INT 7C - SK-UPPS Data Link Interface API - REQUEST BUFFER
        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.
SeeAlso: 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)

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