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1.2.1 - Systems Software

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operating system
a program or set of programs that manages the operations of the computer for the user, acting as a bridge between the user and the computer's hardware
a small program held in ROM which sends instructions to load the operating system by copying it from storage into RAM
user interface
Function of an OS - provides a user-friendly way for a user to interact with the computer
memory management
Function of an OS - such as allocating areas of memory to different files
interrupt service routines
Function of an OS - the suspending of the execution of the running program or process so that an interrupt can be serviced
processor scheduling
Function of an OS - the OS must allocate processor time to all the processes and applications needing to be run
Function of an OS - using settings to keep the computer secure, such as by setting user access rights
Function of an OS - such as disk defragmentation software, anti-virus, disk cleaner, and system monitor
network management
Function of an OS - controlling the connections between other devices, computers and the internet
backing store management
Function of an OS - the OS must keep a directory of where files are stored so they can be accessed and retrieved
peripheral management
Function of an OS - communicating with other devices to check they are connected correctly and use them when needed in the running of applications
In case the user wants to switch from one application to another, each running application, must be stored in memory ...
the physical division of memory into equal-sized blocks
page table
uses mapping to store a link between the physical memory address and the logical address of each process
the logical division of address space into varying length segments which depend on the program structure (each segment may contain one module or routine)
In both segmentation and paging, it is possible to load only one part of a program into memory ...
Virtual Memory
if the RAM on a computer is not large enough to hold all the programs running simultaneously, part of the hard disk will be used as ... to store some of the programs (usually the ones being used the least at that time), this can cause the computer to run more slowly
disk threshing
a problem that may occur when the operating system has to constantly swap information back and forth between RAM and the hard disk (virtual memory), making the computer much slower as accessing secondary storage is slower
a signal from a software program, hardware device or internal clock to the CPU, which occurs when it needs access to the CPU
this is possible because of the fact that a processor can be interrupted, which gives the appearance of running several programs simultaneously by carrying out small parts of multiple larger tasks in turn
lower priority
when the CPU receives an interrupt signal, it suspends the execution of the running program and disables all tasks of a ...
interrupt service routine
depending on the type of interrupt, a different routine will be used in order to deal with it
the operating system module responsible for making sure that processor time is used as efficiently as possible
Objective of Scheduler: maximise ...
Objective of Scheduler: to be ... to all users on a multi-user system
response time
Objective of Scheduler: provide acceptable ... to all users
hardware resources
Objective of Scheduler: ensure ... are kept as busy as possible
round robin
Scheduling Method - each process is allocated a certain amount of CPU time (time slice) to complete their processing, if they are still running when time runs out, time will be given to other processes and will return to this one in order
interval timer
set by the OS, this generates interrupts at specific times, which helps to manage scheduling
first come first served
Scheduling Method - jobs are processed in the order that they arrive, with no system priorities
shortest job first
Scheduling Method - the process with the smallest estimated running time is run next; it must be completed before the next process is carried out, unless an interrupt must be serviced
shortest remaining time
Scheduling Method - the process with the smallest estimated time left until completion is run next; it will be run until another process is added with a smaller remaining time (this is checked after each clock cycle)
multi-level feedback queues
Scheduling Method - uses several job queues of different priority levels; high-level priority tasks are processed first and once they use up their allotted time, they are moved down a priority level
an area of memory which will hold data to be sent to an output device, in order to compensate for the speed difference between the output device and CPU
distributed operating system
an OS that runs across multiple computers / servers, where each computer can run a different task, to work together to complete the same job, thus spreading the load
Advantage of Distributed OS - splits the processing load while still appearing to be one machine, allowing for better ...
Advantage of Distributed OS - if one site were to fail, remaining sites could potentially still be able to ...
Disadvantage of Distributed OS - security of data could be ...
multi-tasking system
a system which can switch between many different jobs and processes so quickly that all tasks appear to be running simultaneously
Advantage of Multi-Tasking OS - allows multiple tasks to be run ...
processor starvation
when a process is starved of processor time
scheduling of processes
Disadvantage of Multi-Tasking OS - what is required to help prevent processor starvation?
multi-user system
a system that allows more than one user to share a computer's resources at the same time, meaning a scheduling algorithm has to be used to allocate processing time to each user
multiple users
Advantage of Multi-User System - better at distributing resources to ...
processor resources
Disadvantage of Multi-User System - what must be shared?
mobile operating system
combines features of a personal computer OS with its own special features, such as managing cellular and wireless connectivity
real-time operating system
systems designed to carry out actions in a guaranteed amount of time (usually fractions of seconds), and so are time critical
time critical applications
Advantage of Real-Time OS - allows ... to be run
Disadvantage of Real-Time OS - there is a lack of ...
embedded systems
special-purpose computer systems built into larger mechanical systems / other devices (often running in real time), such as navigation systems, traffic lights, aircraft flight control systems and simulators
embedded systems will often have limited ...
Advantage of Embedded System - ideal when processor and memory resources are ...
Disadvantage of Embedded System - difficult to ...
Basic Input Output System
hardware components
BIOS is used in modern PCs to load the OS from the hard disk into RAM and initialise and test the system ...
device driver
a computer program that provides a software interface to a particular hardware device, allowing operating systems to access hardware functions without needing to know details of hardware being used
operating system specific
Device drivers tell the OS how to interact with a device and therefore are hardware dependent and ...
virtual machine
a self-contained operating environment, imposed through software, that takes on the function of / behaves like a separate computer, often used to run intermediate code or an operating system
power-on self test
diagnostic testing sequence which checks that the hardware is installed correctly; run by BIOS