Fetch-Decode-Execute Model:

Arithmetic/logic unit (ALU): The part of the CPU that does calculations and logic operations is called the arithmetic/logic unit (ALU).

Inside the CPU are several memory locations called registers. These are extremely fast to access, but there are usually only a relatively small number of them. Some registers play a specific role in the fetch-decode-execute cycle, for example, holding a memory address, an instruction, or a piece of data, while others serve a general purpose.

Some of the registers with a specific role that are found in most computers include the following:

• The accumulator holds the results of calculations performed by the ALU. (All input and output from the CPU passes through the accumulator.)
• The program counter holds the memory address of the next instruction to be fetched.
• Current Instructions (holds the instruction currently being executed.)

The steps in the cycle are controlled by a control unit and synchronized by an electronic clock. You have probably seen the advertised clock speeds of CPUs; for example, a 2.2 GHz CPU means that its clock ticks 2200 million times per second.

The bus that connects the CPU to other devices in the computer is split into three parts: the address bus, the data bus, and the control bus.

• The address bus carries memory addresses between the CPU and memory to identify uniquely a memory location.
• The data bus carries the value to be read from or written to memory, and
• The control bus carries the signals that, for example, determine whether to read or write the data and when to do so.

Fetch-Decode-Execute Cycle:

1. The processor checks the program counter to see which instruction to run next.
2. The program counter gives an address value in the memory of where the next instruction is.
3. The processor fetches the instruction value from this memory location.
4. Once the instruction has been fetched, it needs to be decoded and executed. For example, this could involve taking one value, putting it into the ALU, then taking a different value from a register and adding the two together.
5. Once this is complete, the processor goes back to the program counter to find the next instruction.
6. This cycle is repeated until the program ends.