computer hardware

• A touch screen is the only device which works as both an input and an output device. You view the options available to you on the screen (output) and you then use your finger to touch the option that you have chosen (input).
• The central processing unit (CPU) consists of six main components:

  • c ontrol unit (CU)
  • a rithmetic logic unit (ALU)
  • r egisters
  • c ache
  • b uses
  • c lock
• The purpose of the CPU is to process data and instructions by constantly repeating the FDE cycle.
• Von Neumann architecture is the design upon which many general purpose computers are based.
• • data and instructions are both stored as binary digits
  • data and instructions are both stored in primary memory
  • instructions are fetched from memory one at a time and in order (serially)
  • the processor decodes and executes an instruction, before cycling around to fetch the next instruction
  • the cycle continues until no more instructions are available
• • program counter - holds the memory address of the next instruction to be fetched from primary memory
  • memory address register (MAR) - holds the address of the current instruction that is to be fetched from memory, or the address in memory to which data is to be transferred
  • memory data register (MDR) - holds the contents found at the address held in the MAR, or data which is to be transferred to primary memory
  • current instruction register (CIR) - holds the instruction that is currently being decoded and executed
  • accumulator (ACC) – holds the data being processed and the results of processing
• The CU provides several functions:

  • it fetches, decodes and executes instructions
  • it issues control signals that control hardware
  • it moves data around the system
• The ALU has two main functions:

  • It performs arithmetic and logical operations (decisions). The ALU is where calculations are done and where decisions are made.
  • It acts as a gateway between primary memory and secondary storage . Data transferred between them passes through the ALU.
• Cache is a small amount of high-speed random access memory (RAM) built directly within the processor. It is used to temporarily hold data and instructions that the processor is likely to reuse. This allows for faster processing as the processor does not have to wait for the data and instructions to be fetched from the RAM.
• Registers are small amounts of high-speed memory contained within the CPU. They are used by the processor to store small amounts of data that are needed during processing
• Buses are direct electrical connections that connect the CPU to memory and other components. There are 3 buses connected to the CPU:

  • address (memory addresses)

  • data (instructions and data items)

  • control (commands from the CPU and status messages of devices)
• • memory address held in the PC is copied into MAR.
  • address in the PC is incremented by 1. It now holds the address of the next instruction to be fetched.
  • The processor sends a signal along the address bus to the memory address held in the MAR.
  • The instruction/data held in that memory address is sent along the data bus to the MDR.
  • The instruction/data held in the MDR is copied into the CIR.
  • The instruction/data held in the CIR is decoded and executed. Results of processing are stored in ACC.
  • The cycle then returns to step one.
• There are a number of factors that can affect the performance of the CPU. These include:

  • clock speed (faster speed = better performance)

  • number of processor cores (more cores may = better performance)

  • cache size (larger size = better performance)

  • cache type
• The CPU contains a clock which is used to coordinate all of the computer's components. The clock sends out a regular electrical pulse which synchronises (keeps in time) all the components.
• The frequency of the pulses is known as the clock speed. Clock speed is measured in hertz. The higher the frequency, the more FDE cycles can be performed in any given moment of time.
• CPU clocks can sometimes be sped up slightly by the user. This process is known as overclocking, which can cause long term damage to the CPU as it is working harder and producing more heat.
• A processing unit within a CPU is known as a core. Each core is capable of fetching, decoding and executing its own instructions.

  The more cores a CPU has, the greater the number of instructions it can process in a given space of time. Many modern CPUs are dual (two) or quad (four) core processors. This provides vastly superior processing power.
• • parallel processing allows different instructions from the same program to be run at the same time

  • multitasking allows instructions from more than one program to run at the same time.
• Dual-core does not mean that performance will double as not all instructions can  be run in parallel – many need to wait for other instructions to finish first.  
• Random access memory (RAM) is volatile primary memory. Once the computer is switched off, the data and instructions held in RAM are lost. RAM is given the term ‘random access’ because data can be stored and accessed from any location within the memory.
• RAM is used to hold data and instructions that are currently in use. In a modern PC, RAM is used to hold the operating system and any open documents and programs that are running.
• The contents of RAM can be changed at any time, simply by overwriting them with other data and/or instructions. For example, a user might close one document and open a second, or run a different program. RAM can also be upgraded easily, unlike other types of primary memory.
• Read only memory (ROM) is non-volatile primary memory. Its contents are not lost when the computer is turned off. Smaller in size (MB) than RAM (GB)
• ROM can be read from, but not written to, hence the term 'read only'. This makes ROM ideal for storing instructions and data that are needed for the computer to run. These instructions and data are usually programmed by the computer's manufacturer and cannot be overwritten.
• The BIOS (Basic Input Output System) is an example of a program that is stored in ROM. The BIOS runs as soon as the computer is switched on. It checks that the hardware is functioning correctly, then runs a second program known as the bootup or bootstrap program that loads the computer's operating system from the hard drive into the RAM. The BIOS is always needed so it is stored in ROM. The software stored in ROM is known as firmware.
• Secondary storage is non-volatile, long-term storage. It is used to keep programs and data indefinitely. Without secondary storage all programs and data would be lost the moment the computer is switched off.
• Not all computers require secondary storage. Embedded computers, such as those found in a digital watch or central heating system, do not need to store data when the power is turned off. The instructions needed to run them are stored in ROM and any user data is held in RAM.
• Secondary storage is not able to be accessed directly by  the CPU so data and instructions that are needed by the  CPU have to be passed to RAM and then onto the  processor. 
• Data is stored on storage media. The storage medium is where the data  is stored to and retrieved from. A storage device is  needed to access the data stored on the storage  medium.

  
  

  for example (storage medium - storage device):

  • hard disk - hard disk drive
  • blu-ray disk - blu-ray disk drive
  • magnetic tape - tape drive
  • flash memory - USB memory stick
• • Magnetic devices use magnetic fields to magnetise tiny sections of a metal spinning disk. A magnetised part represents binary 1, a demagnetised represents 0. So tiny that disks can contain TB of data.
  • As disk spins, a read/write head moves across its surface. To write data, the head magnetises / demagnetises a section of the disk. To read data, the head makes a note of whether the section is magnetised.
  • Magnetic devices r fairly cheap, high capacity, durable. But they r susceptible to damage if dropped and vulnerable to magnetic fields. A strong magnet might possibly erase the data it holds.
• Optical media also come in different types:

  • ROMmedia: pre-written data which can't be overwritten. Music, films, software and games.
  • R media r blank. An optical device writes data to them. Can only be written to once, but read many times. Copies of data often made using these media.
  • RW the disc can be written to more than once.
• • Solid state devices use non-volatile random access memory (RAM) to store data indefinitely. Faster access times than other, bc they have no moving parts so more durable.
  • Expensive so smaller capacity. Eg solid-state drive that holds 256 GB might be of a similar cost to a hard disk with several TB capacity.
  • Require little power, so ideal for portable devices where battery life is a big consideration. They are also portable due to their small size and durability.
  • limited number of read write cycles
• • Optical devices use laser to scan the surface of a spinning disc made from metal + plastic. The disc surface is divided into tracks, each containing many flat areas + hollows. Flat r lands (reflected binary 1), hollow r pits (0).
  • When laser shines on surface, lands reflect light back, pits scatter the beam. A sensor looks for the reflected light.
  • easily scratched/broken

  A) land

  B) 1

  C) 1

  D) pit

  
• • When amt of RAM needed is greater than available, part of the computer's secondary storage eg hard disk can store data temporarily.
  • Enables data in RAM not currently being used to be transferred to hard disk. This frees up room in RAM for other programs/data. When needed again, unused data is transferred to the hard disk before original data is transferred back to RAM. This is swapping.
  • Makes computer run slower as secondary storage devices have slower access times than RAM. Processing performance can be improved by increasing the size of the RAM, reducing the need for virtual memory.
• label

  A) not currently used

  B) used

  
• disk thrashing is when data is continually being read from/written to virtual memory (on the hard drive), the hard drive is forced to spin constantly and this can be damaging.
• embedded system is a computer system which is built in to another device eg washing machine, calculator, microwave, printer