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The problem is that in SCSI it involves a transition from DATA PHASE to MESSAGE PHASE. With the exception of a few messages that are always seen in a command (like IDENTIFY), most SCSI devices handle the message system in firmware, not hardware. This is because the messaging system for SCSI was never really "designed" - instead it evolved into a relatively complicated set of conditional transitions and events, best implemented in firmware rather than hardware. Going from Hardware to firmware is why you drastically lose performance. You lose some in just protocol overhead, but its executing the protocol in software that is the killer. Jim PS implemented the same way (in firmware) in ATAPI and you would probably have a similar problem - which is why any change involving the real time transfer of data could take years to work it's way into products. This is why the best place to put any such data is at the end of the command, since it is the most likely time for devices to be using firmware anyway (and if not it gives the device the entire command execution time to pre set up the indicator that the host will check at the end of the command). -----Original Message----- From: Pat LaVarre [mailto:[EMAIL PROTECTED]] Sent: Friday, December 07, 2001 11:59 AM To: [EMAIL PROTECTED] Subject: [t13] residue after the fact - a performance disaster? This message is from the T13 list server. > "Mcgrath, Jim" <[EMAIL PROTECTED]> 12/06/01 05:52PM > SCSI did allow for an odd byte indicator > (IGNORE WIDE RESIDUE message), > but I personally have never seen it being used > (it is a performance disaster). Why a performance disaster? I see the annoyance of sometimes having to avoid passing back in, and sometimes having to retransmit back out, the bytes of the residue that did clock across the bus. But I'd call that an annoyance likely to be missing from current Atapi Dma hardware ... not a disaster waiting to happen. No? Thanks again in advance. Pat LaVarre Subscribe/Unsubscribe instructions can be found at www.t13.org. Subscribe/Unsubscribe instructions can be found at www.t13.org.
