Chapter 16 - System Software and Virtual Machines

Meow

Bootstrap

A small program that’s used to load other programs to ‘start up’ a computer

Scheduling

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

Direct Memory Access (DMA) controller

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

Kernel

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

Multitasking

A function that allows a computer to process more than one task/process at a time

Process

A program that has started to be executed

Preemptive

A type of scheduling in which a process switches from running state to ready state, or from waiting state to ready state

Quantum

A fixed time slice allocated to a process

Non-preemptive

A 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

A method by which a system assigns a processor to a task/process based on the priority level

High level scheduler

Makes decisions about which program stored on disk should be moved into memory

Process Control Block (PCB)

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

Process states

The states of a process requiring execution

(Running, ready, blocked)

Round robin

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

Context switching

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

Interrupt Dispatch Table (IDT)

A data structure used to implement an interrupt vector table

Interrupt Priority Levels (IPL)

Values given to interrupts based on values 0 to 31

Optimisation (not compilation)

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

Paging

A form of memory management which divides up physical memory and logical memory into fixed memory blocks

Physical memory

Main/Primary RAM memory

Logical memory

The address space that an OS perceives to be main storage

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

A 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’s part of the memory management unit.

Segments memory

Logical memory being split up into variable-size memory blocks

Segment number

The segment’s index number

Segment map table

A table containing the segment number, segment size, and corresponding memory location in physical memory.

It maps logical memory segments to physical memory.

Virtual memory

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

Swap space

The space on a 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’re actually required

Disk thrashing

A problem resulting from the 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 the processing speed.

Thrash point

A point at which the execution of a process comes to a halt since the system is busy 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.

Least Recently Used (LRU) page replacement

A page replacement algorithm in which the page hasn’t been used for the longest time is replaced.

Belady’s anomaly

A phenomenon which means it’s 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

Clock page replacement

They use a circular queue structure with a single pointer serving as both head and tail. When a page fault occurs, the page being pointed to is inspected. Action taken depends on the R-flag status

Processor management

Decides with processes will be executed and in which order

Memory management

Decides where in memory data/programs will be stored and how

Virtual Machine

An emulation of an existing computer system

A computer OS running within another computer’s OS

Emulation

The use of an app or a device to imitate the behaviour of another program or device. For example, running an OS on a computer which isn’t normally compatible

Host OS

An OS that controls the physical hardware

Guest OS

An OS running on a virtual machine

Hypervisor

A virtual machine software that creates and runs virtual machines.

Lexical analysis

The 1st stage in the process of compilation.

It removes unnecessary characters and tokenises the program.

Syntax analysis

The 2nd stage in the process of compilation.

The output from the lexical analysis is checked for grammatical (syntax) errors.

Code generation

The 3rd stage in the process of compilation.

It produces an object program

Optimisation

The 4th stage in the process of compilation.

It creates an efficient object program

Backus-Naur form (BNF) notation

A formal method of defining the grammatical rules of a programming language

Syntax diagram

A graphical method of defining and showing the grammatical rules of a programming language.

Reverse Polish Notation (RPN)

A method of representing an arithmetical expression without the used of brackets or special punctuation.