Mark Overby MOverby at
Wed Apr 21 07:12:28 PDT 2010

Formatted message: <a href="">HTML-formatted message</a>

Well, except for a few command that use the non-data protocol of course. But
Jim is quite correct on the DMA and PIO protocols, all of them require data
On 4/20/10 6:23 PM, "Jim Hatfield" <James.C.Hatfield at> wrote:
For ATA (SATA or PATA), the Count field should NEVER be zero, because neither
interface supports zero-length data transfer for any protocol (dma, pio,
In ATA, 'N/A' is defined the way it is because there are some ancient
implementations for which there are vendor specific differences.
If a NOTE is needed, perhaps that is beneficial for those not as familiar
with ATA.
Thank You !!!
Jim Hatfield
Seagate Technology LLC
   e-mail:  James.C.Hatfield at
   s-mail:  389 Disc Drive;  Longmont, CO 80503 USA
   voice:  720-684-2120
   fax....: 720-684-2766
On Tue, Apr 20, 2010 at 12:43 PM, Douglas Gilbert <dgilbert at>
* From the T10 Reflector (t10 at, posted by:
* Douglas Gilbert <dgilbert at>
While debugging some USB-3 storage devices (using
the mass storage class rather than UAS **) the
linux community has stumbled across confusion on
the sector_count field in the ATA PASS THROUGH (12)
and (16) commands defined in SAT and SAT-2.
One tool wants to send an ATA IDENTIFY DEVICE command
through the SAT pass-through to a SATA disk inside
an external USB enclosure. The SCSI ATA PASS THROUGH
command fields are set like this to fetch the 512 byte
response from the IDENTIFY DEVICE command:
   PROTOCOL:	4   [PIO DATA-in]
   T_DIR:	1   [data-in]
   BYTE_BLOCK:	1   [transfer unit is 512 bytes]
   T_LENGTH:	2   [use sector_count]
   SECTOR_COUNT: 1 or 0 ??
SAT indicates that SECTOR_COUNT should be 1. However
those familiar with ATA see that SECTOR_COUNT maps
to the COUNT field in the ATA IDENTIFY DEVICE command
and that is marked as "N/A". And they interpret that
to mean 0. According to ACS-2 the "N/A" keyword "has no
defined value and should not be checked by the host or
device". So 1 would seem to be permissible but not
obvious to ATA tools tunnelling through SAT.
The SATL in the USB-3 case failed when sector_count=0
but there seem to be plenty of SATLs out there that
accept it and respond with a block of data.
May I suggest an Annex to SAT-3 to show the encoding
of very common ATA commands like IDENTIFY DEVICE through
the ATA PASS THROUGH commands.
** Folk from Intel recently presented some work on the
    "UASP" transport to Linux community. A suggestion
    that the standard will be called "UAS" seem to fall
    on deaf ears.
Doug Gilbert
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