The CPU and the Life Cycle of Operating Systems
Before we begin looking at the details of a specific Operating System it is important to understand what an Operating System is, and what it does. The CPU is the heart of the computer, responsible for controlling most of the activities that occur within it.
By itself, the CPU can only perform certain simple operations. These operations, known as instructions, are simple and mundane by themselves and cover actions such as add two values together, jump to this location in memory or read one byte from memory address A and place it in memory address B. Although basic, they are the building blocks upon which everything else is created. These instructions have remained fairly static and unchanged since the first x86 processor, with the majority of changes being used to accommodate 32bit addressing1. When a programmer combines these instructions in the right sequence, the CPU can perform extremely complex activities. Whilst sending email, browsing the Internet and editing files are the basic staple of all modern operating systems, the instructions (often abbreviated to code) required to make it happen can stretch to millions of lines.
All programs, regardless of complexity and the programming language they were written in are made up of specifically ordered instructions compatible with the platform the application is targeted at. This means that software written and compiled for e.g. an ARM processor based device (such as an iPaq PDA) cannot run on an Intel x86 processor based device, such as your desktop PC.
An operating system is nothing more than a complex set of programs written to allow an end user to interface with the computer hardware. It is a generic term that applies equally to versions of Microsoft Windows that run on large servers as it does to embedded software that powers mobile phones or watches with organizer features.
All major operating systems can be split into two distinct sections. The first section, the user interface, allows users to work with the computer to perform useful tasks such as loading a word processor to write the book you are holding. The second section consists of an API (Application Programming Interface), which allows software developers to write programs that interface with the operating system to provide enhanced functions and utilities.
While all programs (and thus operating systems) are written for a specific platform, some operating systems are capable running on multiple platforms. The most well known cross-platform operating system is of course Linux, but even some versions of Microsoft Windows are cross-platform, with the ability to run on both Intel and Alpha based processors.
As a reminder to how a processor functions, it is important to understand how binary math works. If you need a refresher, a multimedia tutorial can be found here: http://courses.cs.vt.edu/~csonline/NumberSystems/Lessons/DecimalToBinaryConversion/index.html
1. Recently 64-bit processors have become available to the desktop computer market. A thorough discussion of the practical differences between 32bit and 64bit processors can be found at http://www.xbitlabs.com/articles/cpu/print/64bit.html
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