"reserved" means what, according to t10

Elliott, Robert (Server Storage) Elliott at hp.com
Mon Feb 3 15:49:19 PST 2003

* From the T10 Reflector (t10 at t10.org), posted by:
* "Elliott, Robert (Server Storage)" <Elliott at hp.com>
> -----Original Message-----
> From: JimMcGrath at oaktech.com [mailto:JimMcGrath at oaktech.com] 
> Sent: Monday, February 03, 2003 3:00 PM
> To: LAVARRE at iomega.com
> Cc: owner-t10 at t10.org; t10 at t10.org
> Subject: Re: "reserved" means what, according to t10
> Pat,
> There was some confusion on this topic years ago, but that 
> has not been the case for years.  Essentially all T10/T13 
> standards use the following words:

Not quite...

> ...
> in accordance with a future extension to this standard. The 
> recipient shall not check reserved bits, bytes, words, or 

The "shall not check" is actually "are not required to check" or
"may not check" (meaning "may or may not check" according to 
T10 in 1999) in:
SAM-2, SAM-3
SPC-1, SPC-2, SPC-3
SPI-2, SPI-3, SPI-4, SPI-5
SBC-1, SBC-2
SSC-1, SSC-2
MMC-3, MMC-4 (MMC-1 and MMC-2 used "shall not check")

As I recall from the "may/may not" discussion in 1999, T10 felt it 
was OK if certain command set or protocol standards chose to 
mandate or prohibit checking, but felt that most standards should
take no position.

> fields. Receipt of reserved code values in defined fields 
> shall be treated as a command parameter error and reported by 
> returning command aborted."
> ...
> As you can tell, the wording is essentially identical.
> So while the values should be set to zero by the sender (so 
> that there is a known, legacy value used), the recipient is 
> NOT suppose to check it.  This allows the sender to use the 
> same values for recipients at various levels of standards 
> support.  However, it is up to the sender to recognize that 
> sending values to a recipient that does not support the 
> feature will not lead to the intended result.  The only 
> exception for this is the reserved command codes, which are 
> checked by the recipient.

The standards are very consistent in stating that reserved values 
in defined fields shall be checked.

> Note that hosts are well situated to determine what a device 
> supports, since the device typically indicates its standards 
> compliance level in the returned identification data.  
> Devices are not usually so blessed - it is difficult for a 
> device to determine what a host supports.
> Jim

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