Replies to various threads

From: <(>
Date: Thu Apr 23 12:29:31 1998

[PDP11 risc or cisc]

Pete Turnbull:

> 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.

I'm glad somebody agrees with me on that! IMHO the concept of a GP
register is a RISC sort of thing. And, Allison, if you think RISC
should be register-rich, I claim the PDP11 was for its date, and
certainly was compared to micros of the 1970s.

> 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


I don't like the "one instruction per cycle" definition of RISC - for a start,
what is a cycle? I prefer to think of RISC as an "every cycle is sacred"
philosophy - you don't waste cycles. I'd try to get _memory cycles_ as often
as the hardware permits them - on the 6502, for example, one per cycle (and it
almost manages it!), on 8080/Z80/PDP one every two or three cycles - but I
wouldn't make them all instruction fetches!

> 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

Despite having a 11/44, I have never seen a Commercial instruction Set :-)

> 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.

Agreed. Later PDPs were more CISC, and this reached its maximum in the
Vax. But the basic architecture is IMHO a risc one - very simple and
very powerful.

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

Except the early ones. Allison, are you sure it was the 11/05? I claim
it was the 11/15 (I have an 05). However I will concede that 05 may
have at one time been a name for an 11/20 variant.

> 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
> added.

Fair enough.

> That's my third of a tanner.



PS I shall try and refrain from further comment on this issue - I don't
want to be the one who started a RISC versus CISC flame war!
Received on Thu Apr 23 1998 - 12:29:31 BST

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