Path: utzoo!mnetor!uunet!mcvax!unido!mucmot!ron From: ron@mucmot.UUCP (Ron Voss) Newsgroups: comp.arch Subject: Re: RISC a short answer?? Message-ID: <307@mucmot.UUCP> Date: 2 May 88 09:33:05 GMT References: <1036@nusdhub.UUCP> <21149@pyramid.pyramid.com> Organization: Motorola GmbH Microsystems Munich Lines: 35 Summary: Real estate significant From article <21149@pyramid.pyramid.com>, by csg@pyramid.pyramid.com (Carl S. Gutekunst): > In article <1036@nusdhub.UUCP> rwhite@nusdhub.UUCP (Robert C. White Jr.) writes: >>Can someone give me [short answer style] a description of what "RISC" means. > > Can we add this to the list of frequently asked questions? > > My favorite answer is this: > > RISC (Reduced Instruction Set Computer) is a design philosophy that > trades off decreased complexity in hardware for increased complexity > in software. The instruction set is specifically chosen to provide a > set of primitives that are most usable by compilers. Then these few > instructions are made to run as fast as possible. The compilers are > given the responsibility of building the primitives into the higher- > level constructs used by the language. > > By implication, the compilers becomes as much a part of the design as the CPU > itself. > > This still leaves a lot of room for different implementations. For example, > [good discussion deleted for brevity] > design. So it becomes difficult to generalize. > > It's not that I disagree with Carl, it's just that I think he's left out what for me is the most central issue: Dedicating more of the finite chip real estate to circuits whose chief function is to provide increased processor speed. The most significant consequence is that there is no room left to implement more complex instructions. The theory (proven?) is that there is a net increase in performance, since processors spend most of their time at relatively simple operations, like load, store, add. More complex (now missing) instructions, like "toggle bit n of byte m" may end up running slower in emulation, but the increased speed of simple instructions more than makes up for it.