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From: ehj@mordor.UUCP (Eric H Jensen)
Newsgroups: net.arch
Subject: Re: Tagged architectures
Message-ID: <3729@mordor.UUCP>
Date: Wed, 31-Dec-69 18:59:59 EDT
Article-I.D.: mordor.3729
Posted: Wed Dec 31 18:59:59 1969
Date-Received: Fri, 4-Oct-85 04:48:27 EDT
References: <796@kuling.UUCP> <1713@orca.UUCP> <1599@peora.UUCP> <335@ihlpl.UUCP> <2384@uvacs.UUCP> <412@ucsfcca.UUCP> <926@loral.UUCP> <1683@peora.UUCP>
Reply-To: ehj@mordor.UUCP (Eric H Jensen)
Distribution: net
Organization: S-1 Project, LLNL
Lines: 29
Keywords: tags, memory protection

In article <1683@peora.UUCP> you write:
>> And with a tagged memory system, you must read a memory word before
>> doing a write (to see if the tag allows writing).  This causes alot of
>> extra memory accesses and thus detracts from the performance achievable.
>
>No, it only makes it more expensive.  There are always* RAM parts available
>that are faster than those used for the main memory; it's just a question
>of whether you want to incur the extra cost.  ...

This discussion has me a bit confused ...

I don't understand why anyone would want to use tags for read/write
protection.  Whether you can read/write a memory location is either
determined by your page table entry for the current virtual space or
by some capability you possess.   The above is not related to the
current unitialized data debate because once a location is *written* it
is, by definition, initialized.

Tags are usually used to identify the type of data they are associated
with or the type of data the associated pointer points to (i.e.
integer, flonum, cons cell, gate, initialized, uninitialized, etc...).
It makes no sense to me to put protection information in a tag -
unless of course it's in the tag of a pointer and refers to the
protection of the data being pointed to - but thats capabilities
revisited.
-- 
eric h. jensen         (S1 Project @ Lawrence Livermore National Laboratory)
Phone: (415) 423-0229  USMail: LLNL, P.O. Box 5503, L-276, Livermore, Ca., 94550
ARPA:  ehj@angband     UUCP:   ...!decvax!decwrl!mordor!angband!ehj


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