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> Subject: RE: [t13] RE: countable data bytes > From: Larry Barras [mailto:[EMAIL PROTECTED]] > Sent: Mon 1/13/2003 6:31 PM I'm dividing my reply into "countable data bytes", and "Reading last block", hopefully that helps. > Under Mac OS X SPI (System Programming Interface) > for dispatching CDB's into hardware, the byte count > per transfer is not determined on the basis of the > CDB. The count is a separate field of the command > object structure. Thanks for finding the time to explain this. No surprise yet: as far as I know that's how "everyone" does ATAPI. > For ATAPI devices, if the device byte count does > not agree with the value from the command > structure, the controller driver will attempt to > reconcile the difference if possible. Eh, sorry, lost me, how is "reconciliation" ever "possible"? > Otherwise, the command will usually be returned as > a failed IO to the device driver stack. Rumour tells me that a FireWire device designed to halt if the separate byte count doesn't agree with the command works great on a Mac, not so good on Windows. That suggests Mac works harder to agree always. A correlated rumour is the idea that a USB Mass bus trace gets less STALL's on a Mac bus trace, because STALL's occur there whenever host & device disagree. > Under Mac OS X, IO requests for data from ATA disks > are directly interpreted into an ATA command and > sent to the controller. I'm guessing ATA differs from ATAPI here - ok not fundamentally, but yes in practice? Old proprietary and newly standard commands are more rarely passed thru ATA? > The driver at this layer also manages power states, > resets and configurations and so on. Since there is > no CDB layer for ATA hard drives, there is no CDB > interface possible anyway. I'm not sure I follow you here? An ATA command, its data transfer count & mode, etc. COULD be decided by an app and passed thru, same as ATAPI. No? People just don't do ATA that way much, though they do ATAPI that way all the time. Curiously yours in boundless ignorance, Pat LaVarre
