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        4dos.com v2.1+

        Entry:
          AX = D44Dh
          BH = function
              00h installation check
                  Return: AX = 44DDh
                          BL = minor version number
                          BH = major version number
                          CX = PSP segment address for current invocation
                          DL = 4DOS shell number (0 for the first (root)
                               shell, updated each time a new copy is loaded)
              01h (internal) terminate current copy of 4DOS
              02h ???
                  DX = ???
              03h EXEC program
                  CX:DX -> EXEC record
              FEh deallocate shell number (passed through to root shell)
                  ???
              FFh allocate shell number (passed through to root shell)

        Format of EXEC record:

        Offset  Size    Description
         00h    WORD    offset of ASCIZ program name in same segment as EXEC
                        record
         02h    WORD    offset of DOS commandline in same segment as EXEC
                        record
         04h    WORD    segment of environment for child process
                        (see INT 21/AH=26h)