> On Jul 21, 2016, at 4:22 PM, Peter Corlett <ab...@cabal.org.uk> wrote:
>
> On Wed, Jul 20, 2016 at 09:02:41PM +0200, Liam Proven wrote:
>> On 19 July 2016 at 17:04, Peter Corlett <ab...@cabal.org.uk> wrote:
> [...]
>>> RISC implies a load-store architecture, so that claim is redundant.
>> Could you expand on that, please? I think that IKWYM but I'm not sure.
>
> A load-store architecture is one where the ALU only operates on registers. The
> name comes from having separate instructions to load registers from memory,
> and
> store them to memory.
>
> The converse is register-memory, where ALU instructions can work directly on
> memory. However, this means that the instructions have to do quite a lot of
> work because now data has to be brought in from memory to an anonymous
> register
> to be worked on and then stored back to the same location. This also results
> in
> a proliferation of instruction and addressing mode combinations. Sounds rather
> CISCy, doesn't it?
>
> Meanwhile, a load-store architecture would have to decompose that into simpler
> independent load, operate, store instructions. Hey presto, RISC!
I would point out that RISC is not a goal in itself. Instead, the reason RISC
is interesting is that -- at least at some point in the evolution of technology
-- it was an architecture approach that results in more cost effective
computers. That is, faster for the same money or less expensive for a given
speed.
It's easy to be led to the equivalence "register-memory instructions" ==
"complex instructions". If you're most familiar with Intel x86, or even saner
architectures like VAX (which have memory to memory instructions, with a pile
of addressing modes), that seems plausible. But, for example, the PDP-8 has
register-memory instructions but it has a very simple instruction set and can
be implemented in a tiny amount of logic. Similarly, register-memory
instruction sets were used in many older architectures, and clearly were easy
to do -- if your logic is tubes, you're not likely to implement large execution
units! Even machines like the CDC 6000 peripheral processors, which have a
pile of extra logic for controlling the CPU and surprising features like a
barrel shifter, take only a few thousand gates.
paul
