...

• The idea of bypassing (often called forwarding) is to add special hardware to recognise this hazard, and to route the value direct to where it is needed.

OBSERVE:

Inst$^{\rm n}$

ADD R1,R2,R3

IF

ID

EX

MEM

WB

$^\uparrow$R2+R3 calculated here

SUB R4,R1,R5

IF

ID

EX

MEM

WB

$^\uparrow$R2+R3 needed here

Cycle

1

2

3

4

5

6

• So all we have to do is copy the ALU output back to the ALU input ready for the next cycle.

• Of course we must still make sure R2 + R3 is eventually stored in R1, in case it is referred to again.