On 18/01/2015 05:47, Miles Fidelman wrote:
PhillipJones wrote:
RISC - Reduced Instruction Set Computing Very little Code just
barely enough to get computer to boot burned into the code. System
Code has to tell it practically everything Therefor size is large
and Clunky and slow to load even with tons of system Ram But
advantage is if you want add features without changing processors,
you can. CISC - Complex Instruction Set Computing Most code to do
everything is built into chip. System is small Compact and loads
very fast even on slow systems. Disadvantage where so much
operation code is burned in It is difficult and or impossible add
new features without creating a New Chip with new code burned in.
Ahh, the topic of many long battles.
The argument for RISC is: - if you analyze compiled code, most of it
uses a small subset of a CISC instruction set, so why waste the
silicon - optimize for blinding fast speed on the reduced instruction
set and you come out ahead - I expect it's easier to do things like
preemptive look ahead and pre-fetch with a simpler instruction set
The argument for CISC is: - do it faster in hardware - write
compilers that take advantage of the breadth of the instruction set
There are certainly examples of very successful CPUs in both families
- e.g. SPARC for RISC, Intel's entire line for CISC.
I can also recall a couple of cases where very specific CISC
instruction sets made a lot of difference: - the old DG Nova's
macro-instructions looked very much like micro-instructions, with the
ability to do multiple instructions in parallel (e.g., rotate, shift,
and compare in one macro-instruction cycle) -- very useful for things
I think most modern CPU architectures these days support SIMD in one
form or other.
like optical character recognition - at one firm, I personally helped
develop a set of instructions specialized to high-speed bit twiddling
in a radar processing system (back in the days when 4MIPS was
blindingly fast, it was pretty much impossible to manipulate
nanosecond pulse trains without specialized hardware)
All major CISC cpu families these days are internally RISC (e.g. x64-AMD
from Intel and AMD) with a translation layer for CISC to RISC
conversion. So in a sense RISC has won.
Normally you can't access the underlying RISC core but at least one
manufacturer (I think Taiwan or China based) made a x86 CPU chip where
you could switch to the internal (and very different) RISC instruction set).
Phil
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