Overview of Operating System Concepts

In a previous post, I introduced the concept of Operating Systems (OSs) while discussing the differences between System Software and Application Software. If you're unfamiliar with these topics or if you're new to the study of technology, I strongly encourage you to read about the DifferenceBetween Application Software and System Software before moving forward. As you read on, you will learn about the software mechanisms that form an Operating System and how each mechanism plays a role in the bigger picture. From this, you may even learn a bit about bettering the world. After all,

“Alone we can do so little; together we can do so much” -- Helen Keller

Operating Systems are essentially a modular set of System Software that works together as part of a 'system' to help users perform certain 'operations'. Simple, no? Despite the simplicity of our definition, tasks performed by an OS are anything but simple. In this post, however, I spare my readers the complex details and offer an easy-to-follow breakdown of the typical General Purpose OS. I hope the lessons that follow will help bridge the gap between the world's tech-ninjas and its not-so-tech-savvy. Like many other module-based concepts from science and engineering, OS modules form a hierarchical structure whose lower layers act as a support for the upper layers. We'll begin our discussion with the OS's lowest layer and then work our way up.

At the Operating System's core, we find a software layer known as the Kernel. OS Kernels are small, but very complex, pieces of software. They act as the system's main hardware interface; requiring that all hardware requests be addressed to the Kernel. This layer of abstraction is valuable for practical and integrity-related reasons. To start, computer hardware is very fragile. Direct access by poorly crafted software can easily lead to damaged circuits. However, if all direct access is handled by a well-designed and trusted Kernel, software packages can now obtain hardware resources with limited risk. Kernels offer hardware resources as services to other software on the system. These services are accessed via a common signaling technique known as System Calls.

Not only is hardware access dangerous, it's often considered difficult and tedious. With modern Application and System Software development, programmers write code in 'human-readable' languages that are later transformed into 'machine-readable' languages. However, this is not the case for Kernel programming. Since Kernels communicate directly with hardware, the languages used in their creation (often known as assembly languages) are closely coupled with the hardware they interact with. Non-Kernel software, on the other hand, is programmed using a more flexible language that is hardware independent. In other words, assembly requires its developers to have a detailed understanding of the hardware being accessed and to take responsibility for manual management of system resources (details that other languages abstract from their developers). Using available System Calls, software developers are given the luxury to use 'high-level' languages rather than relying on system-specific assembly languages.

As mentioned above, Kernels also address integrity concerns. Since software must consult the Kernel before performing tasks, Kernel-owned policies act as a central enforcer of user and application privileges. These policies protect the system from certain threats and enhance the way resources are shared within the system. Threats come in many forms, but can seperated between two categories: accidental threats and malicious threats. May policies exist to protect users from themselves and from external forces. For instance, the Kernel is responsible for restricting a user's access to the files and folders he/she owns. This policy can prevent a harmless user from accidentally deleting important system files. Likewise, the same policy restricts malicious users from accessing other users' private information. Another example of Kernel policy is time-sharing. Since computers can only run one process at a time, time-sharing policies were set to allow the OS to 'Context Switch' between running processes. Processes are software instructions that are queued for execution by the OS. Without time sharing, users could only run one application at a time (e.g. they could not download a file while typing in their word processor).

In the same way that cars can be driven by someone that doesn't understand the engine, applications can be run by users that are ignorant of the OS. This is made possible through the use of abstract software layers; as seen in our discussion of the Kernel. The further we abstract software from the hardware, the more flexible it becomes. Throughout OS design, you will find more and more abstractions that further divorce a user from what's 'under the hood'. Application Software is a well-known abstraction that builds and isolates a set of actions that help users complete tasks. But how exactly does your run-of-the-mill Web Browser or Word Processor use Kernel services? Let's dig a little deeper to find out.

It is possible (in most OSs) to use direct system calls for Kernel access. However, though the Kernel has simplified hardware access, its services are still low-level and are somewhat difficult to use. This is especialy true when developing very large and complex software packages. To facilitate this difficulty, Operating Systems include a set of commonly used software routines known as Shared System Libraries. These routines expand programming language(s) (usually C or C++) by invoking complex combinations of Kernel-level commands through single lines of code. This abstraction layer highly simplifies the way developers implement the Operating System's remaining System Software.

Unfortunately, with each additional layer of abstraction, we are forced to sacrifice some level of performance. In many cases, you'll find that the advantages of abstraction outweigh the losses. However, the weight of this decision depends on the software's intended purpose. For example, if you were developing a top-of-the-line computer game, performance is very important and abstraction should be limited. On the other hand, if your goal is to design and create server management tools, you're likely to use a very flexible and highly abstracted scripting language (e.g. languages like bash, perl, or python).

Now that the OS has a powerful set of system libraries, we can investigate the System Software that supports users and their applications. Operating Systems come with software packages that run as Background Processes. Such processes are often referred to as daemons or daemon processes. Daemon services can fill many different roles, buth their primary objective is offer services that enhance Application Software capabilities. Network Services, for instance, set up and manage network and Internet configurations. Other examples include: Window Systems, which supply GUI capabilities; Print Services, which enable access to local and network printers; and System Loggers, which maintains a collection of system activity and error reports.

The last OS concept I'll cover here is the shell. Loosely defined, a shell is the user's interface to the system. The combination of a shell and certain system utilities forms what we like to call an Operating Environment. Today's most common type of shell is the Desktop Shell used on PCs. However, in this post we're going discuss an older, more stable, type of shell that acts as the foundation of modern computing systems; a Command Shell. However, if you're interested in learning more about Desktop Shells, I've written a previous post, Window Managers and Systems VsDesktop Shells and Environments, which goes into great detail about the subject.

In a cold and lonely server room, some IT administrator is sitting at a strange, text-only, user interface. That's right; she's using the system without any graphics! During the time when computer technology first began to blossom, this was the only interface users and administrators had to the computer. Because of its stability and limited resource requirements, many systems administrators still prefer to use this approach today. A text-only shell is known more commonly as a Command Shell and is made of a Command-Line Interface (CLI) and a set of daemon utilities. The CLI job is to interpret commands that users have typed into the terminal. Command Shells tend to come with several 'built-in' commands and operators that allow users to: navigate the file system, print file and folder contents to the screen, redirect input/output of streams to/from files, and manage running processes. CLIs also support Environment Variables which can affect the way shells locate executable files and cane change the default behavior of certain software.

Congratulations! You're now one step closer to understanding exactly how your computer works. Keep in mind, however, that this has been a basic introduction to OS concepts. Each of the modules mentioned in this post can be broken down even further and, not surprising, grow more complex the deeper you go. For now, let's pat ourselves on the back and review what we've learned.

In summary, an OS is made up of System Software that acts as building blocks for other software through the use of abstraction. Abstraction simplifies the way we build particular elements of a system and drives the system through a chain of common interfaces. Depending on our intended purpose, we may find that further abstraction will inhibit our design. In other cases, we may not be limited by system performance and can therefore enjoy the luxuries of a highly abstracted system. We learned that the Kernel drives our hardware and manages system policies while the Shared System Libraries simplify the way System Calls give access to Kernel services. Daemon services and utilities run in the background and support the system by managing tasks on behalf of user applications. Lastly, a shell supplies an interface to the user whereby they can access and run executable binaries from the Operating System. There are several types of shells, including the Command Shell. This type allows its users to invoke command by typing them into the CLI. I hope you've learned a lot from this post and I hope to bring you more knowledge in the future. Till then, bye bye!