Int 7C Fn 0004  - Sk-upps Data Link Interface API - Request Buffer         [N]

   AX = 0004h
   DX = protocol ID (see AX=0007h)

Return: AX = completion code (see also AX=0001h)
       0000h successful
       ES:BX -> Memory Buffer (Mbuf) (see below)

Note:  if the DLI has no free Mbufs, it will set an internal flag for the
     protocol and later call the protocol's Event Upcall with
     EV_BUFFER_AVAILABLE (see AX=0007h) as soon as an Mbuf becomes
     available.  There will only be one "buffer available" Upcall, even
     if this function has been called several times before.

See Also: AX=0005h,AX=0006h,AX=0012h,AX=0013h

Format of Memory Buffer (Mbuf):
Offset Size    Description
 00h   DWORD   -> NEXT Mbuf (linked list)
 04h   WORD    OFFSET of actual data within DATA area
 06h  2 BYTEs  reserved (currently unused)
 08h   WORD    LENGTH of actual data found in DATA area starting at OFFSET
 0Ah  2 BYTEs  reserved (currently unused)
 0Ch   DWORD   -> DATA area (size can be obtained via AX=000Eh call)
 10h   WORD    PROTOCOL ID of protocol currently "owning" the Mbuf
 12h   BYTE    IN USE flag; nonzero if Mbuf still in use by DLI (see AX=0006h)
 13h   BYTE    RECEIVE STATUS (bit 0-7 only; see AX=0007h - Receive Upcall)