Processor: The Fetch-Execute Cycle and the Role of Registers within it

Fetch-Decode-Execute Cycle

The Fetch-Decode-Execute (FDE) cycle, also known as the instruction cycle, is the process that the CPU uses to handle instructions.
During the Fetch stage, the CPU takes an instruction from memory, which is determined by the Program Counter (PC). After fetching the instruction, the PC is incremented to point to the next instruction’s memory address.
In the Decode stage, the Control Unit (CU) decodes the instruction fetched into the CPU. During this stage, the CU determines what operation the instruction is requesting.
The Execute stage involves carrying out the instruction. The Arithmetic Logic Unit (ALU) is often used for this, especially for tasks such as addition, subtraction, multiplication, and division.
The CPU can execute several types of instructions, ranging from arithmetic operations to loading data from memory or storing data into memory.
After execution, the CPU might store the result into a register or memory, depending on the instruction. This is known as the Store or Write-back stage.

Registers are small storage locations within the CPU that provide a way to rapidly store and retrieve data and instructions.
The Program Counter (PC) is a register that holds the address of the next instruction that the CPU needs to fetch from memory.
The Instruction Register (IR) contains the instruction that is currently being executed.
The Memory Data Register (MDR), also known as the Memory Buffer Register (MBR), holds the data being transferred to or from memory.
The Memory Address Register (MAR) holds the address in memory for the next instruction or data.
The Accumulator (AC) is a register where intermediate arithmetic and logic results are stored.
The use of registers is essential for the Fetch-Decode-Execute cycle, as they temporarily store the instructions being fetched, the data being processed, and the results of executions.