I feel Hamacher's book says something different about point 6 in the answer
6. Cant there be a forwarding from EX stage(T3) of I1 to EX stage(T4) of I2 ?
This is not possible . See what is happening in I1 . It is Memory Read .So data will be available in register after memory read only .So data cannot be forwarded from EX of I1.
In 6th edition of Hamacher's book, section 6.4. Data Dependencies, it gives following instructions:
Add R2, R3, #100
Subtract R9, R2, #30
Without operand forwarding, the solution is given as follows:
With operand forwarding, its given as follows:
Author says ALU's output can be given back as a feedback to its input to achieve above.
Based on this, I feel it should be possible for EX of I2 to execute in T4. What I am missing here :( ???