Replies to various threads

From: Pete Turnbull <>
Date: Thu Apr 23 08:45:50 1998

On Apr 23, 13:14, wrote:
> First, thanks to Pete, Allison and others for explaining the PDP11-23
> stuff. I stand corrected, I suppose.

I just like to show off :-)

> A long running discussion. Allison, I don't understand how you can say
> that the PDP11, with its very simple instruction set, is _more_ CISC
> than (say) the 80286, with which you compare it here. To my mind the
> only really CISC feature of the PDP11 is the MARK instruction. I fear
> we may be talking at cross purposes, and may mean different things by
> RISC and CISC - could you give some specific examples, please?

I know that was directed at Allison, but I'd say that key features of RISC
architectures include large numbers of general registers,
one-instruction-per-cycle, and hardware decode rather than microcode, not
just the obvious minimised instruction set.

The PDP-11 architecture has only 7 GP registers (since you can't really use
the PC for just anything) but that's good for the times, and they really
are interchangable, so I'd be willing to argue that it wins on that.

It loses on the one-instruction-per-cycle, though. Instructions take
vastly different amounts of time to execute, depending on what they are,
and they're all several cycles long. Just think about the FP instructions,
or the Commercial Instruction Set. That's not the most CISC thing you've
ever seen? :-) At a more mundane level, the additions of instructions
like ASH is pretty CISC -- in fact the whole idea of extending the
instruction set by altering or adding to microcode is the essence of CISC,
and the antithesis of a Reduced Instruction Set Computer.

And of course it loses on the microcode vs hardware decode.

> For those who think a souped up PDP11 could be a pentium killer, bear in
> mind that there was a 32 bit PDP11. I don't mean the VAX, and I don't
> mean the PDP11-68: I mean the Motorola 68000. AFAICT the two
> architectures are very, very similar. Is it a Pentium killer? The
> 68070 might have been but it's rather faded away now...

Similar, but in many ways quite different. I just had this argument (from
a somewhat different point of view) on another mailing list. The 68K is
much more like a PDP-11 than anything else, but it has a lot of clutter

That's my third of a tanner.

Pete						Peter Turnbull
						Dept. of Computer Science
						University of York
Received on Thu Apr 23 1998 - 08:45:50 BST

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