INT 2fH 1681H: Begin Critical Section

                                                      Compatibility: Win 3.x 
 Expects: AX    1681H
 Returns: none
    Info: DOS programs (including TSRs and device drivers), when running
          under Virtual-8086 mode (as when Enh386-Mode Windows is running)
          can use this call to prevent a task-switch from occurring.

          While in a critical section no other tasks, except hardware
          interrupt handlers, are allowed to run.  Use INT 2fH 1682H (end
          critical section) as soon as possible.

See Also: INT 2fH 16xxH: Enh386 Windows Services & Notifications
          INT 2fH: Multiplex Interrupt
          DOS Functions