A-level CS Unit 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

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

Round robin (scheduling)

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

Context switching

"procedure by which

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

"table that maps logical addresses to physical addresses; it contains page number

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

Virtual memory

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

Swap space

"space on HDD used in virtual memory

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."