Key terms

  • Created by: Sophie153
  • Created on: 07-01-16 12:23
what is sharing?
system must be able to share info/resources between concurrent activities therefore system must be considered as part of a network and servers not in isolation
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what are the long term needs of basic storage?
online files which can be accessed with no delay, archived files(may be far away...), protection system (against different users...), back up system incase of failure
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what is nondeterminacy?
events in a OS rarely happen in any predictable or repeatable order, it must be capable of handling all sitations
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what are desirable OS features?
efficiency (high use of resources,low idle time), reliability(max time between system failures due to software error), resilience (maximise time between system failures due to hardware error), maintainability (must be easy to correct errors), small s
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what is concurrency?
several processes active at the same time (its apparent not actual)-involuntary switching of CPU therefore must assume true concurrency
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what is mutual exclusion?
More than one process competes for common but unsharable resource
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what is synchronisation?
a process requires some other processes to have reached certain point in execution before continuing itself
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What is the kernel?
Innermost layer of a system, contains critical code. it provides process switching and control, communication, synchronisation, lowest level of interaction with hardware
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three main components of basic kernel?
first level interrupt handler, low level scheduler, communication primitives
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what are communication primitives?
semaphores, message passing...
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what is first level interrupt handler (FLIH)
the OS instructs hardware to jump to FLIH whenever interrupt occurs
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what is the low level scheduler?
performs start up and switching
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what is process start up?
allocate and initialise PCB, add this to list of runnable processes and update system tables
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what is process switching?
chooses which processes to use processor, if it isn't current one running then it must save context of process, load selected process and pass control to this current process
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what is the process control block?
process descriptor (mainly used by low level scheduler), for each process it saved contents for hardware registers(postponed dictated by hardware),user ID, process name/no, privaligies, state, resource usage,resource limits, priority
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what are single user systems?§
provides one virtual machine for one user (can provide more than one VM)
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what are embedded systems?
used control particular process/equipment e.g. mobile phone, train, washing machine
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what are general purpose systems?
used for many different tasks, different levels of complexities, often multi-user e.g. UNIX
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what are three common approaches to system structures?
monolithic, layered, client-server
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what is monolithic
one large program- difficult to maintain and enhance
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what is layered?
divided in to module layers (more theoretical), structure resembles an onion inner are most important
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what is client server?
modules have roughly equal status, communicate by sending messages through microkernel to different modules, rest of system in different modules e.g. file system, disk system
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why do we need memory management?
memory is a resource which i competed for, factors such as fragmentation come in to play so need to use tricks for more efficient use
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what is a real address known as?
a physical address
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what is the max memory size known as?
address space
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what id a virtual address known as?
logical address
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what is max size on a virtual machine known as?
virtual address space
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what is static memory management?
process A and B are in memory C won't fit in after A is deleted even though enough memory-fragmentation. process B must be moved to make room
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what is virtual memory a provision of?
address transformation (virtual to real), address checking (legal and accessible), protection (ensure some parts cant be altered or examined)
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what does the base contain in base and limit systems?
the real address corresponding to virtual address zero for current process
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what does limit contain
size of virtual machine
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what are paging systems?
provides a VM that can b bigger than real machine
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what can a virtual address be spilt in to?
page number (to locate PTE) and offset (how far through the page it is)
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what is a page fault?
outside the bounds of the page table , hardware generates synchronous interrupt,
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what is a translation look aside buffer>
records contents of recently used PTEs in fast memory in assumption be needed again soom
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what is a demand fetch?
fetch the page when a page fault occurs,can clusterndjacent pages to reduce overhead
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what is anticipatory fetch?
analysing process behaviour and use spare time and I/O capacity to overlap fetch with execution
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what is placement strategy?
use a free frame
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what is replacement strategy?
if no free page frames, the choice of which frame to replace
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what are page replacement strategies?
Least recently used, least used, first in first out, random, not-used-recently,
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what is contagious organisation?
files are stored in consecutively numbered blocks (info is starting block and number of blocks)
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what is file map organisation?
files are stored in any available block on the disk, space on the disk for file map/file allocation table (FAT)
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what is disk compaction?
it is in continuous organisation, moves files to make space in to one lump
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what does an open function for reading do?
allocate buffer space(area in working memory), locate directory entry for file, check access permissions, check for clashing usage, pass back to unique identifier (file descriptor)
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what will an open function for writing do?
create a directory entry if it doesn't exist, truncate any existing file
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what does a write function do?
allows data to be sent to a file at the current position
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what does a seek function do?
allows current file position to be altered
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what is close function needed for?
if writing, flush any partially filled buffers, write their contents to the disk), deallocate buffer space (return to borrowed memory), update directory every with size, date, time...
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/what is the basic aim of a backup system
capable of restoring part/all of contents in the filing system to a previous state
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what is a complete backup?
represents a complete copy of contents of the filing system, takes a long time and uses a lot of resources, easy to restore from
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what is a partial/incremental backup?
represents only the files for which current backup doesn't exist so cheaper, must be based on recent complete backup, more complex to restore
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what is a differential backup?
contains all the files changed/created since last complete backup
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what are desired features of a backup system?
reliability (must garuntee copies are complete and correct), ability to run on an active file system, privacy and security of backed up files, physical security of backup media
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what are the common backup methods?
hard disk to hard disk, hard disk to tape, hard disk to CD/DVD/ Blu-ray
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What shape is a directory hierarchy arranged in?
tree structure
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what is a network?
something that allows data on one computer to be passed to another
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what does a network do?
it can share resources (disks, printers..), communicate (world wide web, file transfer, telephone), security concerns (limit access to resources...), flexible (easy to expand at low cost), reliable, redundant components/machines
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what is inernetworking?
the connection of different types of physical networks , making them operate as a co-ordinated whole
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what are the two standards?
de jure (by law)-standards adopted by national/international bodies, de facto (from the fact) standards that evolve because everyone uses them
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what are the ways of making a connection across a network?
circuit switching (a physical end-to-end pathway is found which is used for the communication e.g. phone call), packet switching it is a packet of data plus red tape info (admin) and it may arrive out of order
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what is a protocol?
a set of agreed rules to allow nodes to communicate with each other in an unambiguous manner
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what are the 7 layers in the OSI model?
application, presentation, session,transport(ensure it arrives,network(routing info over the larger network), data link (collecting bits into larger units for transmission + management of flow), physical (movements of bits across transmission medium)
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what do application, presentation and session layers do?
providing different aspects of the way application programs (using the network) communicate with, and understand, each other
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wat layers does the internet model have?
application, transport, internet, network interface, physical
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what does application layer do?
users invoke application programs that access available services; they pass data to or from the transport layer for delivery
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what does the transport layer do?
provides communication from one application program to another; so-called end-to-end communication may or may not provide reliable (guaranteed) transport, divides any data streams into packets
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what is the internet layer?
handles communication from one machine (‘host’) to another; is told the destination host; wraps packet in an IP datagram, works out how to deliver and does it; receives similarly keeps it all running!
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what is the network interface layer?
accepts IP datagrams, and transmits them over a specific network; receives similarly may consist of device driver or something more complex (e.g. yet another ‘wrapper’)
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what is the hardware layer?
the physical network interface cable, wireless, avian carrier, etc. technology: ethernet, token ring, etc.
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what is shared bus technology?
all stations on it connect to a single, shared communication channel
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what is broadcast technology?
all stations can potentially receive every transmission
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what is best-effort delivery?
the hardware provides no feedback to the sender about whether data have been delivered
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what does a ethernet frame/packet consist of?
header (contains source, some 'red tape' and destination), data, trailer (error checking info)
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what do IP datagrams consist of?
header (source and destination IP address, protocol, 'red tape'), data
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what happens if datagram doesn't fit into physical frames?
spilt in to fragment-transmitted seperatly then reassembled
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what are the two most common protocols in transport layer?
User datagram protocol (best efforts only, single packets, connectionless) transmission control protocol (reliable, multiple packets, connection)
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what layers uses an IP address?
application, transport, internet
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what layer uses physical addresses?
network interface, physical
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what does the address resolution protocol message contain?
the IP address for which a physical address is required the sender’s IP address the sender’s physical address
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what is multicasting?
, the destination is a set of hosts, possibly at multiple locations one copy of the datagram is delivered to each host
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what is any casting?
the destination is a set of computers, possibly at multiple locations the datagram is routed along a shortest path and delivered to exactly one member of the group (the closest member)
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what is a dedicated computer system?
computer is used for one specific task
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what are embedded processors?
have dedicated processors embedded within e.g. motor systems, washing machines...
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What is a CPU?
controls computer operation. executes lists of instructions(software). CPU writes data to output devices and reads input.
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what is memory?
sotres software instruction or data in cells. Each cell has an address so CPU can locate.
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what is input/output devices (peripherals)?
CPU can communicate with outside world. periphals connected to CPU via standard interfaces
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what is a bus?
computer internal interconnections ar called the BUS. transfers data
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what is a operating system?
software responsible for interfacing computer hardware and controlling the execution of programs e.g. microsoft windows...
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what is a bit?
a single binary digit (1 or 0) primary unit
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what is a byte?
8 bits, most commonly used
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what is a nibble?
4 bits, half a byte
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what is a word?
binary data is stored in memory as groups of bits, each group occupies a single address. thee groups are called words. these are fixed for a given computer.
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what are logic gates?
building blocks of computer circuitry
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what is combinational logic?
output depends on current inputs
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what is sequential logic?
output depends on the current inputs plus the state previously applied
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What is a flip flop?
a logic gate, they're sequential and storage devices (store 1 bit)
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what is serial transmission?
uses one wire to send bits at one time
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what is parallel transmission?
uses 8+ wires to send 8+ bits simultaneously
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what do counters and registers do?
holds current instruction, points to next instruction on the list, holds subroutine info, ALU data registers
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what is the control unit?
generates hardware control signals
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what is memory and bus control?
memory management, holds key addresses
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what is cache memory?
fast accessed area
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what is Arithmetic logic unit?
perform computing operations
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what is the condition code register?
signals for negative numbers, zero, carry, overflow
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what does a program counter do?
points to first instruction on the list
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what is a clock?
main driving signal for the CPU.CPU can do one basic operation in one clock cycle.
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what is a CD-R?
To be used as storage, CDs must be writeable. This is achieved by replacing the aluminium by a layer of photosensitive dye
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what isCD-RW?
In rewriteable CDs, the dye layer is replaced by a special compound which forms a reflective layer (lands) at middle temperatures. At very high temperatures, the compound forms a dark substance (pits).
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what are the long term needs of basic storage?


online files which can be accessed with no delay, archived files(may be far away...), protection system (against different users...), back up system incase of failure

Card 3


what is nondeterminacy?


Preview of the front of card 3

Card 4


what are desirable OS features?


Preview of the front of card 4

Card 5


what is concurrency?


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