the breakpoint at !XL (hex) will cause the STx_C at !XL to fail; the
 LDx_L is at !XL

    Facility: DEBUG, VMS Debugger

    Explanation: The debugger has limited support for debugging of
    critical sections delimited by memory locking/unlocking (e.g.
    LDx_L/STx_C (load-locked/store-conditional) instructions. The
    exception mechanisms used by the debugger causes the lock-flag
    set by the locking instruction to be cleared. This action affects
    the behavior of subsequent instructions that rely on memory being
    locked. Such critical sections are (or should be) coded to retry
    when the LDx_L or STx_C fail.

    User Action: Cancel or deactivate all breakpoint events that
    might trigger while the application being debugged is executing
    the critical section; a breakpoint at the LDx_L is permissible
    (will not effect the the lock-flag); a STEP issued from the load_
    lock instruction without interfering breakpoints or watchpoints
    will completely step over the critical section.