Re: How many times must Apple make such DRASTIC architecture changes?

["Michael J. Mahon" <mjmahon@aol.com>]

Tim Cutts wrote:
> In article <11ah0pqdup2vq4c@corp.supernews.com>,
> Laine Houghton <laine@intergate.com> wrote:
>
> > Aren't OS's being written and tested on the IA-64 and AMD 64 at this moment?
>
>
> 64-bit OS's have been around for a long time, but not in the mainstream.
> DEC alias Digital alias Compaq alias HP created the Alpha back in the
> early 90s.  That has had a full 64-bit address space for years.

Yes, and the MIPS R10000 was the first shipping 64-bit microprocessor.

> As to the IA64, it actually has a 50 bit physical address space (I
> think).  I have an SGI Altix at work which has 192 GB of RAM, in a
> single operating system image, and which a single process can access in
> one go.  SGI have built much larger machines for some customers; I
> believe there is a *single* machine in Japan with 13 TB (yes, terabytes)
> of physical RAM.  The SGI cluster NASA are building is a cluster of
> larger Altixes, in terms of CPU count, but they don't have as much
> memory per node.

And it is easy to extend the physical addressing support without
any impact on software (other than scale), by simply expanding
the width of the TLB and bringing the address lines out.  (Of course,
50 bits is 1K terabytes [an exabyte], so there's no rush... ;-)

As memory densities increase, the physical addressing can easily
expand.  Architecturally, logical addresses are already 64 bits.

> As far as I know, no-one except SGI currently markets machines which can
> take as much RAM as this.  Compaq used to - our Altix' predecessor
> machine was an AlphaServer GS320, which also had 192 GB of RAM.

It's a pretty small market, so if there is any fixed system cost
to support memory that is not installed, it's a cost-performance
loser for most users.

-michael

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