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> Just looking to be educated > What is all this about -:? Here's another try at nutshelling the issue, snipped from some offline conversation associated yesterday with the "ohhh"s I'm hearing: > > UDma indeterminacy ... > pause ... only matters ... in the middle. That would be true only if in advance the sender and the receiver always agreed where the middle ends ... > UDMA is no different than PIO or DMA in regard to number of words True [but not significant]. I think I see you focusing on the exchange of WORDs across the bus. Let me ask you to turn your focus awhile to a slightly larger question: how do the host & the device come to agree how many BYTEs have crossed the bus? For example, suppose we have an op x12 Inquiry for 5 bytes. How can a Usb bridge know it should pass 5 bytes on to its host, not 6? We can't afford to teach the bridge precisely how the ... drive connected at the moment parses every command, even if we knew. With AtapiPio, we don't have to. The drive reports in x1F5:1F4 how many bytes it means to move. Maybe that's x00:05. Maybe x 00:04 + 00:01. Maybe x 00:02 + 00:02 + 00:01. Whatever, we know precisely the correct byte count to pass on. We can handle anything from 0 to xFF:FF:FF:FF, no worries. With any form of Ide Dma, life gets harder. We get only the status INTRQ, no DRQ INTRQ with x1F5:1F4 reports of byte count. By definition, the drive can only ask to move whole "word"s. We lose the ability to distinguish an Inquiry for 5 bytes from an Inquiry for 6, unless the drive helps us out. With Ide UDma and data out, life gets harder still. What's new there is that there is no protocol to tell the sender which terminating pause is the final terminating pause of the command. So that sender can't know to avoid sending more clocks past the receivers request to pause to terminate. So the receiver ends up receiving more bytes than it would have moved in Pio mode. Teaching the bridge and the device to report a count of these extra bytes gets us back as much information as we had in Pio days, so we can precisely reproduce the bridge-to-Pio behaviour that we know works. ... Subscribe/Unsubscribe instructions can be found at www.t13.org.
