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HARD03.TXT
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1993-09-30
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RISC
By: TIM FARLEY
RISC stands for "Reduced Instruction Set Computer", which is a
theory that someone came up with that if you severely reduce the
complexity of operations that a computer can perform, limiting it
to a small set of very simple operations, you can build a much
faster computer.
For instance: the CPU's in your PC-compatible have a very
complex, involved instruction set. The instructions can be from
one byte to umpteen bytes long (I forget the exact maximum). The
execution times vary from one clock cycle to hundreds of clock
cycles per instruction. All of this makes the CPU very complex
and causes it to be very hard to predict the execution times of
software. Optimization of software is also very hard.
On a RISC computer, all instructions are generally designed to be
(nearly, or exactly) the same length in memory, and to execute in
(again, nearly or exactly) the same amount of time. This makes
the internal design of the processor simpler, and it means that
you can process instructions more evenly as you go. This in
theory leads to faster processors and better optimized programs.
What you lose in a RISC machine is neat instructions like, for
instance, the built-in integer MULtiplication and DIVision
instructions in the 386. Instead, these get replaced with
software subroutines that run nearly as fast.
The jury is out on RISC, as it remains unclear whether it has
real benefits. Meanwhile, vendors of non-RISC processors like
Intel have included "RISC like" features in their CPU designs to
increase speed.
For instance, the 486 will "pipeline" instructions so it can
partially execute the next instruction before it is done with
this one. This is a feature that originally came about in RISC
machines.