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  3. September 2001

Re: Tagged memory (was Re: Memory Upgrade)

From: Alexei Pytel <pytel@us.ibm.com>
> > 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.
>
> Are you saying that if you replace TXER instruction by NOOP and then go on
> to use
> untagged pointer, there will be no exception ?
>

in effect: yes.
It is called a counterfeit pointer.
People have known how to make these as long
as there have been AS/400s. They are not my
invention.

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