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[ BC [EMAIL PROTECTED] ] > Hale Landis 04/14/02 16:49 PM > the ATAPI/SCSI CDB tells the device > and the host how much data to move. > Otherwise you have yet another problem: > What happens to data the device > wants to move that is beyond the end > of the application program's I/O buffer? Yes this problem you pose here is very real. Yes implementations vary over how robustly they handle this problem. This problem appears as the "negative residue" H < D cases 2, 3, 7 and 13 in the taxonomy at http://members.aol.com/plscsi/ftf.html#minus There we divide normal life into three cases: copy no data, copy data In, copy data Out. This leaves ten other anomalous cases to discuss: "Fundamentally, the host can ask to copy no bytes, some bytes In, or some bytes Out. Symmetrically, the device can ask to copy no bytes, some bytes In, or some bytes. Two players, each with three independent choices, together make 3x3 = 9 possible combinations. Furthermore, when the host and the device do agree to copy some bytes In or Out, then still either one may ask to copy less bytes than the other. That adds 4 cases, for a total of 13 fundamental possibilities to discuss. "The committee that wrote the specification for Scsi over Usb published a diagram of these 13 cases. Specifically, we mean the diagram that appears as "Table 6.1 - Host/Device Data Transfer Matrix" in the section titled "6.7 The Thirteen Cases" of the 103,609 bytes of http://www.usb.org/developers/data/devclass/usbmassbulk_10.pdf . A copy of a late draft appears at 20000125/13cases.gif . Visually, personally, I prefer http://members.aol.com/plscsi/13cases.gif This is my drawing of a Scsi-over-whatever diagram analogous to that Scsi-over-Usb diagram. I include this diagram in the discusion at http://members.aol.com/plscsi/ftf.html#thirteen I prefer it because I drew it to visually divide the thirteen cases into just four groups: 1. (D = H) The Thin Diagonal (Cases 1, 6, 12) 2. (D < H) Positive Residue (Cases 4, 5, 9, 11) 3. (H < D) Negative Residue (Cases 2, 3, 7, 13) 4. (I != O) Wrong Way (Cases 8, 10) Pat LaVarre
