Notes Websystem Print - Summary Web Systems PDF

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OPERATING SYSTEMS Definition Software the sits between the computer hardware and all programs. Purpose Manages your computer, Runs Programs, Interface between user and hardware, Provides services to programs & users, Protects users and programs from each other Examples Unix based(mac OSX, Linux, Solaris) || Windows NT based(Windows Vista, Windows 10) || Symbian (Nokia phones) Hardware MICOS: Memory, Input, CPU, Output, Storage Kernel Controls hardware devices on a direct level, provides necessary resources and services to devices and manages their usage Application & Services Programs(Do something for user), Services(Run behind the scenes and provide system support) Shell Uses command line to execute commands(Greater access to system for more competent users however harder to learn) GUI Runs on top of OS user-friendly uses mouse, icons, menu and windows

Multiple Interfaces helps with understanding automating and customising Characteristic of Scripting Languages are language inconsistent, untyped, created by one person and run through interrupter OPERATING SYSTEMS MORE INDEPTH Unix used since 1969, most computer that run the internet use it Evolution: -Most are based on System V and BSD -Ad Hoc(When needed) development lead to powerful OS but confusing interface for Users -Original Standard POSIX rejected as it was too complex(hard to transfer between systems), 2002 SUS if specs meet it would be called UNIX like IBM VM/CMS only one used continuously Why has UNIX survived -No own the ideas > easily to implant no hidden secrets -Based on simple concepts > enables use of new ideas and technologies -UNIX is portable -Unix is free, fast, stable, secure > design security for many user systems -Unix as a set of tools approach > Create very powerful Ad Hoc tools due to passing output of commands to one another FILE SYSTEMS => part of OS manages storing and accessing data -Logical File system = user view of files(executable and data), directories and partitions -Physical File system = how items are physically stored & shown Directories Creates logical division in a file system which helps with organisational purposes Partitions Creates physical divisions, Can mount and unmount partitions(USB),partitions independent of each other, aka(Drives) TREES: collection of nodes with a relation -Edge: branch of tree

-Leaf: node with no children -Siblings: node with same parents Disk Physical stack of magnetic platters, divided into cylinders, each cylinder has circular tracks which are dived into sectors. -Head moves concurrently along disk however disk spins with constant angular velocity -Formatting operation which creates physical disk structure Disk Logical Partitions: Partitions are dived parts of disk and each one becomes independent of others Blocks: OS sees all disk space as a array of fixed size logical blocks File Allocation -Block Space allocated to files one or more blocks -Directory table of information OS uses to locate blocks within files -Contiguous supports random access, however fragmentation of unused space will occur. Good for tiny file systems -O(1) “about 1) -Chained good for sequential access, each block points to next, directory entry contains the head block number and length of file -O(n) “roughly n blocks” -Indexed tree based file allocation uses special index data block which contains list of data blocks numbers for file good for large -O(logk(n)) blocks -Inode: Used in Linux, inodes are numbered, inodes store metadata NOT FILE NAME, pointers to directory, inodes use directories(store filename and inodes number for files), -“Single indirect block”, pointer to disk block -“double indirect block”, pointer to more “single indirect blocks” -“triple indirect block” , pointer to more “double indirect blocks” WEB ABD HCI Web Server, program that accepts requests from internet. Web pages, plain text files that use text and tags(HTML) Web browser, Program that uses an engine to see an HTML document Structural Tags: , define compotents and areas on page Presentation tags: , define how information is presented 4 types of selectors available to us -tag -id #name, used only for one element -class .name, can be used for multiple -inline coding within html file 4 ways to implement CSS -Imported, add external CSS file to a set of CSS rules -Linked, link an external CSS file to an html

-Embedded, including css style at the beginning of HTML file -Inline, doing inline with the text e.g. Operating System Process Management Processes, are programs in execution, each run in new process, there are also additional system processes, services and background tasks. Process Management, how OS handles starting, managing, interprocess communication and terminating processes. Managing processes

Concurrent Programming -Multiprogramming “cooperative” process waits for I/O, Multitasking “pre-emptive” sets a timer on all processes ‘fair share’ CPU time, can have priorities set(low, high, real time) Multithreading, Multiple instances of same code, allows O/S to run process on more then 1 CPU Multiprocessing, Multitasking spread amongst 2 or more CPU’s Process States 1 Created, loaded into memory 2 Waiting to be run by CPU 3 O/S runs program 4 If process needs resource, ‘blocked’(waiting) for resource 5 Then waits for OS re-start process with resource ready 6 Once Process is completed terminate the process Interrupts O/S runs process continuously until interrupted CHANGE Interruptions come from Hardware(Mouse movement, network) and Programs(runtime error, pause for input, waiting for data to load) Process Scheduling

INTER PROCESS COMMUNICATION processes often need to communicate with each other called IPC -Operating Systems allow communication via File Sharing, shared memory, signals, sockets, messages, pipes, semaphores, locks 2 way IPC: Processes can communicate both directions Shared Memory, bit of memory which acts like file Name Pipe, special 2 way pipe acts like a file Socket, network or internal interface card Message quote/passing, special programming interface like SMS Semaphore, special flag/file controls access to resources Operating System Resource Management RESOURCE MANAGEMENT Resources, things that process might need to run Kernel, manages all systems resources Resource contention, when two process want to alter same resource at same time Types of resources: Memory, Files and Hardware Deadlock, When processes wait for a resource that is held by another process and that process is waiting for the others resource hence neither can give up their resource waiting 4ever Solutions to ‘Dead lock’ Semaphores, flag held by process changing memory Lock files, file is not readable/writable while data is being written Deadlock occurs when there is mutual exclusion or hold and wait Mutual Exclusion, exists a resource, that is only accessible by one

process at a time Hold and wait, process that hold at least one resource is waiting for another resource Dealing with Deadlocks includes Avoidance/Prevention(O/S) decides which processes may use resources and when, Detection/Management, allows for deadlocks to be formed and then breaks them. Operating System Memory Management Memory management: Allows for: run more process even thought they can’t fit into physical memory, optimise use of RAM, keep track of processes using blocks of memory, provide control to memory, decide where processes are loaded into, allocate/deallocate memory. PHYSICAL AND LOGICAL ADDRESSES -Computers have fixed amount of real/physical space -Kernel has access to this memory via physical addressing -O/S hides physical memory from process so all process see is logical memory. -Typical O/S allocate PAGES(4k blocks of RAM) While executing a process the same logical address may be mapped to many different physical addresses as data and programs are paged in and out. The logical address is larger then physical(RAM) if we have Virtual Memory VIRTUAL MEMORY Virtual memory, makes part of the hard disk like main memory VM: Logical address space(RAM + Disk) Swap file: Part of disk used for VM A page: amount of data that can swap between RAM & Disk at any given time Page Table Maps logical addresses to either physical or virtual memory Paging action of swapping page between RAM and Disk Page Fault When data to be accessed is not in Ram and needs to be swapped from Disk Thrashing OS spends more time paging then running applications Locality guesswork on what needs to be done in RAM Temporal Locality recently used memory is likely to be used again. Spatial Locality locations near used accessed memory are likely to also be used again Computer Science CPU Memory Types: ROM(read only memory) RAM(Computer forgets when its off) Arithmetic Logic Unit, performs actual operations maths & logic Control Unit, directs all input and output in relation to ALU as well as fetches code for instructions, proving control. Registers, like super fast small RAM to hold instructions, storage address or any kind of data Process Function, Fetch Execute Cycle(Fetch > Decode > Read Memory(Registers) and Execute Instructions DATA = DIVA Documents, Images, Video and Audio CPU’s don’t understand regular data e.g. characters, images ,etc O/S designers invented character encoding -A-Z, a0z, 0-9, !@#$%^&*() Latin and other symbols Pre-Historic -EBCDIC, used IBM mainframes, punch cards Historic -ASCII, 7 bits – 128 different characters -ISO-8859 – extension of ASCII to include other alphabets e.g Latin, Cyrillic, Arabic, Hebrew, Greek, Thai Current encoding

Unicode, Expands ASCIII to 16 bits, over 120,000 characters UTF-8, most common, compatible with ASCII UTF-16, every character as 16 bits or 32 bit The first 128 characters same as ASCII for backward compatibility Computer Science Conversions  Number System: how to represent binary, octal, decimal and hexadecimal  Binary to Decimal: write the 2 power from right to left and add only the ones with one  Decimal to Binary: divide the number by 2 and start from right until you get to 1 ( 1 = odd 0 =even)  Binary to Hexadecimal into binary and break it down Brief Adv Logic

Networking AND THE INTERNET Nodes are hosts, hubs, switches and lines are Ethernet ADSL Networks Network File System, part of remote disk made to appear as another hard drive Host, another name for computers Modem, Converts between analog and digital signals NIC (Network Interface Card), Direct connection to local area networks. Hub, connect more then 2 hosts, dumb Switch, a more smart Hub Wireless Access Point, what most people use LAN, Local access networks Network Topology, refers to structure of network Physical Topology, how data is actually transmitted Logical Topology, concerned when data ends up To avoid collision hosts take turns to transmit data using tokens WAN, Wide access Network Router, smarter, router decides next destination of packets via IP -Simple router could be your home -Router use routing rules -Various algorithms exist -The Unix command ‘traceroute’ Fat client, where most processing is done, Thin client just the prestation is done at client and processing is at server Three-tier architecture: Used when there is intermediate stage between user. User -> Webserver -> Database FORMULA: World Wide Web == network + Applications + people

Three common categories of peer to peer: -File swapping -Instant messaging -Distributed processing Web crawling -Creates an index matching combinations of words -For each search, it ranks web sites in relations to other websites American Standard Code, standard code for information interchange Uniform Resource Locator, URL allows identification of resources on internet Telnet, Allows you to login remotely on other networks SSH, Secure shell, provides encrypted versions of common networks unties. SMTP, transfer of emails between servers. File Transfer Protocol, FTP PORTS 1 TCP Port Service Multiplexer (TCPMUX) 5 Remote Job Entry (RJE) 7 ECHO 18 Message Send Protocol (MSP) 20 FTP -- Data 21 FTP -- Control 22 SSH Remote Login Protocol 23 Telnet 25 Simple Mail Transfer Protocol (SMTP) 29 MSG ICP 37 Time 42 Host Name Server (Nameserv) 43 WhoIs 49 Login Host Protocol (Login) 53 Domain Name System (DNS) 69 Trivial File Transfer Protocol (TFTP) 70 Gopher Services 79 Finger 80 HTTP 103 X.400 Standard 108 SNA Gateway Access Server 109 POP2 110 POP3 115 Simple File Transfer Protocol (SFTP) 118 SQL Services 119 Newsgroup (NNTP) 137 NetBIOS Name Service 139 NetBIOS Datagram Service 143 Interim Mail Access Protocol (IMAP) 150 NetBIOS Session Service 156 SQL Server 161 SNMP 179 Border Gateway Protocol (BGP) 190 Gateway Access Control Protocol (GACP) 194 Internet Relay Chat (IRC) 197 Directory Location Service (DLS) 389 Lightweight Directory Access Protocol (LDAP) 396 Novell Netware over IP 443 HTTPS 444 Simple Network Paging Protocol (SNPP) 445 Microsoft-DS 458 Apple QuickTime 546 DHCP Client 547 DHCP Server 563 SNEWS

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