Fetch-execute cycle

The fetch-execute cycle (or fetch-decode-execute cycle) is the way that a processor executes a program’s instructions.

The cycle has three main stages:

1. Fetch data from memory
2. Decode the instruction
3. Execute the instruction

Fetch

• The first thing it does is to actually read the instruction it needs to execute.
• It does this by copying the Program counter (which stores the current memory address of the next instruction) into the Memory address register (MAR), which is used to access memory.
• Then, it copies that address onto the address bus, which it then carries to the main memory.
• The memory then returns the instruction stored at that address, which is copied into the Memory buffer register (MBR).
• Finally, the instruction is copied from the MBR into the Current instruction register (CIR), which is where the processor holds the instruction it’s currently executing.
• The processor also increments the program counter, so that it points to the next instruction for the next cycle.

Decode

• Sure, the processor now has the raw instruction in its CIR, but how does it know what to do with it? The instruction needs to be decoded.
• Decoding the instruction means that we take the raw instruction and figure out what operation it represents, and what data it needs to operate on.
• For example, we may look at an ADD instruction and decode it to mean “add the values in registers A and B, and store the result in register C”.

Execute

• Once the instruction has been decoded, the processor can execute it.
• THat usually means sending signals to the parts of the processor (or the rest of the computer) that need to be involved in carrying out the instruction.
• For example, if the instruction is an ADD instruction, the processor would send signals to the arithmetic logic unit (ALU) to perform the addition, and then send signals to the registers to store the result.
• Often, it involves fetching or writing data to memory.

How frequently does this happen?

Processors have a clock that ticks at a certain frequency (measured in hertz). Each tick of the clock represents one cycle, and during each cycle, the processor can carry out one fetch-decode-execute cycle. So, if a processor has a clock speed of 3 GHz, it can carry out 3 billion fetch-decode-execute cycles per second!

Look at the clock speed to see how quickly it does the fetch-execute cycle.