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PCI, L2, and ISA

To: riscy@sunsite.unc.edu
Subject: PCI, L2, and ISA
From: Bill Broadley <broadley@neurocog.lrdc.pitt.edu>
Date: Sun, 30 Jan 1994 20:02:36 -0500 (EST)
I responded via private email, I'll repost it.

My justification for an L2 cache:

No L2 cache bothered me about the mips design.  It reduced the 60 specint92
60 specfp92 to about 35/35.  About equivalent to reducing a 586 class cpu
to 486 class (int wise).

The mips 4000 runs at 100 Mhz internally and 50 Mhz externally, and
gets almost twice the speed from the L2 cache.

The alpha 21066 runs at 166 Mhz internal and 33 Mhz external I'd expect
it to be even more dependent on L2 cache.  The smallest L2 cache dec ships
is the 256k (even on the 100 Mhz alpha).

Given the Alpha chip supports a L2, and cache ram chips are reasonably
cheap I'd think it would be a big loss to not have an L2 cache.

Typically when buying a pc mothebroard 64k to 256k cache upgrades are
about $30 or so (right?).  I'd guess we could get 512k L1 for well under 
$100 since 33 Mhz doesn't need a particularly fast L2 cache.  If it doubles 
the speed of a $300-$500 cpu it's well worth it.


My justification for PCI:

As for pci it would be a shame to throw away a state of the art bus
just to save the price of the connectors/board space since it's supported 
by the alpha chip.

PCI allows 10 "loads" on the bus.  Each connector, and each chip on the
bus is a load.  This translates to 3 or 4 PCI slots maximum.

So for the price of the connectors, and the increased functionality
of a state of the art bus makes it a good investment, and a shame
to leave out.


My justifcation to NOT have ISA:

ISA will cost the price of the connectors, board space, an ISA<->PCI bridge
chip, and a load on the PCI bus.

The benefits are adding any of the millions of the ISA cards, but this
isn't as good as it sounds.  ISA is as ugly bus you have to worry about
16 MB addressing limit, irq and I/O conflicts.  We can't use any
on board BIOS, we wont have drivers for any ISA cards, and we have
limited software development resources to write them.  

If the design is to expensive I'd vote for the ISA to be the first to go.
Real workstation do fine without it, and it's the last remnant of the
UGLY intel architecture to haunt the riscy design.


-- 
Bill Broadley@{neurocog,schneider3,lrdc5}.lrdc.pitt.edu (in order of preference)
Linux is great.         Bike to live, live to bike.                      PGP-ok
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