Systems Software — OCR A-Level Computer Science

An operating system (OS) is systems software that sits between hardware and application software. It is needed because raw hardware is hard to use: without an OS, each program would have to control devices, share the processor and memory safely and handle errors itself. The OS's functions include managing processes, memory, files, devices and the user interface. Its purpose is to make the computer usable, efficient and secure by managing resources for users and applications. For example, when a student saves a document, the application asks the OS to write it, and the OS chooses where the data is stored and records it in a directory. Learners should state the need, describe each function and link it to the purpose.

Memory management allocates main memory to processes, tracks its use and protects each process's memory. Paging divides logical memory into fixed-size pages and physical memory into equal frames; a page table maps pages to frames, so contiguous memory is unnecessary. Segmentation divides a program into variable-length segments, such as code and data, tracked by base and limit values; gaps can cause external fragmentation. Virtual memory keeps only some pages in RAM and the rest on secondary storage, so a process may exceed installed RAM. A page fault occurs when a needed page is absent, prompting the OS to load it. Example: 4 KiB (2¹² bytes) pages in a 32-bit address space use a 12-bit offset and a 20-bit page number.

An interrupt is a signal, from hardware such as a keyboard or timer, or from software, that asks the processor to pause its current task and deal with an event. Interrupts let the CPU respond to events without constantly checking devices. Within the Fetch-Decode-Execute cycle, the processor checks for a pending interrupt after the execute stage completes, so the current instruction finishes before control changes. If an interrupt is present, the program counter and status information are saved, usually to the stack, and the address of the matching Interrupt Service Routine (ISR) is loaded into the program counter. An ISR is the short program that handles that specific event, for example reading a key code from a keyboard buffer. When the ISR ends, the saved values are restored and the cycle fetches the next instruction from the point where the original program stopped.

Scheduling decides which ready process the processor runs next, and each named algorithm chooses differently. Round robin gives every process a fixed time slice in turn, returning unfinished processes to the back of the queue. First come first served runs processes in arrival order without interruption, so a long job can delay short ones. Shortest job first picks the ready process with the smallest expected run time, while shortest remaining time is its preemptive form, switching when a new process has less remaining time than the running one. Multi-level feedback queues use several queues with different priorities and time slices, moving processes between them according to their behaviour. For processes of 8 ms, 4 ms and 2 ms arriving together in that order, first come first served gives an average waiting time of 6.67 ms, while shortest job first gives 2.67 ms.

This statement asks you to identify five operating system types and state what each is designed to do. A distributed OS manages several networked computers so they appear to users as one system, sharing files and processing. An embedded OS runs a dedicated device, such as a washing machine controller, with a fixed function and limited memory. A multi-tasking OS switches the processor between tasks through time slicing, so several programs appear to run at once. A multi-user OS lets several people log in at once, each with separate accounts and permissions. A real time OS guarantees responses within deadlines, as in airbag or industrial robot control. Learn each type by its defining feature.

This statement asks you to recognise BIOS, the Basic Input/Output System, and describe its role when a computer starts. BIOS is firmware stored in read-only or flash memory on the motherboard, so it is available as soon as power is applied. When the computer starts, it runs a power-on self-test to check key hardware, initialises devices such as the keyboard, display and storage, and then locates a boot device. It hands control to the bootloader stored on that device, which loads the operating system. Because BIOS supplies low-level input and output routines, it sits between hardware and the OS during start-up. Modern PCs often use UEFI firmware for the same job, but this specification names BIOS.

Device drivers are pieces of software that allow the operating system to communicate with and control hardware devices such as printers, keyboards, graphics cards and storage. Each device has its own control interface, so a driver translates the generic instructions the operating system issues into the specific commands that a particular device understands. For example, when an application asks the operating system to print a page, the operating system passes the request to the printer driver, which converts it into the printer's own commands and sends them through the relevant connection. Drivers are usually written by hardware manufacturers and installed on the system, so the operating system can use a device without needing to know its internal workings.

A virtual machine is software that takes on the function of a machine. The statement covers any instance of this, not only one product, and names two examples. First, executing intermediate code, such as Java bytecode run by the Java Virtual Machine. Second, running an operating system within another: a guest operating system runs inside a host operating system, which supplies the hardware it sees through software. Learners should state the general idea, then describe each example, explaining that software performs work the hardware would normally do. For instance, Linux running inside Windows through virtualisation software is a virtual machine.