> TMON/MVS (possibly under a newer name than when I developed this 
> part of it in 1987) sets itself up as an application using the 
> Monitor Call Trace Facility when and only when a short-duration I/O 
> trace is requested by a TMON/MVS user .  It removes all its Monitor 
> Call control blocks when the trace ends.  Unless that code has been 
> modified since I last worked with it 20 years ago. 

  That's good to know.  The Monitor Call interface in z/OS has
never been externally documented (to my knowledge), and the internal
macro for invoking it is PL/S(AS)(X) - only.  Althought it appeared
that the ancients (before my time) who orginally design this interface
were intending to allow multiple concurrent appplications, it looks like
at some they gave up on getting that working, and restricted it to 
one application (GTF) at a time.  But, it was GTF initialization that
enforced the restriction, not the Monitor Call interface code.  So I don't
know what would happen if this TMON function was active at the same time
as GTF.

  In z/OS 1.6, there was a line item to remove this restriction,
to allow multiple concurrent GTFs.  Which eventually (after a number
of PTFs) mostly seems to work.  There were still some remaining
issues relating to GTRACE TEST, and  GTFTRACE under IPCS, when 
multiple GTFs are active, that I am working on fixing in z/OS 2.1 
 
 Jim Mulder   z/OS System Test   IBM Corp.  Poughkeepsie,  NY

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