Descriptor Sense question

Rajiv Madabhushi RMadabhushi at
Mon May 9 19:15:51 PDT 2011

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I have some questions regarding the Descriptor Sense format.  In SPC4 (I'm
looking at rev 29),
4.5.1 Sense data introduction
The RESPONSE CODE field shall be set to 70h in all unit attention condition
sense data in which:
a) the ADDITIONAL SENSE CODE field is set to 29h; or
b) the additional sense code is set to MODE PARAMETERS CHANGED.
I interpret this to mean that the various types of resets (POR, bus reset,
etc.) as well as mode parameters changed should be in fixed format...
Seems clear enough that sense bytes sent for subsequent host commands
following these particular unit attentions may (or may not) be in Descriptor
Format, and the onus is on the host to understand that.
1.	 I was wondering if the drive (target) were coming ready (sending
02/04/01 K/C/Qs), say on a POR, may those sense bytes be sent with Descriptor
During that time, the drive would not be honoring mode sense commands, so I
was curious to know how the host would know...
I suppose looking at the response code (70h vs 72h, for example)...?
My second set of questions is regarding the request sense command itself...
if the drive (target) were not SPC4 compliant, and thus did not support the
D_SENSE bit in control mode page 0x0A, and thus did not support the DESC bit
In the request sense cdb, I see according to table 263 in section 6.29 that
it will ignore the DESC bit and continue sending sense bytes in Fixed Format:
Table 263 - DESC bit
Note: Device servers that are compliant with SPC-3 are capable of ignoring a
DESC bit that is set to one.
So this makes sense to me I think... if the D_SENSE bit in mode page 0x0A is
0 (CLEAR), and a request sense command with DESC bit is 1 (SET)... then the
SPC-3 compliant drive will return sense bytes in fixed format... and the host
will have to honor that, I assume.
2.	 Now if the drive were SPC-4 compliant, but the D_SENSE bit in mode
page 0x0A is 0 (CLEAR), but the request sense command's DESC bit is 1(SET)...
what does that mean?  Does the request sense command's DESC bit take
precedence over the D_SENSE bit in the mode page?
3.	 Next, what about the other configuration... if the D_SENSE bit in
mode page 0x0A is 1 (SET), and a request sense command's DESC bit is 0
(CLEAR)... again, does the command's DESC bit take precedence?
4.	 What about cases where some hosts are SPC-4 compliant, and others
are SPC-3 compliant... If the SPC-4 compliant host were to set the D_SENSE
bit, and expect sense bytes in Descriptor Format... then the SPC-3 host I
assume would not be able to speak with the drive?
Thanks in advance.
-Rajiv Madabhushi
Pliant Technology
(408 321-0320, x108

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