Fetch Decode Execute Cycle Flashcards
What must happen before the FDE cycle can take place?
The program instructions have been translated into machine code
The program instructions have been loaded (from secondary storage) into the main memory (RAM)
What happens in the first part of the fetch stage?
The program counter (PC) keeps the address of the next instruction to be executed. The contents of the PC are copied to the memory address register (MAR), which is connected to the address bus. The address of the next instruction to be executed is placed on the address bus.
What happens in the second part of the fetch stage?
Once the address of the instruction is on the address bus, the control unit instructs a memory read operation to allow the contents of the memory location to be transferred to the processor. The instruction that is stored at that address is transferred using the data bus from the main memory to the processor, and is saved in the memory buffer register (MBR)/memory data register (MDR).
Simultaneously, the contents of the program counter (PC) are incremented by one so that they point to the address of the next instruction that needs to be fetched.
What happens in the third part of the fetch stage?
The contents of the memory data register (MDR) are copied to the current instruction register (CIR). This ensures that the current instruction is kept safe so that the memory buffer register (MBR)/memory data register (MDR) can be used during the execute stage, in order to store additional data that is needed.
What happens in the decode stage?
The control unit decodes the instruction that is kept in the current instruction register (CIR). This involves splitting the instruction into operand and opcode to determine what type of instruction needs to be carried out, checking if additional data are required from memory, and figuring out where these are kept in main memory.
