Computer Science

The processor.

Control unit, Buses, Arithmetic-Logic Unit (ALU), Dedicated registers

The part of the processor that coordinates the activity of all other components.

A series of connectors that transfer signals between internal components.

The binary numbers (8, 16, 32, 64).

Consists of three separate buses carrying control signals, addresses and data.

Control bus, data bus, address bus.

Control signals are sent along the bus between the control unit and the other components of a computer.

Memory read, memory write, bus request, bus grant, clock

Causes data from the addressed location in RAM to be placed on the data bus

Causes data on the data bus to be written into the addressed location in RAM

Indicates that a device is requesting use of the data bus

Indicates that the CPU has granted access to the data bus

Used to synchronise operations by "ticking" between 1 and 0

Arithmetic-logic unit. The problem solving part of the processor that performs arithmetic, logical and shift operations on data.

Add, Subtract, Multiply and Divide

AND, OR, NOT, XOR

Binary shift- moving bits to the left or right within a register.

A register that is used to temporarily store results from the ALU, so that the processor is able to immediately access and reuse these results in subsequent calculations.

Holds the memory address of the next instruction to be executed.

Holds the current instruction, which is split into opcode and operand

The instruction that is executed by the CPU.

The data or memory location used to execute that instruction.

Holds the address in memory where the processor is required to fetch or store data from or to

Temporarily holds data moving between the processor and main memory

To carry out instructions from programs stored in memory.

Components so that they work together to achieve instructions.

Processors operate in defined stages that are used to carry out program instructions. Fetch-> Decode-> Execute-> repeat

The address of the next instruction is copied from the PC to the MAR

The instruction held at that address is copied to the MDR

Simultaneously, the contents of the PC are incremented

Where the value of 1 is added to a byte to indicate the next step

The contents of the MDR are copied to the CIR

The instruction held in the CIR is decoded. It is split into operand and opcode to determine the type of instruction it is. Additional data, if required, is fetched from memory and then the instruction is passed into the accumulator.

The instruction is executed and result held in the accumulator or stored in memory.

Words

Separate

-One directional -The width of it determines the maximum possible memory addresses of the system

2 to the power of 8/ 256

4 GiB (gibi bytes)

-Bi-directional -Width is defined by the number of wires or lines it contains

Data can be transferred to and from memory in a single operation

The architecture of a computer, including: • the word size • and the width of the address bus

Generally, there is a one-to-one correspondence between a machine code instruction and its assembly language equivalent

the width of the address bus

• Clock speed • The number of cores in the processor • The amount and type of cache memory

The Fetch-Execute cycle is triggered by the clock pulses of the system clock therefore the faster the clock speed, the faster a computer can fetch, decode and execute instructions.

Each core is theoretically able to process a different instruction at the same time with its own fetch execute cycle however, the software may not always be able to take full advantage of all of the processors.

In systems designed for parallel processing, each core can work concurrently on different parts of the same task.

A small amount of superfast (but expensive) memory that stores data and instructions that have recently been used by the processor.

Level 2 cache is larger but not as fast as Level 1 cache

Both types are held on the processor chip.

Instruction cache and data cache, so that data and instructions can be fetched simultaneously.

The more cache memory a computer has, the more likely it is that it will not have to fetch the next instruction or data from RAM, as it will already have been loaded into the superfast cache memory from which it can be retrieved much more quickly.

This is a technique used to improve efficiency, for example by overlapping stages in the fetch-execute cycle, or by breaking down the stages in an arithmetic instruction. Starts another step before the first one finishes, like a tunnel (you don't wait for the first car to exit before you enter).

All programs that run on a computer

Applications software and systems software

Analysis Design Implementation (programming, testing and installation) Evaluation Maintenance

A circle

What the current system does, if there is one What the new system needs to do

Interview people who will use the software Use questionnaires to get information from large groups of people Observe how the current system works Look at existing documentation

A "system specification" that is a vital document, used to create the design, and to evaluate the finished product, that defines what the system will do, but not how it will do it.

A description of the data: data type, format, and validations Database design if appropriate Input screens Output screens and reports How the data will be processed How the software will be tested

Coding and testing the software Writing user and technical documentation Installing the software for the user

Testing that is carried out independently of the code used in the program. It looks at the program specification and creates a set of test data that covers all the inputs, outputs and program functions.

Testing that depends on the code logic. Tests are devised which test each path through the code at least once.

This is carried out by the software developer’s in-house team and by the user It can reveal errors or omissions in the definition of the system requirements.

The software is given to a number of potential users, who agree to use the software and report any faults.

When commercial software is being developed (e.g. MS Windows, MS Word, Sage Accounts, etc.)

Real users may try and do things the developer didn’t anticipate.

The user now needs to test every aspect of the software to make sure it does what it is supposed to do It will be evaluated against the original specification document

Acceptance testing.

Corrective maintenance Adaptive maintenance Perfective maintenance

Bugs will usually be found when the software is put into action, no matter how thoroughly it was tested

Over time, user requirements will change and the software will have to be adapted to meet new needs

Even if the software works well, there may be ways of making it even better – faster, easier to use, more functionality

In both, each stage is completed and documented before the next is begun.

Unlike in the lifecycle model, in the waterfall model the customer does not see the end product until it is completed, any change to be made often means the project has to be started again and the process doesn't repeat

If the lifecycle model didn't repeat and wasn't a circle.

The model is simple to understand and use Each stage is separate and self-contained with well defined outcomes and written documentation This makes the project relatively straightforward to manage The model works well for smaller projects where requirements are very well understood

There is not much user involvement after the Analysis stage, when the Specification document is agreed No working software is produced until late in the cycle The user is presented with the finished product and if it is not quite what was required, it is generally too late to make changes

The requirements are very clear and fixed There are no ambiguous requirements The technology is well understood The project is short

A spiral, where there are several prototypes, which each include the stages

Maintenance

A new, more refined prototype.

The software meets all the requirements

The well-defined steps make the project easy to manage Software is produced at an early stage so problems and issues can be identified early The user gives feedback on each prototype and any required changes can be made early in the process Added functionality can be added during the process The end result is more likely to be what the user wants

The process of developing prototypes, getting feedback and refining the prototypes is time-consuming so the finished product takes longer to develop A system is more costly to develop because of the time involved Not suitable for smaller projects

For medium to high-risk projects When users are unsure of their needs and what the possibilities are When the requirements are complex For large projects which may take years to develop, during which time new technologies may develop and significant changes occur

Plan Develop Test Install (optional) Demonstrate

Software is developed in rapid incremental cycles Each version builds on previous functionality Each version is thoroughly tested before release Good for small, time-critical projects Limited planning is needed to get started

Rapid, continuous delivery of useful software leads to customer satisfaction Customers, developers and testers constantly interact with one another Working software is delivered frequently, within weeks rather than months Software is easily adapted to changing circumstances Even late changes in requirements can be implemented

There is a lack of emphasis on necessary design and documentation The project can fail to deliver if the customer is not clear about the desired final outcome Not suitable for novice programmers – experienced programmers capable of making good decisions are required

When new changes need to be implemented – small incremental changes can be made frequently and for little cost In an expanding or developing business where users’ needs are continuously changing and developing

This is a type of agile software development Frequent releases of the software are made in short development cycles It is intended to improve productivity and responsiveness to changing customer requirements

Personnel change Requirements change Technology advances Costs spiral upwards Project gets cancelled

Workshops and focus groups instead of a document. Prototyping is used to continually refine user feedback Each part of the system is produced within a strict time limit – maybe not perfect, but good enough Software components are reused whenever possible