pentium a cisc architecture by shalvin maharaj

8/8/2019 Pentium a CISC Architecture by Shalvin Maharaj

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The Pentium: A CISC

Architecture

Shalvin Maharaj

CS 14711-09-04

Umesh Maharaj:

Umesh Maharaj:

Umesh Maharaj:

Umesh Maharaj:

Umesh Maharaj:

Umesh Maharaj:

Umesh Maharaj:

Umesh Maharaj:

Umesh Maharaj:

Umesh Maharaj:


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Contents

What is CISC

History: CISC

CISC

Advantages of CISC

Disadvantages of CISC

RISC vs CISC

Sources


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What is CISC?

CISC stands for Complex Instruction Set

Computer

CISC takes its name from the the very largenumber of instructions (typically hundreds)

and addressing modes in its ISA.


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History: CISC

The first PC microprocessors developed

were CISC chips, because all the

instructions the processor could executewere built into the chip.

Memory was expensive in the early days of

PCs, and CISC chips saved memorybecause their programming could be fed

directly into the processor.


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History: CISC

CISC chips were improved mainly by

adding more instructions to the processor

design. This also meant that programmingchanged with new CISC designs. CISC

designs grew complex and somewhat bulky


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History: CISC

If this was done it would be easier to write

compilers and code would be smaller so it

would save RAM


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Examples of CISC Processors

Examples of CISC processors are

VAX

PDP-11 Motorola 68000 family

Intel x86/Pentium CPUs


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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.


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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 comparableRISC designs.


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CISC

This increase in transistor count has obvious

implications for the issue of power

dissipation, which is central to mobilecomputing.

The increase in transistor count also makes

it more expensive to produce CISC chips.


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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.


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CISC

Due to the increase in transistor count and

obvious issue of power dissipation Mobile

Pentium III introduced a new powermanagement facility.

This was called SpeedStep. SpeedStep

detects the power source(wall socket orbattery) and adjusts the processor's clock

speed and voltage accordingly.


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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 processorsthat can reduce only their clock speed.


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Advantages of CISC

CISC has varying lengths to reduce wasted

space in memory.

Has developed a process to manage powerwhich 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.


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Disadvantages of CISC

CISC chips are relatively slow (compared to

RISC chips) per instruction.

CISC chips require many more transistorsthan comparable RISC designs .

Harder to pipeline using CISC architecture.

Expensive to produce.


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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 topicpops up again.

Most PC's use CPU based on CISC

architecture. For instance Intel and AMD

CPU's are based on CISC architectures.


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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 hasnt managed to kick CISC out of the picture, some

argue that if it is really the architecture of the future itshould have been able to do this by now.


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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 powerfulinstruction set, which provides programmers with

assembly instructions to do a lot with shorter

programs.


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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 similartasks. But by doing this RISC architecture takes the

burden away form the hardware resulting in an increase

in performance(mainly speed).


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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 andcheaper isnt helping RISC claim supremacy.


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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 chipssupport just as many instructions as

yesterday's CISC chips. The PowerPC 601,

for example, supports more instructions

than the Pentium.


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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.


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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

pentium a cisc architecture by shalvin maharaj

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