[Info-vax] QIO Writes to a mailbox

[Michael Moroney]

Steve Bainbridge <stephen_bainbridge at yahoo.co.uk> writes:

>This code was written several years ago and I've been dumped on to
>investigate and correct the issue we now very occasionally see. I'm
>aware of the obvious missing check on the IOSB, but I can't say with
>any certainty that this is causing the problem.

You *must* check the IOSB status and make sure the operation is complete
to ensure the operation was performed properly.  Especially when using
QIO (no W).

>Whilst it's simple to change the code, replicating the problem has
>proved impossible and releasing into production onto many tens of
>systems takes a long time with a lot of effort - especially when we
>don't know if the problem has been resolved. So we would prefer to try
>and understand why we are seeing the problem, rather than just
>incrementally trying to correct issues.

Not checking the status is almost certainly the cause of the problem.