James C Hatfield james.c.hatfield at
Tue Apr 20 16:23:43 PDT 2010

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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, etc).
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>wrote:
> * 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
> *
> * For T10 Reflector information, send a message with
> * 'info t10' (no quotes) in the message body to majordomo at

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