Operating Systems Flashcards
Purpose(s) of Operating System
- To provide a user-friendly interface between the user and the hardware.
- To manage memory, and processor to ensure they are used efficiently.
- To manage peripheral devices, such as printers and keyboards.
- To protect the hardware and software from incompetence and inexperience.
- To provide similar environment on different machines. This should make programs more transportable.
- To avoid duplication by incorporating commonly used routines.
- Monitor the user and protect data on the computer from unauthorized access.
Batch Processing: Define
The data is collected together before processing which is then carried out without human interaction e.g. Payroll
Single User
A mode operation where the operating system deals with just a single user.
Multi User
A mode of operation where the computer can deal with number of users at the same time. E.g. scales system in a supermaket.
Multi Tasking
The ability of the operating system to have several programs loaded and the user can switch between them.
Multi Programming
That ability of the operating system to have several programs loaded and the operating system can rapidly switch between them giving the impression they are all running at the same time.
Interrupt
A signal sent from a peripheral device to the processor requesting attentions.
Typical causes of interrupt
(Hardware)
- Mouse movement.
- Keyboard key press
- Printer running out of paper.
(Software)
- Program run time error.
- Clock reaching specified time.
Process of Interrupt
(1) –The current state of the processor is stored (on a stack).
(2) –A jump is then made to a subroutine that deals with the interrupts.
(3) –The interrupt is processed.
(4) –The previous state of the processor is retrieved (from the stack).
Priority of Interrupt
Where data loss will result a high level of interrupt is allocated.
Where the user is waiting for a response a medium level of interrupt is allocated.
Other cases a low level of interrupt is allocated.
Polling
Polling is the checking by the OS of a number of devices, in turn, to see if they need attention.
Buffers
A buffer is a portion of RAM allocated as a temporary storage while data is being transferred to and from a peripheral. It deals with speed difference between the devices.
Buffer is used when
- There is a speed difference between the computer and the peripheral device.
- Where data needs to be sent in batches rather than one bit/byte at a time.
Example of Buffer
Keyboard Buffer. When a key is pressed on a keyboard an interrupt is generated that places the character into a buffer. Applications take characters from the keyboard buffer when they require input.
A word processor can send data very rapidly to a printer. A printer, since it is mechanical, can only use the data slowly. To free up the word processor a print buffer may be used.
Double Buffering
A single buffer can suffer from buffer underflow which results in pauses. There is also a problem in allowing the memory to be written to whilst it is being read. To avoid this two buffers are used.
By using double buffering a continuous flow of data from the buffer is possible.
Device Driver
Software that converts OS instructions into the actual commands needed by a piece of hardware such as a printer.
Logical device
The way in which a device e.g. hard disc is referred to, it may or may not match the physical hardware. E.g. one physical disc can be partitioned to appear as several discs.
Security
The process of protecting data from unauthorized access or damage.
Weakness of username/password security
- Can be forgotten.
- Obvious password, guessable.
- Using same password on multiple systems
- Can be observed - key logger.
Alternatives to username/password security.
Biometric (additional hardware):
- Retina scan.
- Finger print.
- voice pattern.
Smart card.
Administrator
A user who has full rights on a multi-user computer. They can create and modify other user accounts.
File Protection
Methods of stopping deliberate and accidental access to or damage of a file. It is achieved by access rights.
File encryption
Scrambling the contents of a file in order to make it unreadable by anyone who does not know how to unscramble it.
Access log
A record of what each user has done on a computer. Maintained by OS.
Record locking
Restricting access to a record whilst it is being updated.
High-level scheduler*
With a batch processing system this is responsible for deciding the order in which jobs are to be started
Based on: -
- Resources needed.
- Processor.
- Print etc..
- Estimated running time.
- Priority (set by user).
Low-level scheduler
Maintains a queue of all of the currently active jobs and decides which task is to currently have access to the processor. The order might depend upon the priority, the required resources and the available resources, also the closeness to complete of a job.
Time slices may be fixed in length or variable (last as long as a task can use it, or longer for more import tasks).
Time slices may be allocated more of often to more important tasks.
Time slice
In a multi programming system the time slice is the amount of processor time given to a task before the processing moves to another task.
Scheduler Strategies - Round robin
Each job is given an equal time slice of the processor, provided it can be executed i.e. is not waiting for input/output. The time slice may be 10ms.
Scheduler Strategies - Run time method
A task is executed until completion or processing comes to a halt (awaiting I/O) or a previous request for I/O has been answered.
Scheduler Strategies - Job Priorities
Some Operating Systems allow jobs to be given different priorities. A high priority job might be be given more processor time by ‘Bigger time slice’ or ‘More frequent time slices.’
Five state Process Model
A model that shows the possible states of a program on a multi-tasking computer.
- New.
- Ready.
- Running.
- Blocked.
- Exit.
Five state Process Model - Stage: New
A process has been created but has not yet been admitted to the pool of executable processes.
Five state Process Model - Stage: Ready
Processes that are prepared to run if given an opportunity. That is, they are not waiting on anything except the CPU availability.
Five state Process Model - Stage: Running
Process that is currently being executed.
Five state Process Model - Stage: Blocked
A process that cannot execute until a specified event such as user input or reading a record from a file completes.
Five state Process Model - Stage: Exit
A process that has been released by OS either after normal termination or after abnormal termination (error).
Memory structure
The main store of the computer consists of RAM (used for holding most running programs and data that is being processed) and ROM (holding at least the instructions to start the computer). On a hand held computer without a hard disc the whole of the operating system and some applications will be on ROM.
Partitioning
On a multitasking computer partitioning refers to dividing up the main store so that each of the current programs can have part of it. The OS must ensure that one program cannot corrupt another program.
Variable/memory Partitioning
Each task is allocated a variable sized section of memory depending on how much memory the task needs.
The task has exclusive use of the partition.
This process allows more than one program to be in the memory at a time.
After many programs start and end memory may become fragmented.
Paging
The dividing of memory into blocks for management purpose.
The main store is often divided into pages. Each task will be given a number of pages. They may in practice not be next to each other but the operating systems memory management system will make it appear to the application that the pages are consecutive. When a task accesses memory the memory manager intercepts the request and routes it to the correct page.
Swapping
The process of exchanging the contents of two storage locations.
In order to allow more programs to be in main memory than the capacity of the main store a very high speed swapping disc may be used to swap out currently suspended tasks (between time slices). In practice only the programs varaibles need to be saved each time since the program will not be changed during execution. When execution is resumed the program and variables are recalled from the swapping disc. (The program must be saved once on the swapping disc).
Virtual Memory (Virtual storage)
Temporary transfer of part of the main store (RAM) onto hard disc to free space in RAM. The contents are then bought back when needed.
Cache
Fast storage (RAM) containing a copy of frequently used data.
Because of the time taken to get to the main memory (RAM) many high speed devices have a small amount of RAM built into them for very fast temporary storage.
This RAM is known as a cache
Such devices include processors and hard discs.
Advantages of digital over analogue
- Data transfer is less prone to interference.
- Values are precise.
Serial transmission
Example: From keyboard to a computer
The bits travel one at a time down a single pair of wires.
Simplex
Data can only flow in one direction.
Broadcast digital radio (DAB) and digital television are Simplex transmission as data only goes from the transmitter to the receiver.
Half duplex
A half-duplex system provides for communication in both directions, only one direction at a time (not simultaneously). An example is a “walkie-talkie” style two way radio or many entry phones.
Full Duplex
Data can flow in both directions at the same time. Example include a mobile phone.
Parallel Transmission
A number of bits (typically 64) are transferred simultaneously through a series (e.g 64) of parallel wires.
Advantage of serial transfer
Simple, smaller, cheaper
Advantage of parallel transfer
Faster
Digital transmission
If data is being sent down a copper or fiber optic cable the pattern of bits can be represented by two different voltages (0v and 5v) or by a light being off or on. Such a transmission system is called baseband.
Modulation
Varying a carrier wave in some way in order to represent binary data, Eg changing amplitude or frequency.
Modulated signals
A more complex arrangement where the binary data is used to vary some other signal is known as a modulated transmission.
Modem performs modulation and it’s reverse demodulation.
Modulation Allows
-Binary data to be sent over systems that were not designed to carry binary data.
E.G: By using two different tones data can be sent over a telephone system.
-Several bits to be sent at the same time by each bit being sent using a different carrier wave.
E.G: A wireless system may use eight different carrier frequencies to enable eight bits to be sent at the same time.
E.G . In a fiber optic system different colourer light may be used, each colour carries a different bit.
ADSL
A system of sending data over telephone lines by using a number of modulated carrier waves.
Multiplexing
Multiplexing is the process of combining serial signals down a single communication channel and splitting them up at the receiver.
Time division multiplexing
Sending multiple sets of data down a single line by switching rapidly from one (sender-receiver) to the next (sender-receiver).
Frequency division multiplexing
Sending multiple sets of data down a single line with different frequencies.
Data packets
When a file is sent across a network it is broken into blocks of data and placed into packets.
Each packet will have the packet number, the address of the machine it came from, the address of the machine it is going to, the data and a checksum to ensure the data has not been corrupted on route.
Baseband
A transmission system that does not use modulation. Eg in a copy cable where 0 and 1 are represented by 0v and 5v.
Biometric test
A security test that confirms identifies the user by measuring a characteristic of their body e.g .fingerprint.
Bio-metric process
- Person’s voice (print) originally captured, digitized and stored.
- On attempted entry, person’s voice (print) captured and digitized.
- Digitized data compared and entry allowed if successful).
Switching (Network)
Switching prevents all data being sent to all parts of a network/ makes efficient use of the data lines.
Circuit switching
A route is negotiated.
The bandwidth on the route is guaranteed.
The data is sent, all data following the same path.
The connection is closed.
Packet Switching
With packet switching each packet is individually routed through the network.
Different packets may follow different paths so the packets may arrive out of order.
Advantages of Packet Switching
Better use of network.
Fault tolerant.
Advantages of Circuit Switching
Speed of transfer is known.
Packets arrive in order.
CSMA / CD
The collision avoidance and detection system generally used on a LAN.
Token Networks
- A token (permission to send) is transferred from machine to machine.
- Only when the token is received can the data be sent, by adding it to the token.
- When the token reaches the receiving machine the data is removed.
Why are Token Networks not used?
Why are Token Networks not used? They need specialist hardware making them expensive.
They tend to have a slower basic data rate that n CSMA/CD
Bridge
It’s piece of hardware that joins two (or more) networks together. The networks must use the same protocol.
Collision Domains
An area of the network that can only transfer one data packet at a time.
Gateway
It joins a network to another network that may have a different protocol.
Router
A router is a device in a network which holds information about the addresses of computers in the network or other networks. A router looks at the destination address of a data packet and forwards the packet to its destination or another router closer to the destination. Router may choose optimum path.
Collision (Files)
In a hash file a collision occurs when the hash function give the same record position for two records.
Protocol
It’s an agreement between two devices about how they should exchange data. To communicate both devices must use the same protocol.
TCP/IP (Protocol)
Transfer Control Program/Internet Protocol. The most widely used protocol for networks. It defines how machines are identified using addresses and how data packets are constructed and forwarded.
IP Address
The unique 32bit value used to identify a machine one the internet.
ISP
Internet Service Provider is a company that provides individuals and other companies access to internet.
HTTP
The protocol used to request and send WEB pages.
ICANN
One of the controlling bodies of the internet.
VLE
Virtual Learning Environment. The use of the Internet to publish teaching resources (e.g Blackboard, moodle/iLearn).
Intranet
The use of internet technology but private and only accessible from within an organisation. Used for:
Notices.
Access to resources e.g. standard documents.
As a parthole to other service.
A feature web page should have (E)
- Should comply with established web standards (legislation).
- Optimum for different accessibility.
- Should be easily navigable/ should be hyperlinks (accept buttons) to allow navigation.
Extranet
Remotely accessing an intranet across the internet. This will require the user to “log in” to gain access.
E-Commerce
Carrying out commercial operations using the Internet.
E-Commerce - Auction Sites
Sites that auctions items and charge commissions for the service.
Advantage is a large number of possible buyers.
Disadvantage is that items have to be sent. Problems of buyer/seller trust.
E-Commerce - On line shopping
Selling goods or services to the public.
Advantage: Easy price comparison, rapid ordering.
Disadvantage: Can’t get the item immediately (tangible item), cannot see/feel/try-on the item (tangible item), website may be fraudulent - may not receive goods or credit card details may be stolen.
E-Commerce - Banking services
Access to banking facilities remotely across the internet Quick and easy access to account. Disadvantage is security.
E-Commerce - Audio/video download
Selling of music/video for downloading (e.g. I-Tunes) or streaming (e.g. Spotify).
Communication - Blog (WEB LOG)
A personal commentary, diary etc. this is frequently updated and made publicly available.
Distributed Databases
Database (data stores) that have different parts (possibly different entities) stored on different servers. Queries (searches) might involve extracting data from a number of servers using the Internet.
Distributed Processing
A software application where different parts of the same application execute on different computers connected via the internet.
Distributed Software
Some software updates itself automatically across the Internet. Particularly used for software that needs regular updates e.g. Virus Checkers, Operating Systems.
Cloud Computing
The use of a third party to host data and applications. All access is through web browser.
URL
Uniform/Universal Resource Locator
Applet
An applet is a small program, written in Java, that runs within a web page. Applets are machine independent.
ASP
Active server pages. These are web pages written on request e.g. the web page that shows the result of search from a search engine.
Bookmarks
In a browser it is a shortcut to a page.
Browser
Displays WWW pages written in HTML.
Cashe
Fast storage (RAM) containing a copy of frequently used data.
Cookies
Small data file stored by a web page site on a browser so it can “remember” when the user goes back to the site.
Domain names
It’s a text based substitution for an IP address.
FTP
File Transfer Protocol, a method by which files can be transferred from a remote computer to a local computer.
MetaTags
Key words added to an HTML to summarize its contents.
Modem
Any device that converts binary data into another form, using modulation, for transmission.
