Consider a three word machine instructionADD A[R0], @BThe first operand (destination) A[R0] uses indexed addressing mode with R0 as the index register. The second operand (source) @B uses indirect addressing mode. A and B are memory addresses residing at the second and third words, respectively. The first word of the instruction specifies the opcode, the index register designation and the source and destination addressing modes. During execution of ADD instruction, the two operands are added and stored in the destination (first operand).The number of memory cycles needed during the execution cycle of the instruction is:a. 3b. 4c. 5d. 6My work till now:- As per my knowledge instruction execution has decode sub phase.So it will take 2 MR in decode phase to fetch the instruction.Then to read indexed addressing mode operand it will take one memory reference and since this is the destination also so in total it will have 2MR and then to read second operand it needs 2 MR. so in total i will go with 6 as the answer here.But i want to clear whether is it actual 6 or is it 4?4 because if we consider that decode phase is not part of the execution cycle. 
COA Finding number of memory references
computer architecture