Int 2F Fn 1901  U - DOS 4.x Only Shellb.com - Shellc.exe Interface         [l]

   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