INT 61 - Banyan VINES - GET SERVER SERIAL NUMBER
        AX = 0004h
        DS:DX -> request block (function 0008h)
Return: AX = status
            0000h server ID returned in request block
            000Fh invalid drive
            0015h drive not ready

Format of request block:
Offset  Size    Description
 00h    WORD    0008h
 02h    WORD    drive number (0=default, 1=A, ...)
 04h  6 BYTEs   buffer for server ID

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