Compatibility: 2.0+ (doc'd: 5.0)
Expects: AH    50H
BX    Process ID (i.e, PSP segment) to make current
Returns:       none
Info: This function tells DOS that the PID  in BX identifies the
current active process.  It is used in TSR programs to inform DOS
which process is running; i.e., after popping up.

Several DOS fns (including file handle operations and memory
allocation functions), refer to and make changes to the PSP of
the current process.

A TSR should call Fn 51H and save its PSP during initialization.
Before popping up, the TSR should use fn 51H to get the ID of the
interrupted program, then use fn 50H to set the PID to the saved
value.  Be sure to change the PID back before popping down.

See Also: PSP (Program Segment Prefix)
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DOS Fn 50H: Set PSP