This message is from the T13 list server.
On Mon, Aug 01 2005, Hale Landis wrote: > This message is from the T13 list server. > > > The history... > > Alexander Krebs wrote: > >This message is from the T13 list server. > >Hi all, > >Could anybody please explain why the Features registers hold the > >sector count whereas the Sector Count registers hold the tag for the > >READ/WRITE FPDMA QUEUED commands? This looks weird to me. > > Many years ago, the ATAPI folks, working outside of X3T9.2 and X3T10, > developed the idea of "tagged command queuing" for ATAPI devices. The > PACKET command uses the Sector Count register as the Interrupt Reason > register and the tag value is need along with the interrupt reason data, > so they put the tag value into this register. I've never seen an ATAPI > device that implements "tagged command queuing". > > Also many years ago, some ATA hard disk folks thought that ATA devices > should also have "tagged command queuing". These ATA folks decided that > since the tag data is needed by host controller hardware at interrupt > time, the tag data should also be in the same bits of the Sector Count. > Of course this means that the ATA sector count must be moved somewhere > else and the only place was to the Features register. There have been a > few ATA devices and host controllers that implement "tagged command > queuing". Ah, makes perfect sense (talking about why it was implemented that way, not a comment on the actual implementation). Thanks for the explanation Hale! > Most recently we can only assume that the SATA Secret Society also > decide to maintain the tag data in the same ATA registers for backward > compatibility. But of course we will never know because we don't see the > minutes of their meetings, or even who attended their meetings, and > perhaps worse, T13 is not allowed to discuss SATA items without the > prior approval of the Secret Society. I've have not seen any indication > that T13 has been given permission to discuss the First Party DMA QUEUED > commands. A question for the T13 officials would be: Has such > approval been given and where is the letter that says this (what is the > T13 doc number for that letter)? commands. But then again, I usually don't bother too much with politics. It tends to interfere with work. That doesn't mean I don't think it's important, but one can only do so much and I have to choose. -- Jens Axboe
