Replies to various threads

From: Pete Turnbull <>
Date: Thu Apr 23 21:51:01 1998

On Apr 23, 14:33, Allison J Parent wrote:
> Subject: Re: Re[2]: Replies to various threads
> <> The PDP-11 architecture has only 7 GP registers (since you can't
> <> the PC for just anything) but that's good for the times, and they
> <> 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.
> Compared to maybe 6800 or 6502, the 8080 had 4 16bit registers (bc, de,
> hl, sp). The z80 added a second set and IX/IY. But that was only one
> aspect.

But you can't easily use both sets of registers at the same time (yes, I
know we sometimes do, but it's a fiddle) and the Z80 is very much a
single-accumulator type of beast. And as for IX and IY ;-)

The original design of the 6502, incidentally, was that all of zero-page be
treated as registers. They just happen to be external to the chip, which
wasn't completely unknown elsewhere in those days. So in that sense it is
possibly the most register-rich design of the era -- but the registers are
hardly general-purpose, and the 6502 is also a single-accumulator design.

> On the instructions RISC systems of the time and even later didn't have
> the addressing modes and often had a distinct register load and store
> instruction. The best example of that difference was an ADD (R1),_at_(r2)+.
> Now compare that to the DG Nova and it is of a stark difference.

If you count all the ways you can index with registers, MIPS processors
have quite a few addressing modes. Not all are used very often, though.

> Of all the micros in my collection, none are RISC save for the PDP-8 and
> 6502 which in my mind come close.

The 6502 has a certain elegance of instruction set. Quite a different
philosophy to the Z80, in many ways, but I like them both. We used to say
that you had to learn how to use the 6502, and when you did, the code was
neat, but on a Z80, you just had to decide what you wanted an instruction
to do, and then pick the one that did that. Exaggeration, of course.

> I have: 1802, SC/MP, 6800, 6809, NEC D78PG11, 8748/9, 8751, 8080/8085,
> z80, z180, z280, z8002, z8001, 808x, 8018x, 80286, 80386, 80486 and the
> micro version of minis 6100(pdp-8), 6120(PDP-8+EMA) TI9900, PDP11(T-11,
> F11, J-11).

Showoff :-) No 8008? I always wanted a 4004 (anyone listening out there,
that's a hint) and an SC/MP. Anybody remember Fairchild F8's?

> Now something with a MIPS chip, ARM, sparc or some such would be a great
> addition of a real RISC processor.

Well, I've got all of those, and my favourite is the ARM. I've had to
write MIPS assembler, and it's not great fun.

Pete						Peter Turnbull
						Dept. of Computer Science
						University of York
Received on Thu Apr 23 1998 - 21:51:01 BST

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