1.1b: The Fetch-Execute Cycle
Exam Board:
Cambridge OCR
Specification:
GCSE - J277
Watch on YouTube:
The Fetch Execute Cycle
The purpose of the Central Processing Unit (CPU).
The actions that occur at each stage of the fetch-execute cycle and how CPU components are used in the cycle.
The purpose of each register (PC, MAR, MDR and ACC) and what they store. The difference between data and an address.
What You Need to Know
The Fetch-Execute Cycle
The purpose of the Central Processing Unit (CPU) is to process data and execute instructions by constantly performing the fetch-execute cycle.
Fetch Stage
Execute Stage
In the fetch stage, instructions are fetched from RAM and transferred into the registers inside the CPU.
It is the Memory Data Register (MDR) specifically that a fetched instruction is stored in.
In the execute stage, instructions are decoded by the control unit and then executed (run).
Any mathematical calculations are performed by the Arithmetic Logic Unit (ALU) and the results are stored in a register called the Accumulator (ACC).
RAM
CPU
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Control Unit
ALU
ACC
Registers
A register is a small storage space for a temporary piece of data, instruction or address in the CPU.
Each register has a specific role. There are four essential registers used in the fetch-execute cycle:
The Memory Address Register (MAR) stores the address of the current instruction to be fetched from RAM, once it has been copied from the PC.
The Memory Data Register (MDR) stores the instruction fetched from the memory location identified by the MAR.
The Program Counter (PC) stores the address of the next instruction to be fetched from RAM.
The address from the PC is copied into the Memory Address Register (MAR).
The PC is then incremented by 1 because it always shows the address of the next instruction.
The results of any executions, such as calculations made by the ALU, are stored in the Accumulator (ACC).
The PC, MAR and MDR are used in the fetch stage, whereas the ACC is used in the execute stage.
The PC and MAR store an address (a memory location in RAM) whereas the MDR and ACC store data (a value).
Questo's Questions
1.1b - The Fetch-Execute Cycle:
1. What is the purpose of the CPU? [2]
2. Draw a diagram with icons and words to show what happens in the stages of the Fetch-Execute cycle. [4]
3. Describe the purpose of each register:
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a. The Program Counter (PC) [2]
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b. The Memory Address Register (MAR) [2]
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c. The Memory Data Register (MDR) [2]
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d. The Accumulator (ACC) [2]