Section 1 - Components of a computer

Components of a CPU

Control Unit, Buses, Arithmetic Logic Unit, Dedicated Registers

Control Unit

Controls and coordinates the activities of the CPU. It receives an instruction, decodes it, manages its execution and stores the resulting data back in memory/registers

Buses

A set of parallel wires connecting two or more components together. There are three main buses in the CPU: the address bus, the data bus, and the control bus. These are known collectively as the system bus

Control Bus

A bi-directional bus, which transmits command, timing and specific status information between system components. Control Lines include: Bus request, bus grant, memory write, memory read, interrupt request, clock

Bus Request

indicates that a device is requesting use of the data bus

Bus Grant

Indicates that the CPU has granted access to the data bus

Memory Write

Causes data on the data bus to be written to the addressed location

Memory Read

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

Interrupt request

Indicates that a device is requesting access to the CPU

Clock

Used to synchronise operations

Data Bus

Provides a bi-directional path for moving data and instructions between system components

Address Bus

Memory is divided up into units called words. Each word in memory has a specific address. The address bus transmits the memory addresses of words that are used as operands in program instructions so that data can be retrieved and sent back to the processor.

Arithmetic-Logic Unit

The ALU performs all arithmetical and logical operations on data.

Registers

Registers are special memory cells that operate at high speed.

Accumulator

All results of operations from the ALU are stored in the accumulator temporarily.

Program Counter (PC)

Holds the address of the next instruction to be executed.

Current Instruction Register (CIR)

Holds the current instruction being executed in operand and opcode.

Memory Address Register (MAR)

Holds the address of the memory location from which the data is to be fetched or to which data is to be written.

Memory Data Register (MDR)

Temporarily stores the data read from or written to memory.

Fetch-Decode-Execute Cycle

Fetch:

* 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, while the contents of the PC is incremented
* The contents of the MDR are copied to the CIR

Decode:

* The instruction held in the CIR is decoded. It is split into opcode and operand. opcode is used to determine the type of instruction and what hardware to use to execute it, and operand is either the address of the data to be used with the operation, or the actual data to be operated on.

Execute:

* The appropriate instruction/opcode is carried out on the operand

Clock Speed

Each CPU cycle starts as the clock changes from 0 to 1. All processor activities begin on the clock pulse. The greater the clock speed, the faster instructions will be executed.

Number of Cores

Each core is able to process a different instruction at the same time, which increases the processor performance.

Amount and Type of Cache Memory

When data is copied to cache, it can be accessed again much faster than from main memory, so the more cache memory the more data can be fetched quickly. There are also three levels of cache, where level 1 is the fastest but smallest, and level 3 is the slowest and biggest.

Pipelining

While the processor is performing operations, the next instruction is fetched and decoded and held in a buffer until they can be executed.

Von Neumann Architecture

Same data bus to transfer instructions and data. Single address bus for transferring addresses for data and instructions. Same word length is used for all memory.

Harvard Architecture

Has physically separate memories for instructions and data. Used in audio & speech signal processing, sonar & radar signal processing, biomedical signal processing, seismic data processing and digital image processing.

Contemporary Architecture

This makes use of different aspects from both Von Neumann and Harvard architecture.

Complex Instruction Set Computers (CISC)

A large instruction set is used to accomplish tasks in as few lines as possible. Complex instructions are built into the machine’s hardware. The code is relatively short and very little RAM is required, however in an average program only 20% of the instructions built into the machine are used.

Reduced Instruction Set Computer (RISC)

Only simple instructions are used that take one clock cycle can be executed. This means that the compiler has to do more work to translate the code into machine code, and more RAM is required. However because of the reduced complexity of instructions, and possibility of pipelining it is still just as fast.

Co-Processor systems

A co-processor is an extra processor to supplement functions of the primary processor. It generally only carries out a limited range of functions.

Graphics Processing Unit (GPU)

A GPU is a specialised electronic circuit which is very efficient at manipulating computer graphics and image-processing. It has thousands of small efficient cores running parallel to handle several tasks simultaneously. They are now finding more generalised uses like machine learning, oil exploration, image processing and financial transactions.

Input Devices

Barcodes are common simple type of input that can be read in several ways including:

* Pen-type readers
* Laser scanners
* Camera based readers

Digital cameras are another type of input device that convert light into a digital image.

Radio Frequency Identification (RFID) are actually a type of both input and output device that can be used to identify and track things using a microchip transponder and antenna. They come in passive and active forms where the passive does not require a battery and rely on the radio waves emitted from a reader to function, but the active form requires a built-in battery.

Output Devices

Output devices include:

* Screens
* Laser printers
* Inkjet printers
* Dot matrix printers
* 3D printers
* Projectors
* Speakers
* Actuators (motors commonly used with sensors to control a mechanism)

Hard Disk

A hard disk is a type of magnetic storage with rotating platters where binary is represented by magnetic or non-magnetic spots representing 1s and 0s respectively. A drive head moves across the disk to access different tracks and sectors. They are relatively cheap, have high capacities, but because of moving parts they are not very robust.

Optical Storage

Optical storage comes in three different formats: read-only (CD-ROM), recordable (CD-R), and rewriteable (CD-RW). They work by having reflective and non-reflective spots called lands and pits, which are then read by a laser, so if the light is reflected back it is a land, if not it is a pit which correspond to 1s and 0s. It can be very cheap to produce and can have reasonably high storage space, but they can be easily damaged from scratches or excessive sunlight.

Solid-State Memory

Solid-State memory is a type of memory that uses flash memory to store data. It is relatively expensive, but can store large amounts of data, has relatively fast read and write speeds, uses less power than a hard disk, and is very robust due to the lack of moving parts. It does however have a limited number of read and write cycles.

Random Access Memory (RAM)

RAM is used to store programs and data currently being used. It is much faster the secondary storage, and is volatile meaning that when the power is switched off, it loses the data it stored.

Read-Only Memory (ROM)

ROM is used to store data that needs to be permanently in memory like the bootstrap loader (the software to boot up the computer). In embedded systems, ROM is used as there is fixed instruction set.

Cloud Storage

Cloud storage is a method of storing data that involves storing data on a remote server, and then accessing it via the internet. This results in reduced hardware costs, improved reliability and performance, and the flexibility of scaling storage requirements up and down over time. The only real limitation of this is that to access your data you need a reliable internet connection.