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Computing GCSE


Week 2 Understand what the processor is and what it does. Execute basic LMC programs. Understand how CPU characteristics affect performance. A computer system is made ... – PowerPoint PPT presentation

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Title: Computing GCSE

Computing GCSE
  • Week 2

2.1.2 Computing Hardware
  • Understand what the processor is and what it
  • Execute basic LMC programs.
  • Understand how CPU characteristics affect

What is a computer system?
  • A computer system is
  • made up of hardware and software components and
    is capable of
  • data input - using input devices
  • data processing - using a microprocessor,
    typically the Central Processing Unit (CPU)
  • data output - using output devices
  • It may also be capable of
  • data storage - so data can be stored for later
  • data transmission - so data can be transferred to
    another computer system
  • As well as the personal computer, this definition
    applies to any equipment which uses computer

What is this?
What does the processor do?
What is the purpose and function of the CPU?
  • The purpose of the Central Processing Unit (CPU)
    is to carry out program instructions.
  • The function of the CPU can be broken down into a
    series of steps that are carried out in a
    continuous cycle.

The fetch step
  • This involves retrieving an instruction from a
    memory address. The address of the instruction is
    stored in a register called the program counter
  • After an instruction is fetched, the PC is
    updated so the CPU knows the address of the next
    instruction it has to fetch.

The decode step
  • This involves the CPU identifying the operation
    code (op-code) part of the instruction which
    tells it which operation to perform.
  • If the op-code requires the CPU to act on some
    data then the second part of the instruction will
    contain either the data or the memory address
    where the data is stored.

The execute step
  • In this step the parts of the CPU are connected
    that are needed to execute the instruction that
    was decoded.
  • For example, if the instruction involved integer
    arithmetic or logical operations then the
    arithmetic logic unit (ALU) would be connected to
    the relevant memory locations so that
  • The data for the calculation can be passed along
    a data bus to the ALU as input.
  • The ALU can execute the required operation
  • The result of the operation can then be passed
    from the ALU along a data bus as output.

The write back step
  • In this step the result of the execute step is
    either written to RAM or, if it is to be used by
    the next instruction, to one of the CPU
  • Some types of instructions alter the program
    counter rather than produce result data. This
    allows programs to carry out iteration loops and
    conditional program execution rather than just
    stepping through the instructions in sequence.
  • Some instructions change the state of single-bit
    flag registers. These TRUE/FALSE registers are
    used to indicate the result of an execute step,
    for example a flag can be set to TRUE if two
    numbers are compared and found to be equal or if
    a subtraction produces a zero or a negative

Little Man Computer
  • http//
  • Run the applet.
  • Go through examples.
  • Work through worksheet 1, 2 and 3.
  • Complete LMC sheet 2
  • Research recent, commercially available CPUs,
    and their characteristics, present your findings
    in a PPt.