INT 7C - SK-UPPS Data Link Interface API(Application Program[ming] Interface) The defined set of calls which a program may make to interact with or request services of the operating system or environment under which it is running.  Because the inputs and outputs of the calls are well-defined, a program using the API can continue using the identical calls even if the internal organization of the program providing the API changes. - REQUEST BUFFER
	AX = 0004h
	DX = protocol ID (see AX=0007h,#03849)
Return: AX = completion code (see also #03846)
	    0000h successful
		ES:BX -> Memory Buffer (Mbuf) (see #03847)
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 #03853,#03854) 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 SK-UPPS Memory Buffer (Mbuf):
Offset	Size	Description	(Table 03847)
 00h	DWORDDoubleword; four bytes.	 Commonly used to hold a 32-bit segment:offset or selector:offset address.	-> 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	DWORDDoubleword; four bytes.	 Commonly used to hold a 32-bit segment:offset or selector:offset address.	-> 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)