Int 21 Fn 50  - DOS 2+ Internal - Set Current Process Id (set Psp Address  [D]

   AH = 50h
   BX = segment of PSP for new process

Notes: DOS uses the current PSP address to determine which processes own files
     and memory; it corresponds to process identifiers used by other OSs
   under DOS 2.x, this function cannot be invoked inside an INT 28h
     handler without setting the Critical Error flag
   under MS-DOS 3+ and DR-DOS 3.41+, this function does not use any of
     the DOS-internal stacks and may thus be called at any time, even
     during another INT 21h call
   some Microsoft applications such as Quick C 2.51 use segments of 0000h
     and FFFFh and direct access to the SDA (see AX=5D06h) to test whether
     they are running under MS-DOS rather than a compatible OS; although
     one should only call this function with valid PSP addresses, any
     program hooking it should be prepared to handle invalid addresses
   supported by OS/2 compatibility box
   this call was undocumented prior to the release of DOS 5.0

See Also: AH=26h,AH=51h,AH=62h