INT 21 - DOS 1+ - TERMINATE PROGRAM
        AH = 00h
        CS = PSP segment
Notes:  Microsoft recommends using INT 21/AH=4Ch for DOS 2+
        execution continues at the 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=26h,AH=31h,AH=4Ch,INT 20,INT 22

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