Learning Objectives
By the end of this section, you will be able to:
- Describe an operating system and its role in computing
- Explain the architecture of operating systems
An operating system (OS) is the core piece of software that typically manages and controls the interconnection of hardware and software on a computer. The OS is loaded upon start-up and is the key piece of software needed to operate any computerized device.
Introduction
There are many operating systems (OSs), and anyone using a modern computer is using one of them. The typical OSs for computers are Windows, macOS, and Linux and for mobile devices, iOS and Android (Figure 6.2). Microsoft Windows is a popular operating system, celebrated for its ease of use and broad software compatibility. Apple Inc.’s macOS, which is the driving force behind Apple computers, ensures tight hardware-software cohesion. Linux, a freely available open-source OS, is acclaimed for its reliability, safety, and adaptability. Linux source code has over 27 million lines of C, while Windows has over 50 million lines. Linux offers a variety of versions or distributions, such as Ubuntu for ease of use, Fedora for the latest features, Debian for stability, and Kali Linux for security tasks. Each version is designed to meet specific needs, from general computing to specialized applications. All the commercial operating systems today manage security for users.
The goal of this section is to explain what an OS is. Traditionally, we think of the OS as the means by which communication between applications that the user wants to run and the computer hardware that is responsible for running them is facilitated. Applications typically do not directly manipulate the hardware; instead, they must ask the OS to do this on their behalf. One of the primary goals of the OS from an application software point of view is isolation. Isolation ensures that the multiple programs that are running concurrently on the same CPU and memory operate independently without interfering with each other’s execution or data. This maintains system stability and security. From a hardware point of view, the hardware is designed to enable programmers to write OSs and applications. In general, the OS mediates a program’s access to hardware resources such as the CPU (for computation); memory (for volatile storage) and disk, flash memory, and so on (for persistent storage); TCP/IP stacks, Wi-Fi, and Ethernet network interface cards, and so forth (for network communications); and keyboard, display, touch screen, audio, game controllers, and so on (for input/output). Isolation is what allows device manufacturers to update their hardware without requiring software developers to rewrite their programs each time.
Link to Learning
To learn more about operating systems, read this informative article on understanding operating systems from How-To Geek.
Monitoring Access
Operating systems play a crucial role in managing and controlling access to hardware devices within computer systems, including peripherals such as keyboards, mice, hard drives, and other critical components. They act as gatekeepers, ensuring that interactions with these devices occur smoothly and securely. One of the key responsibilities of an OS is monitoring hardware access, which involves tracking and regulating which applications or users can communicate with the hardware and how they do so. This capability is vital for maintaining a system’s overall integrity and performance.
In addition to hardware monitoring, the OS is instrumental in enforcing authentication mechanisms. This means the OS helps verify that only authorized users gain access to certain devices, especially those considered sensitive. By managing user permissions and access levels, the OS can prevent unauthorized access to critical hardware resources, safeguarding against potential security breaches or data theft. This aspect of the OS is particularly important in environments where access to information and resources needs to be tightly controlled, such as in corporate or government settings.
The role of an IT administrator is to define which hardware devices are available within a company’s network and determine their sensitivity levels. This task requires a deep understanding of both the technical specifications of the hardware and the security implications of its use.
You may be wondering: How is the OS organized? How are resources shared across users? How is one user or process protected from another? Such questions relate to the characteristics or properties that are considered when designing an OS. Table 6.1 depicts various OS design questions and the properties they are associated with, such as structure, sharing, naming, protection, security, performance, availability, and reliability, among others.
| Property | Question |
|---|---|
| Structure | How is the OS organized? |
| Sharing | How are resources shared across users? |
| Naming | How are resources named, and what is the scope? |
| Protection | How is one user or process protected from another? |
| Security | How is the integrity of the OS and its resources ensured? |
| Performance | How does an OS avoid making all the applications run slowly? |
| Availability | Can the applications always access the services they need? |
| Reliability | How often do things go wrong either with the hardware or with a program? |
| Extensibility | Can new features be added? |
| Communication | How do programs exchange information, including across a network? |
| Concurrency | How are simultaneous activities such as computation and I/O created and controlled? |
| Scale | What happens as demands or resources increase? |
| Persistence | How can data be made to last longer than program executions? |
| Distribution | How can a computation be allowed to span hardware, such as machine/network, boundaries? |
| Accounting | How can a user’s resource usage of an OS be tracked, and how might the user be charged for it? |
| Auditing | Can actions and processes be reconstructed? |
Efficiency Management
By managing system resources (e.g., CPU, memory, disk, and network) efficiently, the OS ensures that no resource is underutilized or overburdened. We start with this question: What level of ease or complexity is involved in developing applications with optimal efficiency on a computer? A well-performing operating system facilitates the development of efficient applications that enhance the user experience by providing faster response times. The OS also impacts user experience positively by managing resource allocation and multitasking efficiently.
At some point, you might work for an organization that is engaged in developing a new app, and the company may ask itself this question: If we were to take the time to develop all the additional software required for our application to boot and run on raw hardware, how much faster would it be? This is the same as asking what the penalty or cost is of developing what the OS provides (i.e., sharing of the hardware among apps, and limited damage when programs have defects). The answer to this question relates to runtime efficiency. With respect to coding time efficiency, the OS includes various abstractions, interface and libraries that application programmers can use to ease the burden of software development. Having to write these from scratch without the support of an operating system would greatly increase the required coding effort and extend the development time associated with application development.
Mechanisms Implementation
In operating systems, a policy is a way to choose which activities to perform (i.e., what needs to be done) and a mechanism is an activity that enforces policies (i.e., how to do it), which often depend on the hardware on which the operating system runs. If a process is granted resources using the first come, first served policy, then that policy may be implemented using a queue of requests. To understand how policies and mechanisms relate, consider this analogy. A car is a mechanism because it enables operations such as going, stopping, and turning that enable a driver to get from point A to point B. Notice that the mechanism (in this case, the car) does not say anything about how to use the mechanism to get anywhere in particular. The driver provides a policy by deciding on a route. Now compare a car with a public bus. In this case, the bus driver determines the policy. The same goes for the self-driving cars that are being developed these days. They serve as the mechanism as there are no decisions about routes being built into cars. Instead, a self-driving car uses machine learning to determine a policy, which includes deciding on a route, selecting the procedure on how to drive, and allowing the driver to override automated driving at any time.
OS-Level and Server Virtualization
The ability of a system or server to run different types of applications used by multiple users at a time on the same computer is called virtualization. Server virtualization places a software layer called a hypervisor (i.e., virtual machine monitor, or VMM) between a machine’s (i.e., server’s) hardware and the operating systems that run on it. The hypervisor creates and manages virtual machines. A virtual machine (VM) is software that is created to run like a physical computer and that operates its operating system and applications like separate physical servers. OS-level virtualization is a basic form of server virtualization. When using OS-level virtualization, there is no need for a hypervisor as the server’s OS handles all resources.
Cloud computing leverages server virtualization and makes it possible for users to commission virtual machines. In this case, the servers reside in distributed data centers provided by cloud vendors. Commissioning a virtual machine is equivalent to gaining access to a virtual computer that runs a chosen operating system while leasing the underlying hardware and only paying per use (i.e., cost of CPU, storage, and network usage).
OS Architecture and Support Layers
As you’ve now learned, the OS is a system software program whose job it is to manage all the programs running on a computer. OSs enable users to run multiple applications at the same time and keep them from interfering with or crashing each other. OSs provide convenient abstractions to handle diverse hardware. They coordinate resources and protect users from each other using a few critical hardware mechanisms. OSs make it easier for developers to create applications by offering built-in features that help with managing errors. These features include fault containment, which prevents errors in one part of an application from affecting the whole system; fault tolerance, which allows the application to keep running even when errors occur; and fault recovery, which helps the system to fix itself or revert to a previous state after an error. The fact that these standard services are provided by the OS means that developers do not have to build these error management features from scratch, which simplifies the development process. Figure 6.3 shows the support layers within the UNIX/Linux system structure. The OS navigates between three layers: the user mode, where the application resides; the kernel mode, which is in effect sandwiched between the user and hardware; and the hardware, which refers to resources such as the CPU and memory. In the next sections, we will describe these modes and the features they support in detail.
Link to Learning
Now that you’ve been introduced to the basics of operating systems, it may come as no surprise to you that operating systems come in all shapes and sizes. There are many different types of OSs (from batch operating systems to network operating systems), and each type handles many different functions (from file management to device management). This OS tutorial explains the different types. As you read along, consider how much you interact with an OS over the course of a single day—or how many different OSs you interact with.
OS Kernel Features
The kernel is the program that is running at all times on the computer and provides basic services for all parts of the OS. Typically, the kernel is not the only program that is running; there is usually either a system program/operating system or an application program running as well. Any program running on top of the OS is considered to be a process from the OS standpoint. It includes the program code, its current activity represented by the program counter, and a set of resources like open files and allocated memory. It also consists of address space, one or more threads of control executing in that address space, and an additional system state associated with it. The operating system uses processes to manage the execution of programs, ensuring they have the necessary resources while keeping them separate from each other to prevent interference.
Software applications have to be compiled and linked with system libraries before they can run as executable programs on a machine. Each running program runs in its own process, and the OS can run, switch, and isolate processes from each other even though they are actually running on the same hardware. For a given application, the “machine” is the process abstraction provided by the OS. Processes provide user-friendly interfaces, rather than raw hardware, and an execution environment with restricted rights controlled by the OS.
An OS virtualizes the machine by providing easy-to-use abstractions of physical resources while masking limitations. In this context, a thread is a path of execution within a process, and a process may contain multiple threads. Thus, multithreading involves executing multiple threads (i.e., execution units that are part of a process and share the same resources) concurrently, which improves overall responsiveness and efficiency.
Industry Spotlight
Operating Systems and Health Care
In the fast-moving world of IT, it is crucial to think globally. This global perspective can be particularly useful when considering how IT and OSs affect health care. Namely, it can help us understand the role of OSs in patient care, medical research, and the efforts being made to make health care more widely accessible. The challenges associated with data sharing, privacy, and health-care disparities impact regions around the world differently.
As the delivery of health care becomes more integrated with technology, the choices made in designing how health-care IT systems use OSs can significantly impact people’s lives across the globe. These choices range from deciding how to take ethical considerations into account when using AI for diagnosis to creating IT solutions that respect cultural differences.
Check out these initiatives from the World Health Organization and Centers for Disease Control and Prevention for examples in the health-care industry.
Imagine you’re considering entering the health-care IT field. Can you think of some ways that thinking globally and understanding the capabilities provided by OSs could improve your ability to solve problems and create solutions that work well for health-care systems in various countries?
Hardware Management
The ISA defines how the CPU is controlled by the software by abstracting the hardware details from the applications. The OS provides an abstract machine interface to the application programs and leverages the physical machine interface to do so. The OS communicates with input/output (I/O) hardware using device drivers, I/O ports, interrupts, direct memory access (DMA), and effective I/O scheduling. It provides abstractions to manipulate files (i.e., streams) and send messages to the network (i.e., sockets). Programming languages provide application programming interfaces (APIs) that leverage these abstractions (e.g., file I/O, socket libraries, and related APIs) so that the application program can access the underlying resources that are managed by the OS. One of the main responsibilities of the OS is to isolate hardware from programs by providing common services and background management functionality, for example, storage manager, network manager, and power manager.
The OS is the only system that should be able to directly access I/O devices (i.e., disks, network cards) and manipulate memory. Moreover, the CPU hardware provides a privileged instruction that can only be executed by the OS. The OS can use these instructions to establish an execution environment that limits access (to, for example, memory). The application cannot remove the restrictions because it must execute privileged instructions to do so.
Certain operations are prohibited when running in user mode, such as changing the page table pointer (i.e., the pointers to memory pages that are cached for faster access), disabling interrupts (i.e., the interrupts that the OS received from I/O devices), interacting directly with hardware, and writing to kernel memory. Carefully controlled transitions between user mode and kernel mode include system calls, interrupts, and exceptions. The system call appears when the program requests a service from the kernel. The system interrupt manages the communication between the computer hardware and the system. The system throws an exception, which is an error that occurs at runtime.
Think It Through
The Ethics of Open-Source OSs
The idea of making all operating systems open-source (i.e., the copyright holder releases the content or product under a license that allows any user to access, modify, and distribute it freely) has both advantages and disadvantages. On the plus side, open-source operating systems can be more secure and innovative because anyone can inspect and improve the code. This openness also encourages a global community of developers to collaborate, potentially leading to a technology that is more user-friendly and accessible for everyone. Additionally, openness aligns with ethical principles of transparency and freedom, as it allows users to understand and control their digital environments fully.
There are, however, downsides too. Open-source projects might struggle to secure consistent funding and professional support, which can lead to them having slower updates and fixes compared to commercial software. There’s also the risk of fragmentation, where too many variations of the system can create compatibility issues and confuse users.
From an ethical standpoint, the use of open-source operating systems globally can democratize access to technology, ensuring that no single company has too much control over our digital lives. It also encourages a culture of sharing and collaboration, which is essential for addressing global challenges like digital divide and ensuring equitable access to technology. However, the success of such a model depends on balancing openness with the need for sustainable development and support systems to keep the technology reliable and up to date.
Should operating systems all be open source? Discuss pros and cons.
Protected Sharing
An OS’s functions should guarantee protection, isolation, and the sharing of resources efficiently via resource allocation and communication. To implement protected sharing, the OS provides common services (e.g., sharing and authorization). There are many ways to leverage OS sharing. One involves the sharing of processors to perform computations concurrently. In this case, these computations will be completed as if only one processor had been allocated to them although in reality multiple processors are performing the computations/tasks in parallel. An OS also allows a computer’s memory, input and output devices, and files to be shared within the tasks and the processes. It can also allow groups of computers to work together within the network and share resources. All of the sharing capabilities are controlled using secured channels.