by designing I/O keys so there is no operation that has a side effect and then waits. See for example (tlba).

See (kor,bdevice). See (devaloc) for description of program designed to hold the bdevice key exclusively.

Tymnet Varian Base Access

Tymnet Lemcom Base Access

GTT and GTTHALT ("Gnosis TIRE/TORE")

Checkpoint Key

CHECKPOINT(1,(8,tod); ==> c;) Take checkpoint
Tod is a TOD value. If it is in the future, c=1. Otherwise, if no checkpoint has been taken since time tod, one is taken. The operation completes when everything stored before time tod has been checkpointed. More than one domain can make this call at once.

IPL(0,tod;PRANGE => c;)
Tod is a clock value in standard epoch. Outcomes:

c=0 The kernel has been restarted {IPL'd} more recently than the time in tod. If it is necessary to restart the kernel to accomplish this, it is IPL'd from the device which contains the IPL range identified by the PRANGE key.

c=1 Tod is in the future.

c=2 PRANGE is not a page range key to a mounted IPL range.

In order to prevent a restart loop, you must use this key this way. First store the clock. Then force a checkpoint. Then call the IPL key passing the stored clock value.

JP(0;PK==>c) Journalize Page
If PK is a Read/Write page key then the current data in that page will be preserved over checkpoint restart {(p1,checkpoint)}. Outcomes:
0 - Current contents are preserved

1 - Page created since last checkpoint, data not preserved

2 - Permanent I/O error on write

3 - Allocation pot for the page can not be read, data not preserved

4 - Page was all zeroes sometime since the last two checkpoints. Data not preserved.

kt+2 - Key is not a Read/Write page key

JP(kt==>X'B09');