12.2 Fundamental of risc  (Page 2/7)

While the RISC processor designers worked out these issues and the manufacturing capability improved, there was a battle between the existing (now called CISC) processors and the new RISC (not yet successful) processors. The CISC processor designers had mature designs and well-tuned popular software. They also kept adding performance tricks to their systems. By the time Motorola had evolved from the MC68000 in 1982 that was a CISC processor to the MC68040 in 1989, they referred to the MC68040 as a RISC processor. And they did it without ever taking out a single instruction!

However, the RISC processors eventually became successful. As the amount of logic available on a single chip increased, floating-point operations were added back onto the chip. Some of the additional logic was used to add on-chip cache to solve some of the memory bottleneck problems due to the larger appetite for instruction memory. These and other changes moved the RISC architectures from the defensive to the offensive.

RISC processors quickly became known for their affordable high-speed floating- point capability compared to CISC processors. The typical CISC microprocessor in the 1980s supported floating-point operations in a separate coprocessor. This excellent performance on scientific and engineering applications effectively created a new type of computer system, the workstation. Workstations were more expensive than personal computers but their cost was sufficiently low that workstations were heavily used in the CAD, graphics, and design areas. The emerging workstation market effectively created three new computer companies in Apollo, Sun Microsystems, and Silicon Graphics.

Some of the existing companies have created competitive RISC processors in addition to their CISC designs. IBM developed its RS-6000 (RIOS) processor, which had excellent floating-point performance. The Alpha from DEC has excellent performance in a number of computing benchmarks. Hewlett-Packard has developed the PA-RISC series of processors with excellent performance. Motorola and IBM have teamed to develop the PowerPC series of RISC processors that are used in IBM and Apple systems.

By the end of the RISC revolution, the performance of RISC processors was so impressive that single and multiprocessor RISC-based server systems quickly took over the minicomputer market and are currently encroaching on the traditional mainframe market.

Characterizing risc

RISC is more of a design philosophy than a set of goals. Of course every RISC processor has its own personality. However, there are a number of features commonly found in machines people consider to be RISC:

• Instruction pipelining
• Pipelining floating-point execution
• Uniform instruction length
• Delayed branching
• Load/store architecture
• Simple addressing modes

This list highlights the differences between RISC and CISC processors. Naturally, the two types of instruction-set architectures have much in common; each uses registers, memory, etc. And many of these techniques are used in CISC machines too, such as caches and instruction pipelines. It is the fundamental differences that give RISC its speed advantage: focusing on a smaller set of less powerful instructions makes it possible to build a faster computer.