Characteristics of a Microprocessor
The microprocessor is the defining trait
of a computer, so it is important to
understand the characteristics used to
describe microprocessors.
This module provides an introduction to
these characteristics.
Clock speed
Also called clock rate, the clock speed
is the speed at which a microprocessor
executes instructions. Every computer
contains an internal clock that regulates
the rate at which instructions are
executed and synchronizes all the
various computer components.
The faster the clock, the more
instructions the microprocessor can
execute per second. The
microprocessor requires a fixed number
of clock ticks (or clock cycles) to
execute each instruction.
Clock speed is stated in either MHz or
GHz. 1 MHz is equal to 1 million cycles
per second, while 1 GHz is equal to 1
billion cycles per second.
At the present time the most common
microprocessors run from 1.8 GHz (1.8
billion cycles per second) to 3.2 GHz
(3.2 billion cycles per second.
Clock speed is a major factor in determining
the power of a computer.
Instruction Set
The possible operations a
microprocessor can performs is based
on its instruction set. Programs are
written for a microprocessor based on
its instruction set. For example, the
SIMP computer understands 10
instructions, and any program written for
it uses those ten instructions in various
ways to accomplish some surprisingly
complicated tasks.
Meanwhile, advanced processors can
have from 150 to over 200 instructions,
allowing for extremely complicated
tasks.
Since software is written with the
instruction set in mind, sometimes a
larger instruction set will equal better
performance. For example, one
difference between Pentium 4 and
Pentium 5 is that Pentium 5 has a larger
instruction set.
When comparing a 2GHz Pentium 4
and 2GHz Pentium 5, if they both run
software designed with the new
instruction set in mind, the Pentium 5
will outperform the Pentium 4, despite
having the same clock speed. However,
if the two are compared while running
older software, which does not use the
new instructions, their performance will
be similar.
Cache
Most programs access the same
information repeatedly while running.
Cache memory is intended to take
advantage of this fact. Memory cache is
a high speed storage mechanism that
holds recently read data and
instructions from main memory, which
eliminates the processor from having to
constantly access main memory.
The program first checks the cache to
see if the desired information is already
present there. If it is, the cache sends
the information back to the
microprocessor, bypassing the main
memory.
95% of the time the processor is
working, it is accessing information from
cache.
There are two kinds of cache, L1 and
L1 cache (also called primary cache) is
built directly into the microprocessor, a
location referred to as "on-die". Since it
is "on-die", it is part of the
microprocessor and, is usually smaller
in size than L2 cache, but since it is built
in it runs at the same speed as the
processor.
L2 cache (also called secondary cache)
is not usually built into microprocessor,
but it is found within the processor's
external packaging. This location is
referred to as, "off-die." Since "off-die"
L2 cache is not included in the
processor's architecture, it can often be
of greater sizes. However, since it is not
included on-die, L2 cache is usually
slower than L1 cache.
Bus Speed
The processor communicates with other
devices via the data bus, sometimes
called the front side bus. Bus speed is
measured in MHz, the same unit used
to measure clock speed. While a
processor might be working at up to 3
GHz, quite often the performance of the
computer is hampered by a slower data
bus speed.
Recently much effort has been put into
making the data bus have speeds more
comparable to the microprocessor.
At the present time data bus speeds
range from 200 MHz up to 1GHz.