[Gllug] Laptops?

[Rich Walker]

Henrik Morsing <henrik at morsing.cc> writes:

>> If you look at the SPEC2000 results for the IBM Power4 chips clocked at
>> 1GHz, you find them getting 610-630 SPECint base and 630-680 SPECint
>> peak. This is the same as a 2GHz Pentium-4 on an Intel motherboard, or a
>> 1GHz Itanium-2 in an SGI box, or an Athlon XP 1800+ on an Epox
>> motherboard, or a 1GHz Alpha box. Now, it's true that the Power4 is a
>> little more heavy-duty than the G4, but it's the same basic core - the
>> substantial difference being ALTIVEC.
>
> Do you have any links to document this? It is my understanding that the
> POWER4 and G4 is pretty far apart although I don't know much about
> small computer systems CPUs. 

Recollections of comp.arch as the main citation. My recollection is that
POWER4 has *much more* infrastructure, bigger caches, more
fault-recovery, and comes on multi-chip modules, as well as being tested
to hell and back. Whereas the G4 is the same core, with all the
interesting mainframe bits removed. 

[fx: goes and checks, comes back red-as-his-t-shirt]

Whoops. Brain-fart day. 

The POWER4 is a full 64-bit mainframe monster. 

The PowerPC970 is the cut-down version of the POWER4 without the massive
multi-CPU multi-chip-module but with the addition of ALTIVEC.

The G4 is Motorola's continuation of the architecture that started with
the 601. Differences between these and the IBM POWER chips are
discussed:

http://www3.sk.sympatico.ca/jbayko/cpu5.html

and I'm not going to quote too heavily.

So: POWER is full-64-bit. PowerPC is cut-down for consumer use. 

> What is ALTIVEC?

ALTIVEC is the vector processing unit that motorola came up with. It's
actually a vector-processing unit, as opposed to the
single-instruction-8-bytes-of-data that MMX or 3DNOW are. 

> Is the SPEC numbers you state for one processor core or for one chip?

The POWER4 does have 1 or 2 cores per L2 cache - the SPEC figures I
quoted were for 1-processor - so I assume 1 core in use. 

> It also seems a bit excessive that IBM charges £8000 for a processor
> that's basically just a G4.

My bad. The POWER4 compares to the G4 like a SUN compares to a Time
Computers box.

The POWER4 is a monster of a thing. Someone described the key difference
between IBM's mainframe technology and everyone else's technology as
"being able to provide pins by the thousand" - and when you consider
that most CPU's wind up being io-bandwidth-limited, an extra thousand
pins can be handy...

>
>> I don't think there are any CPU's left with a CISC core - except
>> possibly the Itanium. In all other cases, a front-end decoded translates
>> the visible ISA into an internal ISA that is essentially RISC-like. The
>> width of this decoder can be a limiting factor for the CPU performance.
>
> As far as I know, mainframes still use CISC processors but I only have
> this from colleagues that should know (but they might not).

Well, the POWER series is a heavy-duty-ISA series - it has instructions to do
compression and decompression on the fly to and from RAM. On the other
hand, it also has huge amounts of simulation data to show that the extra
complexity of each of those ISA components is actually worthwhile -
which is the key difference between CISC and RISC...



-- 
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