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2.2.9.1 RISC vs CISC Design Philosophies
RISC and CISC are two CPU architectures involving how an instruction set is constructed and how it is handled by the CPU itself.
In general Intel base its CPU architectures on a CISC design while Apple base their CPUs on a RISC design.
As we will see both architectures offer advantages and disadvatages and lately some CPUs have been designed to borrow from both architectures.
CISC :
Pronounced sisk, CISC stands for Complex Instruction Set Computer.
Most PC's use CPU based on this architecture. For instance Intel and AMD CPU's are based on CISC architectures.
Typically CISC CPUs are designed to handle an instruction set that :
1. have a large amount of different instructions.
2. each instruction can be quite complex.
The philosophy behind it is that hardware is always faster than software, therefore one should make a powerful instruction set, which provides programmers with assembly instructions to do a lot with short programs.
In general CISC CPUs are relatively slow (compared to RISC CPUs) per instruction since they are more complex, but use little (less than RISC) instructions.
RISC :
Pronounced risk, RISC stands for Reduced Instruction Set Computer.
RISC CPUs evolved around the mid-1980 as a reaction at CISC CPUs.
Apple for instance uses RISC CPUs.
Typically RISC CPUs are designed to handle an instruction set that :
1. have a small amount of instructions.
2. each instruction is simple.
The philosophy behind it is that almost no one uses complex assembly language instructions as used by CISC, and people mostly use compilers which never use complex instructions.
Therefore fewer, simpler and faster instructions would be better, than the large, complex and slower CISC instructions. However, more instructions are needed to accomplish a task.
An other advantage of RISC is that - in theory - because of the more simple instructions, RISC chips require fewer transistors, which makes them easier to design and cheaper to produce.
Finally, it's easier to write powerful optimised compilers, since fewer instructions exist.
RISC vs CISC :
There is still considerable controversy among experts about which architecture is better.
Some say that RISC is cheaper and faster and therefor the architecture of the future.
Others note that by making the hardware simpler, RISC puts a greater burden on the software. Software needs to become more complex. Software developers need to write more lines for the same tasks.
Therefore they argue that RISC is not the architecture of the future, since conventional CISC chips are becoming faster and cheaper anyway.
RISC has now existed more than 10 years and hasn't been able to kick CISC out of the market. If we forget about the embedded market and mainly look at the market for PC's, workstations and servers at least 75% of the processors are based on the CISC architecture. Most of them the x86 standard (Intel, AMD, etc.), but even in the mainframe territory CISC is dominant via the IBM/390 chip. Looks like CISC is here to stay .
Is RISC than really not better? The answer isn't quite that simple. RISC and CISC architectures are becoming more and more alike. Many of today's RISC chips support just as many instructions as yesterday's CISC chips. The PowerPC 601, for example, supports more instructions than the Pentium. Yet the 601 is considered a RISC chip, while the Pentium is definitely CISC. Further more today's CISC chips use many techniques formerly associated with RISC chips.
So simply said: RISC and CISC are growing to each other.
An important factor is also that the x86 standard, as used by for instance Intel and AMD, is based on CISC architecture. X86 is the standard for home based PC's. Windows 95 and 98 won't run at any other platform. Therefore companies like AMD an Intel will not abandoning the x86 market just overnight even if RISC was more powerful.
Changing their chips in such a way that on the outside they stay compatible with the CISC x86 standard, but use a RISC architecture inside is difficult and gives all kinds of overhead which could undo all the possible gains. Nevertheless Intel and AMD are doing this more or less with their current CPU's. Most acceleration mechanisms available to RISC CPUs are now available to the x86 CPU's as well.
Since in the x86 the competition is killing, prices are low, even lower than for most RISC CPU's. Although RISC prices are dropping also a, for instance, SUN UltraSPARC is still more expensive than an equal performing PII workstation is.
Equal that is in terms of integer performance. In the floating point-area RISC still holds the crown. However CISC's 7th generation x86 chips like the K7 will catch up with that.
The one exception to this might be the Alpha EV-6. Those machines are overall about twice as fast as the fastest x86 CPU available. However this Alpha chip costs about 20000, not something you're willing to pay for a home PC.
Maybe interesting to mention is that it's no coincidence that AMD's K7 is developed in co-operation with Alpha and is for al large part based on the same Alpha EV-6 technology.
EPIC :
The biggest threat for CISC and RISC might not be each other, but a new technology called EPIC. EPIC stands for Explicitly Parallel Instruction Computing. Like the word parallel already says EPIC can do many instruction executions in parallel to one another.
EPIC is a created by Intel and is in a way a combination of both CISC and RISC. This will in theory allow the processing of Windows-based as well as UNIX-based applications by the same CPU.
It will not be until 2000 before we can see an EPIC chip. Intel is working on it under code-name Merced. Microsoft is already developing their Win64 standard for it. Like the name says, Merced will be a 64-bit chip.
If Intel's EPIC architecture is successful, it might be the biggest thread for RISC. All of the big CPU manufactures but Sun and Motorola are now selling x86-based products, and some are just waiting for Merced to come out (HP, SGI). Because of the x86 market it is not likely that CISC will die soon, but RISC may.
So the future might bring EPIC processors and more CISC processors, while the RISC processors are becoming extinct.
Conclusion
The difference between RISC and CISC chips is getting smaller and smaller. What counts is how fast a chip can execute the instructions it is given and how well it runs existing software. Today, both RISC and CISC manufacturers are doing everything to get an edge on the competition.
The future might not bring victory to one of them, but makes both extinct. EPIC might make first RISC obsolete and later CISC too.
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