RE: 48-bit RISC

The CPU, OS, and programs are 64 bits.  It's just the system bus that
may not be.  But that's the case in most systems; the system bus width
and bus speed are independent of the CPU.

Take PCs & PC servers.  Higher-end boxes will have 64-bit PCI slots, but
the CPU is still a 32-bit Xeon.  Old desktops mixed 8-bit slots on
16-bit CPUs; later there were 32 bit CPUs with 16-bit bus slots.  You'd
think we'd be fine with current 32 bit PCI and 32 bit CPUs but in
reality there's still a huge difference as PCI is running at 33MHz and
the PC CPU clock is running 8+ times that.  RAMBUS based PCs only have a
16-bit wide memory path but it's very fast and makes up for the narrow
width with high speed.

At some point in the system architecture, on PCs it's in the
northbridge/southbridge chips, the data coming from the system bus
and/or RAM is converted from it's width and speed to the width & speed
of the CPU.  It's really no slight angainst a system to run the bus
widths & speeds asynchronously.

The only design problem is that the older/slower system bus may not be
able to feed the CPUs fast enough.  For individual IOPs/IOAs, this
generally isn't a problem.  When it becomes a problem is when the
aggregate traffic of all adapters exceeds the system bus width.  That
aggregate bandwidth, for instance, is why my 730 can't do a backup as
quickly as we want it to.  My tape drives can handle an aggregate of
120GB/hour, but the system only seems to deliver about 70.

- John

-----Original Message-----
From: Jan Megannon [mailto:jmegannon@xxxxxxxxxxxxx] 
Sent: Tuesday, April 29, 2003 2:01 PM
To: Midrange Systems Technical Discussion
Subject: Re: 48-bit RISC


Hi Dave,

Which I am well aware of. It is just that, if the RISC machines are
_true_ 64-bit, how come one could move cards from the CISC to the RISC
machines?

Cheers.

Jan.

On Tue, 2003-04-29 at 19:58, David Dunfield wrote:
> CISC machines were 48 bit
> 


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