Exam Board:
OCR A-Level
2.1 - Systems Software
Specification:
Computer Science H446
Watch on YouTube:
Operating systems functions
Paging & segmentation
Interrupts
Scheduling algorithms
Types of operating system
BIOS
Device drivers
Virtual machines
This topic looks at how the operating system manages the resources of a computer system. It also includes programs related to the operating system, such as the BIOS, device drivers and virtual machines.
Operating Systems
The operating system performs essential functions to keep a computer running efficiently. It provides a user interface, such as a command line or graphical user interface (GUI), and ensures system security by managing access rights and protecting files.
The OS manages hardware, coordinating devices like the CPU, printers and hard drives and provides built-in utilities for tasks such as file management and disk defragmentation.
It acts as a platform for software, allowing applications to be installed and run. The OS also schedules jobs, handles interrupts from devices, and manages memory, ensuring each process has the resources it needs without conflicts.
Paging & Segmentation
Paging is a memory management method that splits memory into equal-sized blocks called pages. This makes memory use more efficient as programs don’t need to be stored in one continuous block but unused space may be wasted inside a page.
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Segmentation divides memory into segments of different sizes based on program structure. This makes it easier to organise parts of a program, but because segments are different sizes, it can leave small unused gaps in memory that can’t be easily filled by other data.
Interrupts
An interrupt is a signal that tells the CPU to pause its current task and respond to something more urgent.
Interrupts can come from hardware, such as a keyboard press, or from software, such as an error.
When an interrupt occurs, the CPU saves its current state by placing current register values into a stack. The CPU runs an interrupt service routine (ISR) to handle the interrupt event. After the ISR finishes, the CPU restores its state by popping values from the stack and continues the original program.
Scheduling
Scheduling algorithms are used by the CPU to decide the order in which processes are executed. They aim to manage CPU time efficiently and ensure all programs get a fair share of resources. Common scheduling algorithms include:
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First-Come First-Served: Processes are executed in the order they arrive to the CPU.
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Round Robin: Each process gets a fixed time slice in turn, cycling through all processes.
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Shortest Job First / Shortest Time Remaining: These methods identify the process with the shortest estimated runtime to be executed first.
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Multi-Level Feedback Queue: Uses multiple queues with different priority levels. Processes can be moved between queues to avoid starvation.
Types of OS
Different types of operating systems exist depending on the computer system and purpose:
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Multitasking OS: Allows a single computer to run multiple programs at the same time by quickly switching between them.
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Multi-user OS: Lets multiple users access the computer and its resources simultaneously, often via a network.
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Distributed OS: Manages a group of networked computers as a single system, sharing resources and tasks.
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Embedded OS: Designed for devices with specific functions, like microwaves or smart TVs, often with limited resources.
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Real-time OS: Provides immediate processing and responses for time-critical tasks, used in systems like medical devices or industrial robots.
The BIOS
The BIOS (Basic Input/Output System) is firmware stored in ROM that helps the computer start up and manage initial communication between hardware and the operating system.
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During the start-up sequence, the BIOS first checks that essential hardware (e.g. the CPU and RAM) is present and working correctly by running a Power-On Self Test (POST). Signals are transmitted to all connected components.
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The BIOS then runs the bootstrap program to locate and load the operating system from a connected storage device into RAM. Once the OS is loaded, control is handed over to it, allowing the computer to become fully operational.
Device Drivers
A device driver is software that allows the operating system to communicate with hardware devices such as printers, graphics cards or keyboards.
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Hardware and the OS speak different 'languages', so the driver translates OS instructions into commands the device can understand and vice versa.
They are needed because, without drivers, the OS wouldn’t know how to control the hardware or use its features properly. Drivers also allow devices to work with different versions of an operating system and enable updates that improve performance or fix bugs.
Virtual Machines
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A virtual machine is a software-based computer that runs an operating system and applications like a real computer, using part of the host computer’s hardware. It is isolated from the main system, so it can operate safely without affecting the host.
Virtual machines are commonly used for testing software, running multiple operating systems and safely handling untrusted files. They provide flexibility, efficient hardware use and a secure environment for experimentation.
Questo's Key Terms
Operating Systems: user interface, file management, user management
Memory Management: paging, segmentation, virtual memory
Interrupts: interrupt, interrupt service routine, stack
Scheduling Algorithms: first come first serve, round robin, shortest job first, shortest time remaining, multilevel feedback queue
Types of Operating System: multitasking, multi-user, distributed, embedded, real-time
BIOS: ROM - power-on self-test (POST)
Device Drivers: device driver
Virtual Machines: virtual machine, thin clients, servers
Did You Know?
New versions of the Android operating system used to be named alphabetically after sweet treats, from Cupcake, Donut and Eclair to Nougat, Oreo and Pie - the final named update in 2019.
