INT 7C - SK-UPPS Data Link Interface API - SYNC REQUEST BUFFER
        AX = 0012h
        DX = protocol ID (see AX=0007h)
Return: AX = completion code (see AX=0001h)
        if AX=0000h: ES:BX -> Mbuf (see AX=0004h)
Note:   If the protocol wants to be informed when an Mbuf becomes available,
          REQUEST BUFFER (AX=0004h) should be used instead.
SeeAlso: AX=0004h,AX=0005h,AX=0006h,AX=0013h

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