INT 21 - DOS 2+ internal - SET CURRENT PROCESS ID (SET PSP ADDRESS)
        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 DOS 3+, 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 use segments of 0000h and FFFFh; 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
SeeAlso: AH=51h,AH=62h