• Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

Fetch Decode and Execute

Extracts from this document...


Ricki Lambert Computer Systems Fetch, Decode and Execute Introduction In this task I will be describing in detail the function of the fetch-decode-execute cycle and its relationship to the workings of a microprocessor, whilst making detailed reference to the role played by hardware and software in this relationship. The fetch, decode and execute cycle is formally known as the machine cycle. This cycle undergoes numerous steps incorporating the hardware from inside the processor mainly and the software stored in the disk space. To make sure the processor knows where to find a particular programme stored in memory it is assigned a number which is stored in the control logic area of the processor. It is in this area of the processor that instructions are decoded and obeyed. When fetching from the memory the processor uses a two logical systems of retrieval and they are; Fixed address reset system - This system normally loads from the bottom of the memory Vectored system load - This normally starts from the start address from a set place in memory and this can be either at the bottom or top of memory To counteract this, a jump function is used allowing the processor to fetch any instruction from any place in memory. ...read more.


In the diagram below we can see an example of the code working to a real situation, the situation being the programme code written in the microsim package. Fetch Decode and Execute Cycle Including Example To fetch the instructions from memory data has to travel along a series of communications links, these are called busses. There are a few differing types of busses that can be used. Whenever the CPU needs to read or write a byte of data to or from the memory it will specify the address on the address bus. This data is then placed on another communicational channel called the data bus. The data bus is what is known as unidirectional, and this means that the CPU can specify addresses but the memory cannot. This is a stark contrast to the data bus where both CPU and memory can place information on it, this is called bidirectional. This takes us to the control bus where similarities lie with the control unit. Where the control unit is the power behind the processor, the control bus is the power behind the bus system. The control bus will dictate whether the information is to be read or write, to be sent from memory to processor and visa versa and also whether all information on other busses is ready. ...read more.


Now that the two values have been extracted from the memory this means the fetch part of the cycle has been completed. The data is now placed in to the instruction register so the control unit can begin the process of decoding the values in to some form of useable data. This data is what will present us with an answer. This is done by the arithmetic logic unit. When in the ALU, the decoded information is able to be processed. Processing this data is dependant on the opcode that is presented when in the decoding stage. This will determine how the data is processed, whether it is add, subtract and so on. When processed the data is either stored in the main memory or projected to an output device. But how does the CPU no where to send it? Based on the conditional feedback from the ALU the CPU is able to determine where the information is to be sent to. It is this command that determines what bus route to send the required / processed data through. Things get a lot trickier when the jump command is issued. However if using my example diagram, instead of adding the programme counter by two it will dictate where the next instruction is to go by altering the programme counter to suit. This can be done in various ways. ...read more.

The above preview is unformatted text

This student written piece of work is one of many that can be found in our AS and A Level Computer Science section.

Found what you're looking for?

  • Start learning 29% faster today
  • 150,000+ documents available
  • Just £6.99 a month

Not the one? Search for your essay title...
  • Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

See related essaysSee related essays

Related AS and A Level Computer Science essays

  1. Smart Card System

    Typical criteria we define in access lists are packet source addresses, packet destination addresses, or upper-layer protocol of the packet. However, each protocol has its own specific set of criteria that can be defined. For a single access list, we can define multiple criteria in multiple, separate access list statements.

  2. CP3 - Proposed Solution to a Realistic Problem - Apartment Administration software

    To enable my program to work on other operating systems translation of the code would have to be used, I do not have enough time to do this or learn how to do it. The program I intend to create will be able to run properly on the following specification

  1. Computer Aided Design Package

    After finding out more on DeltaCAD, TurboCAD and AutoCAD I have found out that the latest versions of these three CAD packages are DeltaCAD 5.0, TurboCAD 10 and AutoCAD 2005. DeltaCAD is made by a company called Midnight Software, TurboCAD is made by a company called IMSI and AutoCAD is

  2. With diagrams compare and contrast the relative advantages and disadvantages of digital transmission over ...

    into one of Onyx Internet's Points of Presence (POP), this type of service provides a fast link out onto the Internet. Leased Lines are suitable for heavy Internet users and they can easily be upgraded as Internet usage or traffic flow increases.

  1. Control Unit, Memory Unit, and Arithmetic Logic Unit. The CPU or Central Processing ...

    The CPU will also use this place to store the results of any processing it does. It is made up of individual memory locations, each capable of storing a byte of data. The parts of the operating system, which the computer is using at the time, also need to be stored in memory.

  2. Investigate Data Communications.

    ANSI publishes national standards but does not develop them, standards are developed by other groups that are accredited to develop standards for ANSI consideration. A lot of this work is done by the Institute of Electric and Electronics Engineers (IEEE), who developed the IEEE 802 local area networks standards.

  1. The Value of Data and the Use of Databases

    * Theft of data * Accidental misplacing of data * Theft of the equipment that data is on * Sabotage by an employee Hardware which could be used for back-up purposes: * Floppy disks * Zip disks * CDROM * CD-RW * Creating a mirror hard drive * Backing up to Internet Storage EDI (Electronic Data Interchange)

  2. Different ways of data capture

    marks made in a pre set position on a form e.g multiple choice exams, lottery tickets. 10. Bar Code Reader (Scanner) - 4 Main pieces of info on a bar code are: *First 2 digits indicate country product is registered e.g UK = 50 *Next 5 represent manufacturers code.

  • Over 160,000 pieces
    of student written work
  • Annotated by
    experienced teachers
  • Ideas and feedback to
    improve your own work