A final consideration when building subsystem break processing
capabilities into your program has to do with whether your program
will run in Native Mode, Compatibility Mode, or both.
Native Mode vs. Compatibility Mode |
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So far, the discussion in this chapter has assumed that all
processing is to take place in Native Mode (NM). There are times,
however, when you will want to allow subsystem break processing
to take place in programs that will run in Compatibility Mode (CM),
or in both CM and NM. It is possible to do so by following some
common sense rules when arming the [CTRL]Y trap.
If your program is to run in Native Mode only, you should
use a Native Mode subsystem break procedure as well. XCONTRAP will
execute in Native Mode and pass a 32-bit value as the external address
of the subsystem break handler. This situation presents the most
straight forward case for subsystem break processing. If the subsystem
break character is received during execution of the program, control
will simply be passed from the main application, which is running
in Native Mode, to the break handler, which will also run in Native
Mode. If your program is a Compatibility Mode application, the situation
is considerably different. In this case, since XCONTRAP will execute
in Compatibility Mode, it is only possible to pass a 16-bit external
address in the call. This means that your trap handling routine
must be a CM procedure as well.
If your program executes in both NM and CM the situation becomes
somewhat more complex. HP recommends that you use a Native Mode
trap handler in this case, which would mean that you would have
to call XCONTRAP in Native Mode to pass a 32-bit address for the
trap procedure. It is possible, however, to use a Compatibility
Mode trap handler. (The discussion of the XCONTRAP intrinsic in
Chapter 8 “Intrinsics Reference” includes instructions
for specifying a 16-bit address in the 32-bit plabel
field.)
If you have supplied a Native Mode trap procedure, and the
program is running in CM when a subsystem break signal is received,
MPE/iX will switch to NM to allow the trap procedure to run. The
reverse is also true; that is, a program running in NM with a CM
trap handler will switch to CM if a subsystem break occurs.
For more information on handling [CTRL]Y
traps, see the Trap Handling manual.
