Generic selectors
Exact matches only
Search in title
Search in content
Post Type Selectors

Discuss the execution of a complete instruction.

  1. The execution of a complete instruction is explained by a simple addition instruction.
  2. Consider the instruction, ADD [M], R1 which adds the contents of memory location (address is specified) to the register R1.
  3. The execution of this instruction requires the following steps :
    a. Fetch the instruction.
    b. Fetch the operand.
    c. Perform the addition.
    d. Load the result into R1.
  4. The first step of fetching the instruction is common to execution of all instructions.
  5. The remaining steps depend upon the operation to be performed.
  6. Instruction execution proceeds as follows :
    Step 1 : The instruction fetch operation is initiated by loading the contents of the Program Counter (PC) into MAR and sending read request to memory to read the instruction from memory (Now waiting for response from the memory).
    Step 2 : Memory read is requested.
    Step 3 : The processor is to be delayed until the MFC (Memory Function Completed) signal is received. Once MFC is received the word fetched from the memory is transferred to instruction register.
    Step 4 : The contents of memory are transferred by memory read operation.
    Step 5 : The contents of R1 are transferred to one of the inputs of ALU. Now both operands are available in ALU inputs.
    Step 6 : Addition operation is performed.
    Step 7 : The result is transferred to R1.
    Step 8 : It indicates that the end of the execution of the current instruction, and it causes a new fetch cycle to begin by returning to step 1.

Leave a Comment