Format of MBBIOS PACCOM buffer:
Offset	Size	Description	)
 00h 504 BYTEs	data area
1F8h	WORD	length of data in data area
1FAh	BYTE	flags/status
		bit 7: don't discard buffer after transmitting data
		bit 6: buffer has been transmitted
1FBh	BYTE	reserved (0) for additional flags/status
1FCh	WORD	user data
1FEh	WORD	MBBIOS-internal pointer to next buffer