Question

Problems in this exercise refer to the following sequence of instructions, and assume that it is executed on a five-stage pipelined datapath:

ADD X5, X2, X1
LDUR X3, [X5, #4]
LDUR X2, [X2, #0]
ORR X3, X5, X3
STUR X3, [X5, #0] Now, change and/or rearrange the code to minimize the number of NOPs needed. You can assume register X7 can be used to hold temporary values in your modified code.

Answer 1

To minimize the number of NOPs, we can modify the code by using a temporary register. The modified code will execute without any NOPs.

Step-by-step explanation:

The given code represents a sequence of instructions being executed on a five-stage pipelined datapath. The goal is to minimize the number of NOPs (no operations) needed. To achieve this, we can make use of temporary registers. Here's the modified code:

1. ADD X7, X2, X1
2. LDUR X3, [X7, #4]
3. LDUR X2, [X2, #0]
4. ORR X3, X7, X3
5. STUR X3, [X7, #0]

In the modified code, register X7 is used as a temporary register to hold the value of X5 from the original code. This eliminates the dependency on the result of the ADD instruction, enabling the remaining instructions to be executed without any NOPs.

Answer 2

Below is a possible rearrangement of the given code:

assembly

ADD X5, X2, X1

STUR X5, [X7, #0] ; Store the result of ADD in a temporary register

LDUR X3, [X7, #4] ; Load the result of ADD from the temporary register

LDUR X2, [X2, #0]

ORR X3, X5, X3

STUR X3, [X5, #0]

Based on the code, To reduce the amount of time where the processor is doing nothing, one need to prevent problems and make sure that instructions don't rely on the outcomes of other instructions that haven't finished yet.

So, in the code above, the change helps to reduce the amount of waiting time during the process by using a special register to hold the in-between answer from the addition and preventing problems with the data flow.