paulkoning at comcast.net
Wed Jul 20 10:44:00 CDT 2016
> On Jul 20, 2016, at 9:56 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
>> From: Paul Koning
>>> I always felt that RISC meant 'making the basic cycle time as fast as
>>> possible by finding the longest path through the logic - i.e. the
>>> limiting factor on the cycle time - and removing it (thereby making the
>>> instruction set less rich); then repeat'.
>> "Making the cycle time as fast as possible" certainly applies, in
>> spades, to the 6600. The deeper you dig into its details, the more
>> impressed you will be by the many different ways in which it does things
>> faster than you would expect to be possible.
> My formulation for RISC had two parts, though: not just minizing the cycle
> time, but doing so by doing things that (as a side-effect) make the
> instruction set less capable. I'm not very familiar with the 6600 - does this
> part apply too?
Depending on what you mean by "less capable", I don't know that I would agree with that. For example, I doubt that anyone would argue MIPS isn't a RISC architecture. Yet MIPS is certainly very capable, and it certainly has a rather large instruction set. The key point is that those instructions are, by and large, conceptually straightforward, and lend themselves to efficient (small cycle count, small transistor count) implementation. Also, RISC does not use, or need, microcode.
In that sense, the 6000 certainly qualifies. It has load/store, integer and float arithmetic on registers, boolean ops, and basic transfer of control instructions. That's about it. And the implementation is certainly straightforward. A 6600 has a fair number of gates, but that stems from its multiple functional units, memory scheduling, and intense emphasis on speed, not from the inherent complexity of its instruction set. A 6400, which is a single functional unit implementation of the same instruction set, is a whole lot smaller.
I recommend the excellent (and rather short) book by Thornton, one of the 6600 designers: http://bitsavers.trailing-edge.com/pdf/cdc/cyber/books/DesignOfAComputer_CDC6600.pdf It will take you through the design all the way from transistor considerations and circuit elements to the instruction and memory scheduling machinery.
>>> RISC only makes (system-wide) sense in an environment in which memory
>>> bandwidth is plentiful (so that having programs contain more, simpler
>>> instructions make sense)
> I should have pointed out that programs of that sort take not just more memory
> bandwidth, but more memory to hold them. In this day of massive memories, no
> biggie, but back in the core memory days, it was more of an issue.
I don't think that's necessarily all that big a delta. Again using MIPS as an example, its program sizes are not that much larger than, say, the PDP11 for the same source code.
>> I think a lot of machine designers, though not all, were seriously
>> interested in making them go fast.
> Again, RISC has two legs, not just making the machine fast, but making them
> fast by using techniques that, as a side-effect, make them inscrutable, and
> difficult to program. The concept was that they would not, in general, be
> programmed in assembler - precisely because they were so finicky.
It is true that a few RISC architectures are not very scrutable. Itanium is a notorious example, as are some VLIW machines. But many RISC machines are much more sane. MIPS and ARM certainly are no problem for any competent assembly language programmer. Alpha is a bit harder but definitely doable too.
The burden on compiler optimizers tends to be higher. But I would argue that's true for any high performance design: those have multiple pipelines, caches and prefetch, and lots of other stuff that affects performance. The code optimizer has to know about these things. (So does the assembly language programmer, if assembly is used for performance rather than for esoteric bare metal stuff like boot or diagnostic code.) But, with the possible exception of extremely bizarre designs like Itanium, that's all perfectly doable.
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