Fetch Execute Cycle

Fetch Execute Cycle
Submitted by:
Anna Mazhar
10 C Red

Steps of Fetch Execute Cycle
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Fetch
– The next instruction must be located, moved across at least one system bus to the processor
Execute
– The instruction must be carried out, this often requires information be moved around in one or more registers and system memory.

Registers
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Program Counter (PC)
– The location of the next instruction.
Memory Access Register (MAR)
Current Instruction Register (CIR) or (IR)
– The current instruction.
Accumulator (AX)
– Used for short term storage, and in many instructions

Busses
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Address Bus
– Moves locations of data to different registers, particularly between the Instruction Pointer, Memory Address
Register, and Memory
Data Bus
– Moves contents of memory addresses

Steps:
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1. Fetch instruction
In the first step, the processor fetches the instruction from the memory. The instruction is transferred from memory to instruction register.
The memory places the instruction on the data bus. The processor then copies the instruction from the data bus to the instruction register.
2. Decode instruction
In this step, the instruction is decoded by the processor. The processor gets any operand if required by the instruction. The processor will fetch the constant value 0 from the next location in memory before executing the instruction.
The processor transfers the instruction from instruction register to the decode unit. The instruction tells the computer to store 0 into Accumulator register. The decode unit now has all the details of how to do this.
3. Execute instruction
In the last phase, the processor execute the instruction, it stores 0 in register AX.
Finally it adjusts the instruction pointer to point to next instruction to be executed.

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