Fetch Decode and Execute

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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.

Fetch Decode and Execute Explained

Fetch: The fetch command is the process in which instructions are retrieved from the main memory and sent to the CPU to be processed. The data must then travel along the bus to the CPU. This is all data that is used in memory, and for this to be translated into information that the user will perceive useful it needs to be decoded.

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Within the microsim programme we devised a system fro printing characters on the screen by saving them into registers and sending them from there to the VDU unit. If we use this as an example we can start to see what really goes on.

When the information is typed in to the command screen it is placed into a register. To be able to print the command on the screen the information needs to be sent to the visual display. This information is fetched from the register and placed in to the printable area.

Decode: When ...

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