"reserved" means what, according to t10

Elliott, Robert (Server Storage) Elliott at hp.com
Mon Feb 3 12:01:56 PST 2003

* From the T10 Reflector (t10 at t10.org), posted by:
* "Elliott, Robert (Server Storage)" <Elliott at hp.com>

> -----Original Message-----
> From: Pat LaVarre [mailto:LAVARRE at iomega.com] 
> Sent: Monday, February 03, 2003 12:21 PM
> To: t10 at t10.org
> Subject: "reserved" means what, according to t10
> Eh?  What do T10 folk think "reserved" does mean?
> The story I heard was as follows.  How inaccurate is
> it?
> I first grew curious when I saw Rob E say "defining
> fields to be reserved generally means they must be
> tested for zero", 

That's what the letter ballot comment said, not me :-)

This is the definition SCSI standards like SAM, SPC, SPI, SAS
all use for "reserved".  Checking reserved fields and bits for
zero is allowed but neither mandated nor prohibited.

3.3.9 reserved: A keyword referring to bits, bytes, words, 
fields and code values that are set aside for future 
standardization. A reserved bit, byte, word or field shall 
be set to zero, or in accordance with a future extension 
to this standard. Recipients are not required to check 
reserved bits, bytes, words or fields for zero values. 
Receipt of reserved code values in defined fields shall 
be reported as error.

In SAS, when we have either:

a) bits that we want the recipient to ignore (so future 
features can be added which are known not to break 
existing implementations); or

b) bits that we want the recipient to check (so future
features can be added which are known to break existing

we define the bits as being part of a field, and specify
those rules for that field.  For example, see the FEATURES
and COMPATIBLE FEATURES fields in the OPEN address frame
(section 7.7.3 in sas-r03b).

Rob Elliott, elliott at hp.com
Hewlett-Packard Industry Standard Server Storage Advanced Technology

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