The CPU itself may not seem like much to look at; it’s a chip you can hold in your hand. Inside, there is a complete set of tools for running calculations, such as the program counter, the CPU register, and the arithmetic logic unit (ALU). They are used to perform instruction cycles, which can be separated into four steps: fetch, decode, execute, and write-back. For this reason, the instruction cycle is also called the fetch-decode-execute cycle.

Every time we run a program, the CPU “fetches” instructions and data from the random access memory, or RAM. It “decodes” the instructions to understand what it needs to do. Based on how the processor’s instruction set architecture (ISA) is designed, the instructions may be in the CISC or RISC format.