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CPU ARCHITECTURE

The processor (really a short form for microprocessor and also often called the CPU or central processing unit) is the central component of the PC. This vital component is in some way responsible for every single thing the PC does. It determines, at least in part, which operating systems can be used, which software packages the PC can run, how much energy the PC uses, and how stable the system will be, among other things. The processor is also a major determinant of overall system cost: the newer and more powerful the processor, the more expensive the machine will be.

When the Hungarian born John von Neumann, first suggested storing a sequence of instructions - that's to say, a program - in the same memory as the data, it was a truly innovative idea. That was in his "First Draft of a Report on the EDVAC", written in 1945. The report organised the computer system into four main parts: the Central Arithmetical unit, the Central Control unit, the Memory, and the Input/Output devices. More than half a century later, nearly all processors had a "von Neumann" architecture.

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For some years two families of microprocessor dominated the PC industry - Intel's Pentium and the Apple/IBM/Motorola alliance's PowerPC - each CPU being a prime example of the competing CPU architectures of the time, CISC and RISC.

CISC - Complex Instruction Set Computer

CISC is the traditional architecture of a computer, in which the CPU uses microcode to execute very comprehensive instruction set. These may be variable in length and use all addressing modes, requiring complex circuitry to decode them.

For a number of years, the tendency among computer manufacturers was to build increasingly complex CPUs that had ever-larger sets of instructions. In 1974, John Cocke of IBM Research decided to try an approach that dramatically reduced the number of instructions a chip performed. By the mid-1980s this had led to a number of computer manufacturers reversing the trend by building CPUs capable of executing only a very limited set of instructions.

RISC - Reduced Instruction Set Computer

RISC CPUs keep instruction size constant, ban the indirect addressing mode and retain only those instructions that can be overlapped and made to execute in one machine cycle or less. One advantage of RISC CPUs is that they can execute their instructions very fast because the instructions are so simple. Another, perhaps more important advantage, is that RISC chips require fewer transistors, which makes them cheaper to design and produce.

There is still considerable controversy among experts about the ultimate value of RISC architectures. Its proponents argue that RISC machines are both cheaper and faster, and are therefore the machines of the future. Sceptics note that by making the hardware simpler, RISC architectures put a greater burden on the software - RISC compilers having to generate software routines to perform the complex instructions that are performed in hardware by CISC computers. They argue that this is not worth the trouble because conventional microprocessors are becoming increasingly fast and cheap anyway.

To some extent, the argument is becoming moot because CISC and RISC implementations are becoming more and more alike. Many of today's RISC chips support as many instructions as yesterday's CISC chips and, conversely, today's CISC chips use many techniques formerly associated with RISC chips. Even the CISC champion, Intel, used RISC techniques in its 486 chip and has done so increasingly in its Pentium family of processors.

CPU Architecture Principles - Binary Logic »

Last Update: Fri Mar 27th 2009