Fetch, Decode, and Execute Cycle (FDE Cycle)

What is the FDE cycle?

• aka Fetch Decode Execute Cycle or instruction cycle

• The fundamental cycle that describes how instructions an d executed

• The speed of the CPU is largely determined by how quickly and efficiently it can execute this cycle

What are the 3 main steps in the FDE cycle?

• Fetch: The CPU fetches an instruction from memory.

• Decode: The CPU decodes the instruction, which means it intreperets the instruction into a set of low-level operations that the CPU can execute

• Execute: The CPU executes the instruction, which means it carries out the low-level operations that were specified in the decoded instructions

What occurs in the Fetch phase?

• The CPU receives a language instruction from main memory (typically RAM)

• This involves the CPU sending a request to the memory to fetch the instruction

• The address of the instruction to be fetched is stored in the MAR, and the fetched instructions is then transferred to the IR

What occurs in the Decode phase?

• The CPU interprets the machine language instruction fetched during the previous phase

• This phase is managed primarily by the CPU’s control unit, which decodes the instruction by analysing these components

  • Operational code: dictates the type of operation

  • Operands: the data to be operated on

  • Addressing modes: determine how to locate the operands

• Involves checking the validity of the instruction and ensuring that the operands are within an acceptable range

• Also calculates the effective address of the operands, which is the actual memory address where the operands are located

What occurs in the Execute phase?

• The CPU performs the operation specified by the instruction decoded in the previous phase

• The specific operation carried out is dedicated by the operation code

• Common operations include the following:

  • Arithmetic operations: Addition, subtraction, multiplication, and division, typically performed by the ALU

  • Logical operations: Logical functions like AND, OR, and NOT, also managed by ALU

  • Memory access operations: Load (reading data from memory into a register) and store (writing data from a register to memory) operations, which involve interaction with the systems’s memory.

  • Control operations: Conditional and unconditional jumps which alter the flow of execution based on specific conditions.

What are the role of buses in FDE cycle?

What are the role of buses in FDE cycle?

What are the role of buses in FDE cycle?

What are the role of buses in FDE cycle?