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the limited physical space of memory by using other storage in the computer o e.g. a hard disk can act as virtual memory because the OS can swap parts of a process that are not currently in use from the main memory to the disk and then swap it back when it is needed o means that a vast quantity of memory is available o slower to access secondary storage than main memory Interrupts • interrupt - a signal sent to the processor to request immediate attention o can happen because: ▪ a hardware device has data to process a hardware failure has occurred Component 1.2 revision notes 2 ● ▪ a hardware device has completed a task ● Interrupt service routine - a program designed to respond to an interrupts request 1. The processor receives an interrupt 2. The processor completes the fetch decode execute cycle that it was on 3. the current contents of the processor registers are saved to memory 4. the origin of the interrupt is identified so the appropriate ISR is called 5. lower priority interrupts are put on hold 6. PC is updated with the address of the first instruction of the ISR 7. ISR completes execution 8. processor registers reloaded with values saved to memory 9. lower priority interrupts are re-established 10. PC is set to point to the address of the next instruction that was due to be executed before the interrupt Scheduling scheduling - controlling which tasks are running and how long they take up the processors time • round robin - processor time is divided equally among all running tasks o each time period is called a time slice o ensures that every task gets processor time without a long wait o does not scale up well • First come first served - processors run from start to finish in the order in which the processor receives their request to run Component 1.2 revision notes 3 o easiest to implement o starvation may occur - when a process cannot complete its execution because it is constantly denied processor time o can be a long wait before a process can run o can work well in systems with only a few concurrent processes • Shortest job first - processes are queued and the process that needs the shortest time to complete goes first o reduces wait times o means that longer processors may not be executed • Shortest time remaining - processes with the shortest time to completion go first o can delay longer processes from executing • Multi-level feedback queue - uses queues of tasks where each queue has a different level of priority o if a process used too much CPU time, it is moved to a lower priority queue o if something has been idle for a long time, it is moved to a higher priority queue to prevent starvation o means that all tasks are given processor time but the more important jobs are dealt with sooner o ensures that no tasks dominate the processors time Types of OS distributed OS - o where separate computers are joined together via network connections to work together as a more powerful node Component 1.2 revision notes 4 ● o the distributed OS coordinated communication between these computer nodes o useful when processor intensive tasks need to be completed Embedded OS - o used in computers that only serve a specific purpose o used to provide hardware reliability and ensure efficient use of resources • Multitasking OS - o where the OS needs to switch between tasks so that the user can run multiple applications concurrently • Multi-user OS o provides the facilities for multiple users to access the same system at the same time without affecting other users • Real time OS o supports applications that need to process data to produce a particular output as quickly as possible BIOS • initialises and tests the hardware in a computer system so that it is ready for use by the other software on the computer non volatile Device drivers Component 1.2 revision notes 5 • A program that controls the operation of a specific type of device that is part of a computer system provides an interface that allows the OS and other software to interact with the device without having to deal with the hardware implementation of the device Virtual machines • where systems can run as software processes within other software that is already running without significant performance degradation • A host system can run software to behave as if it were running on a hardware system • can be used to: O run alternative operating systems o support incompatible software o create a test system o run multiple servers • some languages use virtual machines to act as an intermediary between the system and source code o at run time, the original program is translated into intermediate code which is then passed to a VM o the VM then runs this code and determines how to control the host computer Application software • software that is developed for a specific use o accepts user input, processes it and generates output • General purpose software - software that can be used for a variety of tasks o includes office applications o can be used for a number of different tasks Component 1.2 revision notes 6 Special purpose software - software that can only be used for one particular task o usually developed to fulfil a particular business need • Bespoke software - software that can perform a certain task but where the above types do not offer the required functionality o expensive and high maintenance System software • controls the hardware of a computer system • sits between hardware and application software o OS systems like windows and macOS are examples of system software • utility programs - programs that help a computer run as efficiently as possible Open and Closed source software Open source The source code is available Software is usually free Anyone can make modifications and contribute to the development of the software Support is often provided by users not the developers Types of translators Component 1.2 revision notes Closed source The source code is not available May have to pay a fee to use the software All changes are controlled by the developers Support is provided by the developers 7 high level languages - used to write programs that are independent of the computer architecture that they will run on • low level language - assembly language programs Compilers produce executable code in a form that cannot be read by a human takes the source of code and then the produces the code through the stages of compilation can be well optimised by the compiler no translation required at execution time code is processor specific have to wait for the whole program to compile before receiving errors Interpreters translates and executes each line of source code one by one Component 1.2 revision notes very useful at the development stage of a program source code runs slower than compiled code errors are reported as soon as they're encountered debugging is easier requires an interpreter to run the code Interpreter takes up space in memory Assemblers translate assembly code programs into executable code platform specific simple translation process Stages of compilation 1. Lexical analysis - remove any non-program elements • removes white space and comments • words are shortened using tokenisation 2. syntax analysis - checks that the tokens are in the correct order and follow the rules of the programming language 3. code generation - the object code is created before linked libraries are included 4. code optimisation - identifies redundant or repeated code and removes it 8 ● ● Linkers, loaders and libraries library - prewritten code libraries o give access to specialist features o means that code can be reused o library code has usually already been tested so is efficient and reliable dynamic link libraries - a library of prewritten code that can be shared by multiple programs • linkers - resolves cross references between code files and creates a single executable program • loaders - a part of the OS that copies executable code into RAM and prepares a program for execution Approaches to software engineering • The waterfall lifecycle - stages of a project are carried out sequentially o the entire of the last stage has to be fully completed, documented, discussed, agreed and signed off before the next stage can begin o requires a lot of investment in the early stages o requirements are fixed o can be suitable for simple projects o easy to plan with clear lines of responsibility o rigid o requires too much documentation o time consuming o testing is at the end so if defects are found there may not be time to address them Component 1.2 revision notes 9 • The spiral model - uses the same steps as waterfall but incorporates project cycles ● o a prototype is formally reviewed to inform the next cycle o identifies and manages risks o emphasis on risk management means it is suitable for large scale high risk projects o project teams need a risk management expert o types of risk: schedule risk, budget risk, technical risk, external risk agile methodologies - advocates building prototypes, testing and incorporating feedback as soon as possible o highest priority is to satisfy the customer through early and continuous delivery of valuable software o suitable for many projects o if it is to be used for large projects they must be decomposed o gives rise to scope creep - where the client keeps adding to the requirements Rapid application development - an agile approach to software development based around prototyping o puts less emphasis non rigid planning and more emphasis on an adaptive process o well suited for problems where the requirements are unclear o not suitable for projects where the users are hard to contact as it relies on continuous feedback o projects can take a long time • extreme programming - an agile approach that focuses on the refinement of the code o there should be frequent releases in short development cycles Component 1.2 revision notes 10 ● o the user is part of the development team o programming is done collaboratively o code quality is high o requires a high degree of communication ● Programming paradigms imperative languages - where programmers solve a problem by writing a set of instructions that state how a problem should be solved Declarative languages - define what should be achieved by the program • functional languages - a type of declarative language that has no mutable data structures Object oriented languages - where programmers use classes to define objects in order to model the problem to be solved Addressing modes opcode - the part of the instruction that specifies which operation the processor should perform • operand - the value or set of values relevant to the opcode • immediate addressing - where the operand is the value to be used in the operation • direct addressing - where the operand specifies the address of a memory location Component 1.2 revision notes 11 • indirect addressing -where the operand holds the address of a memory location that holds the address of the contents to use in the second operation • indexed addressing - where the operand specifies an address. The contents of the index register are added to the address to give a second address which will be used in the operation Assembly language • used mnemonics to represent binary Operation in machine code 0001 0101 0110 assembly code equivalent ADD LDA Component 1.2 revision notes BRA meaning Adds a value into the accumulator fetch's a value from a main memory location and loads it into the accumulator branch to a specific instruction Advantages Allow a programmer to create optimised programs when a computer has limited resources it allows the programmer to directly control how the resources Disadvantages Harder to write programs specific to a particular instruction set Do not have libraries of functions that can be imported Object oriented programming • an approach to systems development that focuses on objects and the way they interact objects in the real world they are part of our everyday lives • attributes - a data item that helps define or describe the object 12 • instantiation - the process of making an object from the class definition encapsulation - binding together the attributes and methods that manipulate the data to protect the data polymorphism - the ability of a method to exhibit different behaviours depending on the object on which the method is invoked • inheritance - when a subclass gains methods or attributes from its parent class ● Component 1.2 revision notes 13