Compatibility: 3.0+
See: Switcher API Overview for background info.

4b01H (build notification chain)

4b02H (is switcher installed? / get service addr)

4b03H (allocate switcher ID)

4b04H (free switcher ID)

4b05H (identify instance data)

Switcher Services and Notifications (interact with the switcher)

These functions are used by application programs, especially TSRs, which
have special needs with regard to task switching; for instance, programs
which must prevent the DOS Shell from swapping them out or switching to
another program (say, via Alt+Tab).  They also enable a TSR to maintain
instance data--a unique data area for each DOS session.

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INT 2fH 4BxxH: Task Switcher Functions