The Pentium: A CISC Architecture

Umesh Maharaj: Umesh Maharaj: Umesh Maharaj: Umesh Maharaj: Umesh Maharaj: The Pentium: A CISC Architecture Shalvin Maharaj CS 147 11-09-04

Contents • What is CISC • History: CISC • CISC • Advantages of CISC • Disadvantages of CISC • RISC vs CISC • Sources

What is CISC? • CISC stands for Complex Instruction Set Computer • CISC takes its name from the the very large number of instructions (typically hundreds) and addressing modes in its ISA.

History: CISC • The first PC microprocessors developed were CISC chips, because all the instructions the processor could execute were built into the chip. • Memory was expensive in the early days of PCs, and CISC chips saved memory because their programming could be fed directly into the processor.

History: CISC • CISC chips were improved mainly by adding more instructions to the processor design. This also meant that programming changed with new CISC designs. CISC designs grew complex and somewhat bulky

History: CISC • Because the price of hardware was decreasing and the price of software was increasing. Researchers decided to take the burden off of the software and use the hardware to do all the dirty work. The idea was to make the job easier for the compiler by having more instructions that were like high-level language statements.

History: CISC • If this was done it would be easier to write compilers and code would be smaller so it would save RAM

Examples of CISC Processors • Examples of CISC processors are • VAX • PDP-11 • Motorola 68000 family • Intel x86/Pentium CPU’s

CISC • CISC takes its name from the the very large number of instructions (typically hundreds) and addressing modes in its ISA. To accommodate this variety of instruction complexities, CISC instructions are of varying lengths (often ranging from 8 - 120 bits).This is as opposed to RISC which has a fixed instruction set.

CISC • The varying length is used to reduce wasted space but there is a disadvantage which follows. • Variable length instructions are more difficult to process, so CISC chips require many more transistors than comparable RISC designs.

CISC • This increase in transistor count has obvious implications for the issue of power dissipation, which is central to mobile computing. • The increase in transistor count also makes it more expensive to produce CISC chips.

CISC • The complexity of CISC also makes it more difficult to pipeline than RISC, which again increases the required processor logic and, hence, transistor count. • And CISC chips are relatively slow (compared to RISC chips) per instruction, but on the other hand uses little (less than RISC) instructions.

CISC • Due to the increase in transistor count and obvious issue of power dissipation Mobile Pentium III introduced a new power management facility. • This was called SpeedStep. SpeedStep detects the power source(wall socket or battery) and adjusts the processor's clock speed and voltage accordingly.

CISC • For example when a computer is powered by AC mains current it runs at full speed (650 MHz depending on the model)and full voltage. But when a battery is used the clock speed is reduced to 500 MHz and the corresponding voltage is decreased. This gives the Mobile Pentium III a distinct advantage over processors that can reduce only their clock speed.

Advantages of CISC • CISC has varying lengths to reduce wasted space in memory. • Has developed a process to manage power which adjusts clock speed and voltage. • Uses less instructions to perform similar instructions than RISC • Provides programmers with assembly instructions to do a lot with smaller programs.

Disadvantages of CISC • CISC chips are relatively slow (compared to RISC chips) per instruction. • CISC chips require many more transistors than comparable RISC designs . • Harder to pipeline using CISC architecture. • Expensive to produce.

RISC vs. CISC • RISC vs. CISC is a topic quite popular on the Net. Everytime Intel (CISC) or Apple (RISC) introduces a new CPU, the topic pops up again. • Most PC's use CPU based on CISC architecture. For instance Intel and AMD CPU's are based on CISC architectures.

RISC vs CISC • Many claim that RICS is the architecture of the future. • But even though RISC has been in the market since 1980, it hasn’t managed to kick CISC out of the picture, some argue that if it is really the architecture of the future it should have been able to do this by now.

RISC vs CISC • Typically CISC chips have a large amount of different and complex instructions. 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 shorter programs.

RISC vs CISC • On the other hand by making the hardware simpler, RISC puts a greater burden on the software. Software needs to become more complex and Software developers need to write more lines of code to perform similar tasks. But by doing this RISC architecture takes the burden away form the hardware resulting in an increase in performance(mainly speed).

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. But because of the burden placed on software some argue that RISC is not the architecture of the future. And conventional CISC chips becoming faster and cheaper isn’t helping RISC claim supremacy.

RISC vs CISC • 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.

RISC vs CISC • Yet the 601 is considered a RISC chip, while the Pentium is definitely CISC. And today's CISC chips use many techniques formerly associated with RISC chips. • So simply said: RISC and CISC are growing to each other.

Sources • http://ouray.cudenver.edu • http://en.wikipedia.org/wiki/CISC • http://groups.engr.oregonstate.edu/tlc/v2/Component_Summaries/cpu/cpu_how_works.html • http://friday.editthispage.com

The Pentium: A CISC Architecture