pipeline bypassing

Question

A 5 stage pipelined processor has instruction fetch (IF), operand fetch (OF). Instruction decode (ID), perform operation (PO) and Write operand (WO) stages. The IF, ID, OF and WO stages takes 1 clock cycle each for any instruction. The PO stage takes 1 clock cycle for ADD and SUB instruction, 2 clock cycles for MUL instruction and 4 clock cycles for DIV instructions respectively. The number of clock cycles needed to execute the following sequence of instruction where operand forwarding from WO to PO and PO to OF is used

Comments

12??

---

answer is 13 I also dont know how

Answer 1

1

2

3

4

5

6

7

8

9

10

11

12

I1

IF

ID

OF

PO

WO

I2

IF

ID

OF

PO

PO

WO

I3

IF

ID

X

OF

PO

WO

I4

IF

X

ID

OF

PO

PO

PO

PO

WO

Total cycles=12

Comments

for I3 instruction how it is possible to take OF stage before the completion of PO stage of I2 in 6th cycle...???

---

because of split phase :data prepared in first half and read in second half of the same cycle

---

but for i2 instruction when the PO phase will complete then only we would be able to do OF in I3...in between how can we do this operand forwarding technique will be implemented only after completion of the stage.,,.. even if you say the split phase is reason that in one phase data is prepared...but suppose if PO is for 4 cycles then how you split work....??

---

if PO is of 4 cycles then the data will be forwarded in the last cycle as the first 3 cycles will be used for internal processing of ALU. I was also confused regarding this point but then i referred hamacher 5th edition operand forwarding data path diagram. i suggest you to plz refer. u will get a clear picture.

---

yes similarly in i2 data must be forwarded after 6 th cycle and I3 OF starts from 7th cycle isn't??

---

no processing unit is a combinational ckt. it does not need a cycle to reflect its result in its output so it can be done in 6th cycle

---

hmmm maybe I need to brush up my concepts....during my classes I haven't heard of this split phase inside the cycle....nywz thnkew so much

---

neither me heard the phrase split-phase but read it 1-2 days back. go for hamacher book  for pipelinig...it has nice content

---

thanks again :)

---

everything will be clear to u by https://gateoverflow.in/8218/gate2015-2_44

---

Is Operand Forwarding same as that of bypassing?

Anyone please clarify.

---

I don't think so?

What do you mean by bypassing?

---

I feel it is a property used by Operand Forwarding?