Int 21H, AH=51H

2 and higher

GET CURRENT PROCESS ID (GET PSP ADDRESS)

AH = 51h

 BX = segment of PSP for current process

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 DOS 3.0+, 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
supported by OS/2 compatibility box
identical to the documented AH=62h
this call was undocumented prior to the release of DOS 5.0
        

AH=26h,AH=50h,AH=62h

Text based on Ralf Brown Interrupt List Release 61