COMPUTER SYSTEMS

• Hardware


  Physical components of a computer system. Includes internal components EG hard drive, sound card and external components EG printer, speaker
• Software


  Program code, sequences of instructions which are executed to perform a task
• Application software


  Programs that complete a specific task for the user EG word processor, web browser, spreadsheets
• System software


  Operates, controls and maintains the computer and its components. Includes operating system, utility programs, library programs, and translators
• Operating system


  Allows user to control the computer by providing virtual machine to hide the true complexity. Also manages and controls computer resources EG memory management, processor scheduling, interrupt handling
• Interrupt


  Signal sent to processor when an important event occurs
• Utility programs


  Housekeeping tasks EG data backup, defragmenting hard drives, data compression and encryption
• Library programs


  Contain useful functions that are frequently used by a program. Can be used by programmers to simplify program development processes. If making use of one, programmer must import it within their program code
• Translators


  Software that translates between different types of language. 3 types: compilers, assemblers and interpreters
• Development of programming languages

  1. Limited speed and memory of early computers forced programmers to write programs with low level languages which directly manipulated processor, required a lot of effort to code, and were error prone

  2. High level languages allow instructions to be communicated to a processor making programming easier
• Low level languages


  Programs written are specific to the processor and directly affect the processor. 2 types: machine code and assembly language
• Machine code


  Only uses binary, making programs long and unreadable for humans therefore prone to errors and difficult to debug. Directly manipulates processor so very powerful and programmers are not constrained, and no need to translate before executing so useful for embedded systems and real time applications where speed is important
• Assembly language


  Made to simplify writing programs. Uses mnemonics EG ADD, MOV in place of binary machine code so is more compact and less error prone
• Each assembly language instruction has a 1-to-1 correlation to a machine code instruction
• High level languages


  Not platform specific but must be translated by compiler or interpreter before they can be executed. Dont use binary or mnemonics, use english instructions and mathematical symbols so much easier to learn and understand and debug
• High level languages


  • Allow use of built-in functions which save time programming
  • Have named variables, indentation and commenting which makes it easier to debug
• Imperative HLL


  Formed from instructions that specify how a computer should complete a task
• Declarative programming


  Describes what a computer should do
• Translators


  Translate high level languages or assembly language to machine code so they can be executed. 3 types- assembler, compiler, interpreter
• Assembler


  Translates assembly language into machine code. Because they have 1-to-1 relationship it is quick and straightforward. Assemblers are platform specific so must exist for each different type of processor
• Compiler


  Translated HLLs into machine code. Take high level program as their source code, check for errors then translate the entire program at once. If source code contains errors it will not be translated. Produce machine code so are platform specific. Once translated, compiled program can be run without requirement of any other software- not the case with interpreters
• Interpreter


  Translate HLLs into machine code line by line. Have procedures that can be used to translate each kind of program instruction. Rather than checking for errors before translation, they check for errors as they go so a program with errors can be partially translated until error is reached. Once translated, program source code and interpreter must be present- poor protection of source code compared to compilers so original code is difficult to extract
• Source code


  Input of a translator. Assembly language code for assemblers, high level language code for compilers and interpreters
• Object code


  Translators output, produced from the source code
• Intermediate language


  When compilers dont produce machine code straight away. Often bytecode, and allows for platform independence
• Compilers that use intermediate language


  1. Translate HLL code into intermediate language EG bytecode and then use virtual machine to execute bytecode on different processors

  2. Each processor instruction set has its own virtual machine

  3. Using intermediate language allows interpreter to translate source code just once while still being able to execute translated code with variety of processors
• Half adders


  2 inputs, 2 outputs, 2 logic gates
• Full adders


  3 inputs, 2 outputs
• Edge triggered d type flip flop

  Logic circuit which can be used as a memory unit for storing the value of a single bit. Has 2 inputs, one for data and one for clock signal and one output, which holds the value for the stored bit. The clock signal is generated by the computer and alternated between 0 and 1 at a set frequency. The value of the stored bit is set to the value of the data input with each change of the clock signal. The clock can be used to synchronise multiple flip flops when they form a larger system EG shift register