Re: Tagged memory (was Re: Memory Upgrade)

What about resetting tag bit when any store is made to the pointer area,
other than by a pointer instruction?
Is it also software ?

    Alexei Pytel




                    "Leif Svalgaard"
                    <leif@attglobal.net       To:     <midrange-l@midrange.com>
                    >                         cc:
                    Sent by:                  Subject:     Re: Tagged memory 
(was Re: Memory
                    midrange-l-admin@mi        Upgrade)
                    drange.com


                    09/27/2001 01:55 PM
                    Please respond to
                    midrange-l





From: Alexei Pytel <pytel@us.ibm.com>
> > 2) the tagged pointer protection is not hardware, but is done in
> > software, so need not contribute to make memory more expensive.
>
> With all respect, the tagged pointer protection *is* hardware.
> It could in theory be done by software, but it would incur unacceptable
> overhead.
>

Well, the RISC code to check if a pointer is valid is:
LQ R6, addr  ;  load pointer into R6, R7. set bit in exception reg if no
tag
TXER nn ; trap if no tag

to me that is software. The trap can be disabled by replacing the TXER
instruction by a NOOP.

The overhead is the TXER instruction and is not unacceptable.



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