• The process ID number is actually the segment address of program's PSP. This in conjunction with INT 21,50 is useful for TSR's to access their own file handle table in their respective 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
• INT 21,62 is highly recommended for DOS 3.x due to a possible bug when activated from a TSR. DOS may switch to the wrong internal stack which may cause a problems with TSR's if called during an INT 28.

see INT 21,62 (Get PSP segment) for DOS 3.x applications