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

   AX = 1901h
   BL = SHELLC type
       00h transient
       01h 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