INT 21 - DOS 1+ - TERMINATE PROGRAM
        AH = 00h
        CS = PSP segment
Notes:  Microsoft recomments using INT 21/AH=4Ch for DOS 2+
        execution continues at address stored in INT 22 after DOS performs
          whatever cleanup it needs to do
        if the PSP is its own parent, the process's memory is not freed; if
          INT 22 additionally points into the terminating program, the
          process is effectively NOT terminated
        not supported by MS Windows 3.0 DOSX.EXE DOS extender
SeeAlso: AH=31h,AH=4Ch,INT 20,INT 22