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100 CHAPTER THREE
These instructions are decoded at very high speed within the computer and are exe-
cuted immediately.
Years ago, computer companies built computers the size of refrigerators and they
tried to sell customers on the richness of their instruction set. The truth is, almost
nobody cares about that. People buy computers based on almost every other reason
other than this. So why should we care?
If we’re building a robot and we’re watching our budget, we should select the com-
puter carefully. Some computer chips will match our requirements better than others.
Looking at it the other way, given a more powerful computer chip, we can often make
savings by tailoring the robot’s algorithms to the power within the computer. So let’s
take a look at some of the wrinkles that have come along in instruction sets.

RISC

RISC stands for Reduced Instruction Set Computer. The imaginary 8-bit instruction set
shown earlier is similar to a RISC instruction set. The instructions are elemental and
can generally only perform one small computation at a time. RISC machines were sup-
posed to get their power from blinding speed, even compared to computers with more
complex instruction sets.
RISC computers were touted a decade ago as a major advance in computer hardware,
designed to significantly speed up computations. The technology did not, by any means,
take the computer world by storm. Many RISC computer designs are still around, such
as MIPS, ARM, and others. They generally have smaller, simpler semiconductor dies
and can be incorporated into ASICS much more easily than larger computer cores. But
they have found not found their niche because of their speed. Rather, they’ve found their
place in low-power designs and in the relative transportability of the designs. One of
the best ways to see what an RISC computer can do for the robot design is to simply
look at several other designs the computer chip has been used in.
Here are some web sites on the history of RISC computers, discussing them in
greater depth. In addition, some articles show the advantages of RISC over Complex
Instruction Set Computers (CISCs), which we will talk about shortly. Some of the arti-
cles are years old but still have a relevance:

http://copland.udel.edu/ anita/risc.html
www.cs.washington.edu/homes/lazowska/cra/risc.html
www.appliedembeddeddesign.net/design_riscCisc.asp
www.ccs.neu.edu/groups/honors-program/freshsem/19951996/utopia/risc.html