• 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...

Introduction

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.

Middle

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.

Conclusion

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

    categorised according to * Means by which the data is stored, optically or magnetically * The technique used for storage of the data, sequential storage or direct access storage * The capacity of the medium, how much can be stored on it * Portability of the medium, can it be moved around easily * Access times to the data stored.

  2. Smart Card System

    b) Better Control The administration control in the system let the supervisor have the better control of the student computer. c) Access Control As compared with the existing access control methods in the college laboratory, that is the user needs to login manually.

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

    When a word is given to the UART for Asynchronous transmissions, a bit called the "Start Bit" is added to the beginning of each word that is to be transmitted. The control information consists of additional bits added to each character, START BITS- that indicates that transmission is about to

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

    There is noway of improving the speed of using the manual system unless staff were employed and this is not suitable considering the income of the business. Having a centralised system will also mean maintaining, updating and adding records to the system will be easier and faster.

  1. Computer Aided Design Package

    RAM 30MB Optical drive CD-ROM x8 speed Hard drive 9GB Modem 56K The computers they currently have can't be upgraded to use any of the new CAD packages because everything in the current computers would all need to be replaced to make it compatible with any new CAD packages.

  2. Investigate Data Communications.

    CCITT The COMITE` CONSULTATIF INTERNATIONALE DE TELEGRAPHIE ET TELEPHONIE, These are an International committee that defines out communications protocols and standards. This body standards are set mainly to do with modems, faxes, data communication formats ( The way that data is passed between the system).

  1. The Value of Data and the Use of Databases

    is the electronic transmission of business data such as purchase orders and invoices from one firm's computerised information to that of another firm. EDI was developed to provide an interface between two separate computer systems. Each company could have its own way of doing things, but by using EDI, they

  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