A level CS (9618) 16.1 Purpose of an OS

Bootstrap

"a small program that is used to load other programs to 'start up' a computer."

Scheduling

process manager which handles the removal of running programs from the CPU and the selection of new processes.

Direct memory access (DMA) controller

device that allows certain hardware to access RAM independently of the CPU.

Kernel

The core of an OS with control over process management, memory management, interrupt handling, device management and I/O operations.

Multitasking

function allowing a computer to process more than one task/process at a time.

Process

a program that has started to be executed.

Preemptive

type of scheduling in which a process switches from running state to steady state or from waiting state to steady state.

Quantum

a fixed time slice allocated to a process.

Non-preemptive

type of scheduling in which a process terminates or switches from a running state to a waiting state.

Burst time

the time when a process has control of the CPU.

Starve

to constantly deprive a process of the necessary resources to carry out a task/process.

Low level scheduling

method by which a system assigns a processor to a task or process based on the priority level.

Process control block (PCB)

data structure which contains all the data needed for a process to run.

Process states

running, ready and blocked; the states of a process requiring execution.

Round robin (scheduling)

scheduling algorithm that uses time slices assigned to each process in a job queue.

Context switching

procedure by which, when the next process takes control of the CPU, its previous state is reinstated or restored.

Interrupt dispatch table (IDT)

data structure used to implement an interrupt vector table.

Interrupt priority levels (IPL)

values given to interrupts based on values 0 to 31.

Optimisation (memory management)

function of memory management deciding which processes should be in main memory and where they should be stored.

Paging

form of memory management which divides up physical memory and logical memory into fixed-size memory blocks.

Physical memory

main/primary RAM memory.

Logical memory

the address space that an OS perceives to be main storage.

Frames

fixed-size physical memory blocks.

Pages

fixed-size logical memory blocks.

Page table

A table that maps logical addresses to physical addresses; it contains the page number, flag status, frame address, and time of entry.

Dirty

term used to describe a page in memory that has been modified.

Translation lookaside buffer (TLB)

this is a memory cache which can reduce the time taken to access a user memory location; it is part of the memory management unit.

Segments memory

variable-size memory blocks into which logical memory is split up.

Segment number

index number of a segment.

Segment map table

table containing the segment number, segment size and corresponding memory location in physical memory: it maps logical memory segments to physical memory.

Virtual memory

type of paging that gives the illusion of unlimited memory being available.

Swap space

space on HDD used in virtual memory which saves process data.

In demand paging

a form of data swapping where pages of data are not copied from HDD/SSD into RAM until they are actually required.

Disk thrashing

problem resulting from use of virtual memory. Excessive swapping in and out of virtual memory leads to a high rate of hard disk read/write head movements thus reducing processing speed.

Thrash point

point at which the execution of a process comes to a halt since the system is busier paging in/out of memory rather than actually executing them.

Page replacement

occurs when a requested page is not in memory and a free page cannot be used to satisfy allocation.

Page fault

occurs when a new page is referred but is not yet in memory.

First in first out (FIFO) page replacement

page replacement that keeps track of all pages in memory using a queue structure. The oldest page is at the front of the queue and is the first to be removed when a new page is added.

Belady’s anomaly

phenomenon which means it is possible to have more page faults when increasing the number of page frames.

Optimal page replacement

page replacement algorithm that looks forward in time to see which frame to replace in the event of a page fault.

Least recently used (LRU) page replacement

page replacement algorithm in which the page which has not been used for the longest time is replaced.