Registers And The F-E Cycle
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What does the Program Counter do?
• A register that tracks the RAM address of the next instruction to be executed.
What does the Memory Address Register do?
• A register that tracks the RAM address of the instruction that is to be fetched.
What does the Memory Data Register do?
• Stores the instruction that has been transferred from RAM to the CPU.
What does the Current Instruction Register do?
• A register that stores the instruction that has been fetched from RAM, and is about to be decoded or executed.
What does the accumulator do?
• Stores the result of mathematical or logical calculations.
Describe each stage of the Fetch-Execute Cycle (7/8 marks)
• • The PC displays the address in RAM of the next instruction to be executed. This value is copied into the MAR. • • The PC is increased by 1. This prepares the CPU for the next instruction to be fetched. • • The CPU checks the address in RAM which matches the address held in the MAR. • • The instruction in RAM is transferred to the MDR. • • The instruction in the MDR is copied into the CIR. • • The instruction in the CIR is decoded and executed. Any result of an execution is stored in the ACC. • • The cycle repeats by returning to the first step and checking the program counter for the address of the next instruction.