CO1 WEEK 5 FDE CYCLE
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What are the three parts of the FDE cycle?
Fetch-instruction fetched from memory Decode- the cpu decodes the instruction and prepares other areas of chip Execute- preforms the instructions
What is the FDE cycle?
PC->MAR-address of instruction executed (held in PC)is copied to MAR Memory address->MDR - instruction held at that address is placed in MDR PC-Increments by 1 - MDR->CIR - Contents of MDR copied to CIR frees up MDRSon Execute phase -CIR decode and execute - Instruction in CIR is decoded and then executed
Describe FETCH part of the cycle.
Value from PC loaded into MAR. Fetch Signal sent down the control bus and this sends value in MAR down the address bus. Contents of memory of specified location sent down data bus stored in MDR. Copied to CIR. PC then incremented.
Describe DECODE part of cycle.
Contents of CIR sent to Control Unit. It decodes the instruction. If necessary also brings data for instruction to use and stores in appropriate registers.
Describe EXECUTE part of cycle
CU and/or ALU carries out the instruction stored in CIR and cycle begins again!