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33 Cards in this Set

  • Front
  • Back

What is serial transmission

Where data is sent one bit at a time down a single channel. or along the same data line.

If synchronous then whole packets are sent at a time down a data line. (sata/usb)

if asynchronous then data is sent one character ata time with a stop and start bit along with a parity bit

What is parallell transmission

Parallell transmission a number of bits making up a a charactr is sent down a data line simultanously with each bit having its own channel. used for buses to link the cpu with I/O devices.

Serial vs paralell

- parallel is effected by channel width and bit rate

- it is faster than serial

-limited to shortter distances as it can be corrupted

-requires a large amount of wires (16+) so is epensive

- expensive to insure that all bits arrive at the same time at recivi9ng end

- in serial it is slower than paralell

- but can travel longer distances

- onyl two wires used so is cheaper


Where data is sent in one direction only down a channel

Half duplex

Data can be sent both ways down a channel but not at the same time. only one way at a time (two way radio)


Data can be sent both ways down a channel simultanously (at the same time)


This is where a number of signals/data sources are combined into one complex singal which is transmistted down down a single high speed route to a single .destination. at the recioving end the seaparate signals are recovered using a demultiplexor.

Whst is a protocol and why are they used

a sete of rules whcih specidy how data should be transmitted accross a network they are nececary because they enable devices and computers on different netwroks to communicate with eachother.

Cicruit switching

A dedicated path is set up between the transmitter and the reciver. data is sent in order down a single path. all data follows a single route to the destination. whilst the transmission process is being used path cannot be used for any other data.

Packet switching

Data making up the message to be transmistted is broken down into packets of a fixed length. the packets are sent out and may take more than one path/different routes to their destination. packets arrive out of order and are re assembled accoring to their order/sequence number.

Pakcet vs circuit

apcket has better security as only the sender and the reciver have access to the complex set of data. data can travel down different routes so less likely to be interceoted.

- makes more efficent use of data lines as there is no waiting between transmission gaps

- in ciriut all data is arrives in order and follows a singe route so little overhead needed to construct oringla message/data

- meed setup of transmission which wastes network resources/


a collison is where two sets of data are detcted on the netwrok simultanously (at the same time) to solve this the computer waits a psedorandom amount of time before attempting to sent the backept again.


a router is a device which holds all the address of all the computers on its network, it is a layer 3 devixe which operates at the packet level and can send the correct packet to the correct computer deirectly as each device in the betwork is on a segment of 1 machine so collisons are drastically reduced.

what is a switch

it is a device which connects segnents of a network together, it recives signal from one segment to the other and forwards it to correct segment. reduces collisons as each computer in a segment sould be transmitting at one time.;

Roles of the operating system

To ensure computers security

to manage memeory management to switch between procsses

to allow interrupt handling

to allow a backing store management

to allow cheduling to allocate processor time efficeintly.

What is a multiprogramming os

MuLtiprogramming in a computer system is hwere more than one job or process is held in the main memeory at one time and can be processed at (apprently) the same time. it promotes fficent useof the cpu as the cpu is not idle whilst it is waiting for a slower peripheral or some event to occour. ahci9veed by a system of interurots.


inituates collection of programs and data for non interatcive processing together.

- make optimum use of computer recousrces

makes use of [processor when computing resources are less busy so is efficient.

What is multitasking

multistaking - where more than one task/process is avilable/can be to be run at the same time

it is diffwrent from multiprograming as multiprogramign means that the process is stored in emmeory but not running in parallel. the programs may just be open. applications are not performing a task.


this is wher a number of users are connected to a centralised computer and have indiviual control. the central server deicdes what user needs processing (reuest) and allocated time slices indiviually to them.

standalone machine

security is improved as less chance of unathorised access via a network. it is on a network segment of 1 machine

not susepbile to netwrok or power fqailure

but not suseptible to share data or software

blocked ready running

a process is blocked if it is not running as it is waiting for some event such as intput or output from a peripheral

a process is rrady if it is not running because its is waiting for alloation of a time slice by the scheudler program

a process it running if it is actually utiizing the cpu.

what is an interrupt

an interrupt is a signal generated by a device which causes a break in executiion of the current program/ or routine. when an interrupt arises the os suspends to current process and saves any data to registers. an interrupt service routine is run

afterwards the os resumes the current routine or deals with another interrupt.

what happens when a high prioerity input arises

the os suspends to currwnt interrupt service routine

the os runs a high prioerity interrupt routine

after itb has finished servcing the high prioerity interrupt the original interrupt service routine is run

examples of high [priority inp=terupt routines include imminent power failure or imminent loss of data

examples of interrupt

user generated interupts such as ctrl break function

iminent power failure

hardware generated such as a printer being out of paper

as run time error such as dividing by 0

operating system geernated interrupt such as the end of a time slice

a time slice

a time slice is an amount of time allocated to a process to run by a scheduler program


the process by which time slices are allocated. the operatign system may page jobs in and out of the memeory, it allows a job a pre determined time slice for it to run

paging jobs in and out of memeory allows between use of memeory as the time slice may finish after sa process has finished runnign so prevents memory being tied up or cpu being idle waiting for next time slice


thwe sewquential checking by sending signals to compennts or devices to ensure each job gets its apprioraite time slice.


is the division of main memory to allow more thna one job to be resident in main memeory at one time. main ememory is separated into paertitions where jobs are paged in and out.

sequantial add and delete

a equrntial file is where records are stored in key field order and accessed in this way

to ad a file

a copy of the old master file is taken, all records from the old mater file are copied accross until the point where the new record needs to be added. the new record is added from the unsorted transaction file. all the rest of the records are copied accross and th3 old master file is discarded.

do the same for deletion but just dont copy it accross. if more than one records neededs to eb added the transcation file should be sorted becfore the updating process is run


- earier to prgram and fewer overheads

- siitable if thre only access that is ever needed is sequential.

indexed sequntial

records are stored in order of key seuwnce and an index is used to allow serialo access to the records. there are two components to a record. one component stores the bulk of the record anywhere ohysically on the disk and the other ocmonenet stores the key friend/s and the pointer which points to the disk address in memeory. an index allows data to be accessed directly.

a multi level index may be used there is a main index which points to another sub index from here the data can be accessed directly or the pointer may be to another sub index. the process may extend to several levels

advantabges is that it is faster than sequerntial record access.

\no opverhead of has algorith so less oveheads needed and les smemeory intensive is all or most records are reuqired

random access

random access files use a hashing alogorthm wuith hash fields to output a physical location or a bucket to access. or add a record. to add a phycial location is clauclatied by inputting key firleds or has fieldsinto a hashing alogirthm. wich perform a clauclation and outputs it.

if the di9sk block is full then a record will be stored in the overflow atrea. the overflow area is searched and soprted linierly and xuisting records are accessed in the same way. once the overflow area is full then the record indexes will be re claculated.

allows fast access suitable for files of any zise,

generation of files

in a payrioll system or a rugularly updatesd system. the master file is updated with the trenslation file. the old master file becomes the father and gth new master file which has been updated is the son. when the son file is updated again with the new transaction file, the oroginal master file is the grnadfather. the son now becomes the father and the most recoent is the son. the purpiose of generation of siles is that is one version becomes corrupted a tranaction log can be used with one of the 3 lateste versions of the file to update it and restore it.