USING LOADABLE DRIVERS WITH XDT

     It is sometimes convenient to be able to  trap  to  XDT
inside  a  loadable  driver.  Most suggestions include a BPT
either in the source code or temporarily inserted with  ZAP.
If  the  driver is used solely for data acquisition and con-
trol (as most of ours are) such that it will never be called
by  PIP etc.  then the following technique can be used.  The
key to this is to allow the MCR SET/BUF command put  a  flag
into U.CW4.  The driver then checks this flag and condition-
ally executes a BPT.  In this manner the BPT can remain hid-
den  until required and there is no chance that an incorrect
location will be zapped.  The trap will also be  selectively
executed  for  any  unit of a multi-unit driver.  It is also
much easier to use the SET /BUF command.

     For example:

     $XXINI::

        TST     U.CW4(R5)       ;TURN ON WITH SET /BUF=XX:1

        BEQ     .+4             ;NOT SET

        BPT                     ;GO TO XDT