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Monday, 21 October 2013

What Is Fetching ? What Is Instruction Cycle ?

Fetching :-

Fetch is a term used to describe the process of a software program,script, or hardware device retrieving data and then moving it to an alternate location or displaying the data.

Instruction Cycle :-

Instruction Cycle is the cycle which is used to fetch,decode,execute the instruction of the computer which was given by us.Instruction Cycle has mainly 4 phase that are Fetch Phase,Decode Phase,Decision Phase and Execution Phase.

Flow Chart/Diagram of Instruction Cycle:-




where AR=Address Register,
PC=Program Counter,
IR=Instruction register,
MRI=Memory Reference Instruction,
RRI=Register Reference Instruction,
I/O I=Input-Output Instructions.

Fetch Phase:-In Fetch phase,SC=0,AR<--Pc,IR<--M[AR] then PC<--PC+1.

Decode Phase:-In decode phase  we decode operation code and perform AR<--IR and I<--IR operations.

Decision Phase:-In decision phase we decide where the instruction goes.

Execute Phase:-In this phase we execute instructions.

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