Tagged memory (was Re: Memory Upgrade)

[thomas@xxxxxxxxxxx]

Leif:

I agree it's technically feasible to handle this using conventional memory. 
Various forms of 'tag RAM' are used elsewhere for various reasons, even for 
Intel processors, I think most commonly for cache memories. There are always 
alternatives.

And it might indeed be valid to state the reason as being pure profit for IBM. 
That's simply presenting the reason from a non-technical viewpoint, although I 
have to say that 'for profit' is the ultimate reason any business does these 
things anyway and should be understood without saying. To answer Dale's 
question that way would just leave him hanging.

However, I can't totally accept your second point for a couple reasons which 
I'd gladly abandon if you can elaborate on what you wrote. First, the 'tagged' 
memory is a hardware fact; the tag bits physically exist. They might be 
manipulated by software, I can't ultimately prove or disprove that though I 
don't doubt it nor would I argue that you can't do it; but 'tags active' is a 
PPC processor mode and that mode affects instruction execution related to those 
bits.

Whether IBM's implementation is expensive due simply to profit motive or to 
technical requirement is a separate question that can only be answered by IBM. 
I'm sure some chip engineers would argue one way and others would argue the 
other way. If nothing else, economies of scale argue that profit is a 
determining factor here.

Tom Liotta

On Wed, 26 September 2001, "Leif Svalgaard" wrote:

>
> From: <thomas@inorbit.com>
> On Tue, 25 September 2001, "Draper, Dale" wrote:
> > Why exactly is it that IBM does not use PC standard memory chipsets?
>
> Maybe someone else can say it better but AS/400s need special memory. Every
> 64 bits of AS/400 memory are accompanied by a special-purpose 65th bit, the
> 'tag' bit. In short, this extra circuitry provides critical function such as
> 'tagging' pointers in memory so that they cannot be modified except through
> approved interfaces. This is a major part of hardware security enforcement
> and the kind of thing that sets AS/400s apart. Such memory can be used on
> various other PowerPC systems that can run in 'tags inactive' mode, buth
> other memory cannot be used in AS/400s.
>
> ÝÝ> Tom, you are overstating the case a bit:
> 1) the tag bit could be stored in a byte, meaning that no special memory
> bandwidth
> is needed.
> 2) the tagged pointer protection is not hardware, but is done in software, so
> need
> not contribute to make memory more expensive.
>
> As we all know, the reason AS/400 memory is so expensive is simply that IBM
> charges for it what the market will bear (as per its monopoly position);
> this is called "milking the cow".
>
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--
Tom Liotta
The PowerTech Group, Inc.
19426 68th Avenue South
Kent, WA 98032
Phone  253-872-7788
Fax  253-872-7904
http://www.400Security.com


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