Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!mips!apple!motcsd!mcdcup!mcdchg!att!cbnewsc!santol From: santol@cbnewsc.ATT.COM (shawn.r.antol) Newsgroups: comp.sys.ibm.pc Subject: Re: Why The Move To RISC Architectures? Summary: Just my small opinion on RISC vs. CISC Message-ID: <14535@cbnewsc.ATT.COM> Date: 20 Mar 90 23:50:35 GMT References: <28011@cup.portal.com> Organization: AT&T Bell Laboratories Lines: 67 Someone wrote: > Also, since the 80386 has a more complex instruction set and does > more work in a given instruction than does a typical RISC chip, > does comparing MIPS figures between RISC and non-RISC > architectures really tell you anything of worth? Absolutely not. The figures must be taken with respect to some type of standard, ie. application program that is common to both architectures - compiled by the same compiler. Good luck finding this. The problem in comparing the two is easily explained (see below). > Finally, why is everyone so excited about RISC? Why the move to > simplicity in microprocessor instruction sets? You would think > that the trend would be just the opposite - toward more and more > complex instruction sets - in order to increase the execution > speed of very high-level instructions by putting them in silicon > and in order to make implementation of high-level language > constructs easier. Simplicity in design means LESS board space for the CPU. Of course nobody is going to switch just because of board space, but imagine using the *extra* space for caching, floating point hardware, etc. The RISC architecture is well know for its very high MIPS rating. Most notably, one machine instruction during EVERY clock cycle. Compare that with 3-25 clock pulses for a CISC architecture and you can easily see that the RISC can outperform the CISC in SOME applications <--Very important! Consider doing an integer multiply in CISC: Load the registers and let the CPU do it! in RISC: Load more registers, AND write the routine to multiply. It takes more machine cycles but might do it faster! The RISC/CISC debate is a very hotly debated subject and I can't wait to see all of the email this article will cause :-( One reason people prefer CISC is that it has a large number of unique machine language instructions. However, studies have shown that say, for a CPU with 300 instructions, only 20-45% of the unique instructions are utilized by people coding in assembly language. So you say, "who in the hell codes in assembly language these days?" Although there are quite a few who do/need to code in assembly, the truth is that compilers are taking over the show and all in all, the space required to put the added complexity on the CPU does not outweigh the benefits of having complex instructions because the compilers use even LESS of the unique instructions available in CISC. One very important point is that as architectures are performing more and more parallel processing, the COMPILER becomes the determining force for speed in code execution. It is my opinion that one needs to closely define what his/her application is and decide if RISC or CISC is better. Clearly there is no "winner", but "what is the application". I do not advocate one over the other because they both have benefits/faults the other doesn't have. Only the future can tell us which will preside. Shawn R. Antol AT&T Bell Labs Naperville, IL - USA voice: 708-979-5622 email: att!ihlpb!santol #include