Operating System Concepts

1. Operating Systems Manage Computer Resources

The operating system controls the hardware and coordinates its use among the various application programs for the various users.

Resource allocation. The operating system acts as a resource allocator, managing CPU time, memory space, storage, and I/O devices. It resolves conflicting requests and ensures efficient and fair distribution of resources among programs and users. This management is crucial for the overall performance and stability of the computer system.

Control program. The OS also functions as a control program, managing the execution of user programs to prevent errors and improper use of the computer. It is particularly concerned with the operation and control of I/O devices, ensuring that they are used correctly and efficiently. This control is essential for maintaining the integrity and security of the system.

Kernel as the core. The kernel is the one program running at all times on the computer, forming the core of the operating system. It is responsible for managing the system's resources and providing services to user programs. System programs, associated with the OS but not part of the kernel, and application programs, which include all other software, operate on top of the kernel.

2. Interrupts are Fundamental to OS Operation

Hardware may trigger an interrupt at any time by sending a signal to the CPU, usually by way of the system bus.

Hardware signals. Interrupts are signals sent by hardware to the CPU, alerting it to events that require immediate attention. These events can range from I/O operations completing to hardware faults occurring. The CPU responds by suspending its current activity and transferring control to a fixed location containing the interrupt service routine.

Interrupt vector. The interrupt vector is a table of pointers to interrupt routines, indexed by a unique number associated with each interrupt request. This allows the CPU to quickly dispatch to the appropriate handler without needing to poll all possible sources of interrupts. Both Windows and UNIX-based systems use this mechanism.

Interrupt handling. Modern operating systems require sophisticated interrupt-handling features, including the ability to defer interrupt handling during critical processing, efficient dispatch to the proper handler, and multilevel interrupts to prioritize urgent tasks. These features are provided by the CPU and interrupt-controller hardware.

3. Multiprocessor Systems Increase Throughput

The primary advantage of multiprocessor systems is increased throughput.

Parallel execution. Multiprocessor systems, including multicore systems, offer increased throughput by allowing multiple processes or threads to run simultaneously. This can significantly reduce the time required to complete complex tasks. However, the speed-up ratio is not linear due to overhead in keeping all parts working correctly.

Symmetric multiprocessing (SMP). In SMP systems, each CPU processor performs all tasks, including operating-system functions and user processes. This model allows for dynamic sharing of processes and resources among processors, leading to more efficient resource utilization.

Multicore efficiency. Multicore systems, where multiple computing cores reside on a single chip, can be more efficient than multiple chips with single cores due to faster on-chip communication and reduced power consumption. This makes them ideal for mobile devices and laptops.

4. Dual-Mode Operation Protects the OS

In order to ensure the proper execution of the system, we must be able to distinguish between the execution of operating-system code and user-defined code.

User vs. Kernel mode. To protect the operating system from errant users and errant users from one another, most computer systems provide hardware support for distinguishing between various modes of execution. At the very least, we need two separate modes of operation: user mode and kernel mode.

Privileged instructions. The hardware allows privileged instructions to be executed only in kernel mode. If an attempt is made to execute a privileged instruction in user mode, the hardware does not execute the instruction but rather treats it as illegal and traps it to the operating system.

System calls. System calls provide the means for a user program to ask the operating system to perform tasks reserved for the operating system on the user program’s behalf. A system call is invoked in a variety of ways, depending on the functionality provided by the underlying processor.

5. Resource Management is a Core OS Function

A computer system has many resources that may be required to solve a problem: CPU time, memory space, storage space, I/O devices, and so on.

Process management. The operating system is responsible for creating and deleting processes, scheduling processes and threads on the CPUs, suspending and resuming processes, and providing mechanisms for process synchronization and communication. These activities are essential for managing the execution of programs in a multitasking environment.

Memory management. The OS keeps track of which parts of memory are currently being used and by which process, allocates and deallocates memory space as needed, and decides which processes (or parts of processes) and data to move into and out of memory. This ensures efficient utilization of memory and prevents processes from interfering with each other.

File-system management. The operating system is responsible for creating and deleting files and directories, supporting primitives for manipulating files and directories, mapping files onto mass storage, and backing up files on stable storage media. These activities provide a uniform, logical view of information storage for users.

6. Virtualization Abstracts Hardware

Virtualization is a technology that allows us to abstract the hardware of a single computer (the CPU, memory, disk drives, network interface cards, and so forth) into several different execution environments.

Creating illusions. Virtualization allows us to abstract the hardware of a single computer into several different execution environments, creating the illusion that each environment is running on its own private computer. These environments can be viewed as different individual operating systems that may be running at the same time and may interact with each other.

Emulation vs. Virtualization. Emulation involves simulating computer hardware in software, typically used when the source CPU type is different from the target CPU type. Virtualization, in contrast, allows an operating system that is natively compiled for a particular CPU architecture to run within another operating system also native to that CPU.

Virtual Machine Manager (VMM). The VMM runs the guest operating systems, manages their resource use, and protects each guest from the others. It provides services and resource management to virtual machine processes, allowing multiple users to run tasks on a system designed for a single user.

7. Distributed Systems Share Resources

A distributed system is a collection of physically separate, possibly heterogeneous computer systems that are networked to provide users with access to the various resources that the system maintains.

Increased functionality. Access to a shared resource increases computation speed, functionality, data availability, and reliability. Some operating systems generalize network access as a form of file access, with the details of networking contained in the network interface’s device driver.

Network types. Networks vary by the protocols used, the distances between nodes, and the transport media. TCP/IP is the most common network protocol, and it provides the fundamental architecture of the Internet. Networks are characterized based on the distances between their nodes, including LAN, WAN, MAN, and PAN.

Network operating systems. A network operating system is an operating system that provides features such as file sharing across the network, along with a communication scheme that allows different processes on different computers to exchange messages. A computer running a network operating system acts autonomously from all other computers on the network.

8. Kernel Data Structures are Essential for OS Implementation

In this section, we briefly describe several fundamental data structures used extensively in operating systems.

Arrays and Lists. An array is a simple data structure in which each element can be accessed directly. Lists are perhaps the most fundamental data structures in computer science. Whereas each item in an array can be accessed directly, the items in a list must be accessed in a particular order.

Stacks and Queues. A stack is a sequentially ordered data structure that uses the last in, first out (LIFO) principle for adding and removing items. A queue, in contrast, is a sequentially ordered data structure that uses the first in, first out (FIFO) principle.

Trees and Hash Functions. A tree is a data structure that can be used to represent data hierarchically. A hash function takes data as its input, performs a numeric operation on the data, and returns a numeric value. This numeric value can then be used as an index into a table to quickly retrieve the data.

Last updated: August 15, 2025