DOS Internal - Shellc.exe interface

        Entry:
          AX = 1901h
          BL = 00h if SHELLC transient
               01h if SHELLC resident
          DS:DX -> far call entry point for resident SHELLC.EXE
        Return:
          ES:DI -> SHELLC.EXE workspace within SHELLB.COM

        Note: SHELLB.COM and SHELLC.EXE are parts of the DOS 4.x shell