Purposes of an Operating System (OS) (Cambridge (CIE) A Level Computer Science): Flashcards

Device drivers.

Through scheduling, which allows multiple processes to be managed efficiently.

That the CPU switches rapidly between processes.

The OS may use virtual memory on disk to simulate extra memory, which allows more programs to run than would otherwise fit.

They are much slower than the CPU.

A DMA controller allows data transfer between memory and devices without CPU involvement.

False.

DMA allows transfer without CPU involvement, which frees the CPU to perform other tasks.

It frees up the CPU to perform other tasks while data is being moved.

It sends an interrupt to the CPU.

It provides a user-friendly interface through GUIs, and uses device drivers to handle communication with specific hardware, so users do not need to know technical commands.

A GUI makes tasks like file transfers easy, for example drag-and-drop instead of the command line.

A hard disk, at around 100 Mbps, against around 1 Mbps for a laser printer.

Interrupts, DMA, buffering and driver management.

File system management, and read/write optimisation via caching and buffering.

Multitasking is the operating system managing system resources so that the user has the impression that multiple programs are running at the same time.

One. It processes billions per second and switches between tasks so quickly that it gives the illusion of programs running simultaneously.

The OS splits tasks and allocates system resources based on a priority.

A process is a program in execution, including the program code, current data, register values and memory space.

As a separate process.

Running, ready and blocked.

The process is actively being executed by the CPU.

The process is prepared to run but is waiting for the CPU to be available.

The process is waiting for an event or resource, such as I/O completion.

Scheduling is deciding which tasks to process, for how long, and in what order.

Pre-emptive allocates the CPU for time-limited slots and allows processes to be interrupted. Non-pre-emptive allocates the CPU for unlimited time slots, and a process cannot be interrupted until it completes or reaches its burst time.

Round Robin and Shortest Remaining Time First.

First Come First Serve and Shortest Job First.

Low-priority processes can be neglected if high-priority processes arrive frequently.

If a process with a long burst time is running, shorter processes are neglected.

A pre-emptive algorithm that distributes processor time equally, giving each process a time quantum. A process not finished by the end of its quantum moves to the back of the queue.

A non-pre-emptive algorithm that prioritises processes that arrive at the queue first. The current process blocks all others until it completes.

A pre-emptive priority algorithm using multiple queues so tasks of equal size are grouped together. Processes start in the highest priority queue and trickle down if they exceed the time quantum.

A non-pre-emptive algorithm where processes are continuously sorted by burst time, shortest to longest, with shorter jobs at the front of the queue.

A pre-emptive version of Shortest Job First, where processes with the shortest remaining time have higher priority.

Benefit: all processes get a fair share of the CPU, and it is predictable. Drawback: choosing the right time quantum is difficult, and long processes get high turnaround and waiting times.

Knowing the burst time of processes in advance.

False.

Suitability depends on the specific scenario and system requirements. A drawback in one scenario may not be a drawback in another.

An interrupt signal occurs; the current process is paused and its state saved; the kernel identifies the source and priority; the appropriate Interrupt Service Routine (ISR) is called; the original process is restored and resumed.

Its state, including the registers and the program counter.

The routine that is called to handle a particular interrupt, once the kernel has identified its source and priority.

False.

The original process is restored and resumed.

They trigger a context switch between processes, supporting pre-emptive multitasking.

That an I/O task has finished, such as a file write or printer job, which unblocks waiting processes.

Urgent external events, such as a power failure or mouse input.

They are generated by programs to request system-level services, such as memory allocation.

The scheduler, which is part of the kernel, uses interrupts to decide which process should run next.

Memory management is a fundamental role of the operating system, dealing with the allocation and deallocation of the computer's primary memory.

Its data is loaded from storage into active memory so that it can run smoothly.

It is a limited resource in the system.

Efficient allocation enables multitasking, and it maintains security by not letting programs access memory reserved for other programs.

Memory management maintains security by stopping programs accessing memory reserved for others.

Paging, segmentation and virtual memory.

Paging is a method of chunking primary memory into equal-sized blocks.

False.

Paging divides memory into fixed, equal-sized blocks. It is segmentation that uses variable sizes.

Memory is dynamically allocated. Pages are taken away from applications not in active use and granted to applications that are in active use.

Internal fragmentation is the wasted space that builds up inside pages over time, because unused space in a page cannot hold other unrelated data.

The first three pages hold 192KB, so the fourth page has 8KB used and 56KB unused.

Because other unrelated data cannot be stored in that page.

Benefit: it facilitates efficient memory management and enables the use of virtual memory. Drawback: it can lead to internal fragmentation.

Segmentation is a method of chunking memory into blocks that correspond to different types of data needed by an application.

No. They are sized depending on their allocated data.

A video editing application may have a segment for video data, audio data and special effects.

As segments fill up the memory, physical gaps reduce the maximum size of new segments that can be allocated.

Benefit: it allows intuitive and efficient memory access, allocating only the space an application needs. Drawback: it can result in external fragmentation.

When a computer is running low on primary memory, secondary storage acts as an extension of the main memory.

An illusion of a larger memory, which enables applications to continue to multitask.

Virtual memory is created temporarily.

Accessing data in virtual memory is considerably slower than RAM. Solid-state drives are faster than hard-disk drives, but neither is as fast as RAM.

False.

It is considerably slower. Neither solid-state drives nor hard disks are as fast as RAM.

Performance issues.

Disk or secondary storage is used to extend the RAM available, so the CPU appears able to access more memory than the available RAM. Only the data in use needs to be in main memory, so data can be swapped between RAM and virtual memory as necessary.

Paging divides memory into fixed size blocks. Segmentation divides memory into variable sized blocks.

The operating system divides the memory into pages. The compiler is responsible for calculating the segment size.

Paging.