Path: utzoo!attcan!uunet!ncrlnk!ncrwic!encad!entec!jlohmeye
From: jlohmeye@entec.Wichita.NCR.COM (John Lohmeyer)
Newsgroups: comp.periphs
Subject: Re: SCSI-II ID limits
Message-ID: <371@entec.Wichita.NCR.COM>
Date: 18 Jun 89 05:39:08 GMT
References: <4589@ihuxz.ATT.COM> <3170007@hprnd.HP.COM> <8399@xenna.Encore.COM> <369@entec.Wichita.NCR.COM> <8585@xenna.Encore.COM>
Reply-To: jlohmeye@entec.Wichita.NCR.COM (John Lohmeyer)
Organization: NCR, E&M Wichita - KS
Lines: 29

In article <8585@xenna.Encore.COM> terryk@xenna.UUCP (Terence Kelleher) writes:
>Seems like extending identify to allow 32 units is a good idea.  Why
>are those bits reserved?
>
>Terry

They were reserved in case we needed them! Like anything else that is
reserved... 8-)  Actually, we didn't need them at the time, so they were
reserved for future standardization.  The term "reserved" always means
for future standardization (that is, until we (the committee) can think
of a good purpose).

It is not exactly just a matter of designating these bits for an extension
to the logical unit field.  We will also have to consider the impact of
this change on such commands as COPY, which has some 3-bit logical 
unit fields in its segment descriptors.  Nothing is easy. 8-(

By the way, there used to be 3 reserved bits in the IDENTIFY message.  One
was used to specify a target routine instead of a logical unit (so a command
can be directed to the target itself instead of a logical unit on the target).
This is just to show that reserved bits do get used occasionally...

---

-- 
John Lohmeyer         J.Lohmeyer@Wichita.NCR.COM
NCR Corp.             uunet!ncrlnk!ncrwic!entec!jlohmeye
3718 N. Rock Rd.      Voice: 316-636-8703
Wichita, KS 67226     SCSI BBS 316-636-8703 300/1200/2400 24 hours