OCR Computer Science A Level Past Paper Notes PDF

OCR Computer Science A Level 1.2.2 Applications Generation Advanced Notes www.pmt.education Specification: 1.2.2 a) Nature of applications 1.2.2 b) Utilities 1.2.2 c) Open source vs closed source 1.2.2 d) Translators ○ Interpreters ○ Compil...

OCR Computer Science A Level 1.2.2 Applications Generation Advanced Notes www.pmt.education Specification: 1.2.2 a) Nature of applications 1.2.2 b) Utilities 1.2.2 c) Open source vs closed source 1.2.2 d) Translators ○ Interpreters ○ Compilers ○ Assemblers 1.2.2 e) Stages of compilation ○ Lexical analysis ○ Syntax analysis ○ Code generation ○ Optimisation 1.2.2 f) Linkers, loaders and use of libraries www.pmt.education  Nature of applications Software can either be categorised as applications software or systems software. Applications software Applications software is designed to be​ used by the end-user to perform one specific task​. Application software requires systems software in order to run. Examples: ​desktop publishing, word processing, spreadsheets​, ​web browsers. Systems software Systems software is​ low-level software that is responsible for running the computer system smoothly​, interacting with hardware and generally providing a platform for applications software to run. The user does not directly interact with systems software but it ensures high performance for the user. Examples: ​library programs, utility programs, operating system, device drivers. Utilities Utilities are a key piece of system software integral to ensuring the ​consistent, high performance​ of the operating system. Each utility program has a ​specific function​ linked to the ​maintenance of the operating system. Examples include: - Compression Operating systems provide utilities that enable files to compressed and decompressed. This is used when compressing large files to be transmitted across the Internet and is commonly used to compress scanned files. - Disk defragmentation As the hard disk becomes full, read/write times slow down. This is because files become fragmented as they are stored in different parts of memory. The disk defragmenter utility​ rearranges the contents of the hard drive​ so they can be accessed faster, thus improving performance. - Antivirus Antivirus is responsible for ​detecting potential threats​ to the computer, alerting the user and removing these threats. www.pmt.education  - Automatic updating This utility ensures the operating system is kept up to date, with any updates being automatically installed when the computer is restarted. Updates tackle bugs or security flaws so this ensures the system is less vulnerable to malware and hacking threats. - Backup The backup utility automatically creates routine copies of specific files selected by the user. How often files are backed up is also specified by the user. This means that in the event of a power failure, malicious attack or other accident, files can be recovered. Open source vs closed source Source code is written by a programmer and refers to ​object code before it has been compiled​. When software is described to be ‘open source’ or ‘closed source’, this refers to whether or not the source code is accessible to the public. Open source Closed Source Definition Open source code can be used Closed source code requires the by anyone without a license and user to hold an ​appropriate is ​distributed with the source license​ to use it. Users ​cannot code​. access the source code​ as the company owns the copyright license​. Advantages Can be modified and improved Thorough, regular and by anyone well-tested updates Technical support from online Company owning software community provides expert support and user manuals. Can be modified and sold on High levels of security as developed professionally. Disadvantages Support available online may be License restricts how many insufficient or incorrect. No user people can use the software at manuals. once Lower security as may not be Users cannot modify and developed in a controlled improve software themselves environment www.pmt.education Whether a user chooses to use open source or closed source software ultimately depends on the ​suitability of the software to the task they will be using it for​. The user must also consider: - Costs - ​implementation, maintenance, training of staff, license - Functionality - ​features available, ease of use Translators A translator is a program that ​converts high-level source code into low-level object code​, which is then ready to be executed by a computer. There are three types of translator that convert different types of code and work in different ways. Compiler Compilers translate high-level code into machine code ​all at once​, after carrying out a number of checks and reporting back any errors. This ​initial compilation process is longer​ than using an interpreter or an assembler. If changes need to be made, the whole program must be recompiled. Once code has been compiled to produce machine code, it can only be executed on certain devices - compiled code is specific to a particular processor type and operating system​. Code can be run ​without a translator​ being present. Interpreter Interpreters ​translate and execute code line-by-line​. They stop and produce an error if a line contains an error. They may ​initially appear faster​ than compilers as code is instantly executed, but are ​slower than running compiled code​ as ​code must be translated each time it is executed ​with an interpreter. This feature makes interpreters useful for ​testing ​sections of code and pinpointing errors, as time is not wasted compiling the entire program before it has been fully debugged. Interpreted code​ requires an interpreter in order to run​ on different devices. However, code can be executed on a range of platforms​ as long as the right interpreter is available, thus making interpreted code​ more portable​. www.pmt.education Assembler Assembly Code Assembly code is considered to be a low-level language as it is the ​‘next level up’ from machine code​. Assembly code is ​platform specific​, as the instructions used are dependent on the instruction set of the processor​. Assemblers translate assembly code into machine code. Each line of assembly code is equivalent to almost one line of machine code​ so code is translated on almost a one-to-one basis. Stages of compilation When a compiler is used, high-level code goes through four stages before it is turned into object code that is ready to be executed. Lexical Analysis In the first stage of compilation, ​whitespace and comments are removed​ from the code. The code below... while flag = False: print “not found”; #terminates when item is found Is translated into... while flag =False: print“not found”; The remaining code is analysed for keywords and names of variables and constants. These are ​replaced with tokens​ and information about the token associated with each keyword or identifier is stored in a ​symbol table​. Syntax Analysis In this stage, ​tokens are analysed against the grammar and rules of the programming language​. Any tokens that break the rules of the programming language are ​flagged up as syntax errors​ and added to a list of errors. www.pmt.education Examples of syntax errors: ​undeclared variable type, incomplete set of brackets. An ​abstract syntax tree is produced​, which is a representation of the source code in the form of a tree. Further detail about identifiers is also added to the symbol table. Semantic analysis​ is also carried out at the syntax analysis stage, where logic mistakes within the program are detected. Examples of semantic errors: ​multiple declaration​, ​undeclared identifiers Code Generation The abstract syntax tree produced in the syntax analysis stage is used to produce machine code. Optimisation This stage of compilation searches through the code for areas it could be made more efficient. The aim of optimisation is to​ make the code faster to execute​ although this stage can significantly ​add to the overall time taken for compilation​. Insignificant, ​redundant parts of code are detected and removed​. Repeated sections of code may be grouped and replaced with a more efficient piece of code which produces the same result. There is a danger, however, that excessive optimisation may alter the way in which the program behaves. Linkers, Loaders and Use of Libraries Most programs use external pieces of code within them, including subroutines and libraries from outside sources. Linkers This is a piece of software that is responsible for​ linking external modules and libraries included within the code​. There are two types of linker: Static Modules and libraries are ​added directly​ into the main file. This ​increases the size of the file​. Any updates to modules and libraries externally will not affect the program. This means a specific version of a library can be used. Dynamic Addresses of modules and libraries​ are included in the file where they are referenced. When the program is run, the loader retrieves the program at the specified address so it can be executed. The advantage here is that​ files remain small​ and​ external updates feed through to the main file​; there is no need to rewrite the code. www.pmt.education Loaders Loaders are programs provided by the operating system. When a file is executed, the l​oader retrieves the library or subroutine from the given memory location​. Use of Libraries Libraries are ​pre-compiled programs ​which can be incorporated within other programs using either static or dynamic linking. They are ​ready-to-use and error free​, so ​save time developing and testing modules. Another advantage of libraries is that they ​can be reused within multiple programs. Libraries are often used to provide a specialised range of functions which would otherwise require a lot of time and effort to develop, so​ save programmers from having to ‘reinvent the wheel’​ and instead make use of others’ expertise. Popular libraries provide mathematical and graphical functions. www.pmt.education