INT 21 - DOS 2+ - TERMINATE AND STAY RESIDENT
        AH = 31h
        AL = return code
        DX = number of paragraphs to keep resident
Return: never
Notes:  the value in DX only affects the memory block containing the PSP;
          additional memory allocated via AH=48h is not affected
        the minimum number of paragraphs which will remain resident is 11h
          for DOS 2.x and 06h for DOS 3+
        most TSRs can save some memory by releasing their environment block
          before terminating (see AH=26h,AH=49h)
SeeAlso: AH=4Ch,AH=4Dh,INT 20,INT 22,INT 27