• the process ID number is actually the segment address of a program's PSP. This is useful for TSR's to access their own file handle table inside their PSP.
• this function cannot be called while in an INT 28 handler in DOS 2.x unless the critical error flag is set or stack corruption will occur
• safe in DOS 3.x INT 28 handlers since they use a different stack by default
• available in OS/2 compatibility box

see INT 21,51, INT 21,62