Topic 3 computers

• The CPU processes instructions

  When you run a program, it is the

  CPU which runs the instructions
• Von Neumann architecture

  Program instructions and the data the programs

  are using are both stored in the same memory
• Components of the CPU

  Control Unit

  Arithmetic-Logic Unit (ALU)

  Registers
• Registers

  where the CPU stores small amounts of data and results that it is operating on
• Arithmetic logic unit (ALU)

  Performs two sorts of operations on data:

  Arithmetic operations:

  addition, subtraction, multiplication, division

  Logical operations comparing one data item to

  another
• Control Unit

  The control unit coordinates and controls

  all of the activities taking place within

  the CPU

  It decodes instructions and executes them

  It receives signals from the system clock

  It directs the timing and control of other

  parts of the CPU, much like the conductor

  of an orchestra
• Clock speed is the measure of how quickly the CPU can process instructions ( how many per second) . measured in Hz or GHz
• A bus is a set of parallel wires connecting two or

  more components of the computer
• Address Bus: stores the address of the memory or device

  controller to be read from or written to
• Data Bus: carries data throughout the computer system
• Control Bus: uses control signals to control all activities

  within the CPU
• The cpu operates by repeating three operations

  fetch

  decode

  execute
• FETCH – causes the next

  instruction to be fetched

  from main memory
• DECODE – the control unit decodes the

  instruction to work out

  what the instruction is
• EXECUTE – the instruction

  is executed
• Secondary storage is not directly

  accessible by the CPU

  It is non-volatile, meaning

  it will keep data even if there

  is no power

  Secondary storage devices

  may be internal or external

  to the computer
• storage methods

  magnetic

  optical

  solid state
• Magnetic: Mechanical parts move over the disk’s

  surface to read and write data magnetically, or a

  drive head reads a magnetic tape
• Optical: Lasers read and write data using light
• Solid State: Data is recorded onto solid memory

  chips without any moving parts
• Optical storage

  Advantages:

  Cheap, very easily portable, takes up little space physically

  Disadvantages:

  Less storage capacity compared to other types

  Easily damaged / scratched, requires a CD reader

  Slow write speeds

  Uses:

  Songs, videos and other multi-media storage, backup and

  archiving of data
• Advantages /disadvantages

  of SSDs

  Advantages:

  Highly durable, no moving parts, very fast read/write speeds,

  no noisy fan or drive arm, faster start up times

  Disadvantages:

  More expensive than magnetic hard disks, similar storage

  capacity as magnetic disks

  Uses:

  Higher end computers

  Laptops

  Smartphones and tablets
• Flash memory

  Low cost, portable, no moving parts, durable

  This makes them ideal for a range of offline devices:

  Cameras

  Mobile phones

  USB memory sticks
• Operating Systems

  Operating systems manage computer hardware,

  users and the resources used by software

  They are responsible for managing:

  File management

  Process management

  Peripheral management

  User management

  Also, the user interface, memory management

  and multitasking
• Process management

  Processes can be thought of as programs that need

  to be run on the CPU

  A CPU can only run one program at a time, so each program

  must be given a small amount of time to use the processor

  The operating system is responsible for managing processes
• Memory management

  To run a program, the computer must copy the program from storage into main memory

  Data used by the program is copied into main memory

  The operating system keeps a record of where each program and

  its data are located
• Multi-tasking

  Multi-tasking is where an operating system manages many tasks happening at the same time
• Interrupts

  Interrupts are signals sent to the CPU by external

  devices to indicate an event that needs immediate

  attention They tell the CPU to suspend its current activities and execute

  appropriate instructions
• File management

  File management is carried out by the operating system

  The following features are available:

  Naming files

  Allocating files to folders

  Moving files

  Saving files

  Copying files

  Deleting files
• User management

  The operating system is responsible for user login and passwords

  stores in a file or database usually in an encrypted form
• utility software keeps a computer working well
• Utility software

  Defragmentation software

  Data compression and backup software

  File repair software

  Anti-malware software
• Utility programs perform extra functionality and housekeeping tasks that keep computers running efficiently

  They are often packaged as part of the operating system
• When you save a large file it may not fit on the disk in consecutive memory locations

  The file is referred to as ‘fragmented’

  In order to read the file, the drive head will need to move to more locations

  This makes it far slower to access files and programs
• Defragmenting the hard disk reorganises files so they are stored together

  Read times are far faster

  Free space is also in one place so new files do not need to be fragmented
• Anti-malware software prevents malware such as viruses. File repair

  software will retrieve data from corrupted files.
• Backup software makes a copy of data that is stored at a different

  location. Data can be restored in the event of theft, fire or flood.
• Data validation

  Data validation routines can ensure that data entered is of the right type – for example, an integer

  Validation cannot ensure that the user has not entered a wrong value, or made a spelling mistake in a name

  It can only ensure that the data is reasonable and conforms to

  a set of rules
• Range check

  A number or date is within a sensible/allowed

  range
• Length check

  Text entered is not too long or too short – forexample, a password is between 8 and 15 characters