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.

- -

INT 2fH 1681H: Begin Critical Section