UNAVOIDABLE STALLS

EXAMPLE:

LW R1,32(R6)

IF

ID

EX

MEM

WB

ADD R4,R1,R7

IF

ID

MEM.....

SUB R5,R1,R8

IF

ID

......

AND R6,R1,R7

IF

ID

Cycle

1

2

3

4

5

6

7

8

9

$^\uparrow$ADD not OK

$^\uparrow$SUB OK

It is not possible for the ADD to proceed since it needs the value being read from memory at the beginning of the MEM cycle which will fetch it.