Download The Pentium: A CISC Architecture

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The Pentium: A CISC
Architecture
Shalvin Maharaj
CS 147
11-09-04
Contents
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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
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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/Co
mponent_Summaries/cpu/cpu_how_works.
html
• http://friday.editthispage.com