This message is from the T13 list server.
In emails this weekend Pat seemed to be saying that T13 had removed/deleted some functionality from ATAPI DMA that would allow a device to indicate the number of words/bytes transferred (or indicate the "residue" words/bytes, I'm still not sure what Pat means by "residue") for DMA data transfer commands. So I reviewed what SFF-8020 (actually INF-8020) has to say about PACKET commands in DMA mode. I found nothing to indicate that ATAPI had the functionality Pat is requesting. So to say T13 removed it is not correct. In fact, except as noted in the next paragraph of this email, INF-8020 makes it very clear that there are no Byte Count values used at any time during PACKET DMA commands. However (and note to Pete), ATA/ATAPI-x is missing something in the PACKET command subclause for "data transmission". Here it says that the Byte Count is used only in PIO mode. That is not totally true. For a overlapped/queued PACKET command doing DMA data transfer, when the host issues a SERVICE command and the device wants to continue the data transfer, the Byte Count is used. This is how the host knows the transfer length to put into the host DMA engine. But of course to talk about trying to use overlapped/queued DMA commands (ATA or ATAPI PACKET) without having a byte/word counter in our wonderful x86 PCI bus ATA host adapters is mostly a waste of time because without that counter the host has no idea where to start the next part of the command's data transfer. So my next question for Pat is: What did T13 remove? *** Hale Landis *** www.ata-atapi.com ***
