*pxhand
*handler
Private exception handlers (68000 Tripos only)

    In 68000 Tripos, the field in the TCB called TCB.PXHAND is provided
to hold the machine address of a user supplied private exception handling
routine that will be called whenever a fault (TRAP, CHK, illegal instruction
etc.) is encountered while the task is running at interrupt level 0.  For
convenience and to maintain upward compatibilty the standard Tripos fault
handler is invoked if the PXHAND field is zero.  CREATETASK initialises this
field to zero.

    On entry to the private exception handler:

        S  (=A5) = MC address of the current TCB
        R  (=A6) = MC address of an area containing the following:
                        the return link into the kernel,
                        the old value of S,
                        the old value of R,
                        the old value of SR,
                    and the old value of PC.

From this state executing RTS will re-enter the kernel in CALLDBG and
cause the standard tripos entry into DEBUG, however the handler may, if
it chooses , handle the exception itself and return directly rather than
via CALLDBG.


