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 L2.

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.