Explain the meaning of the term memory disambiguation and write a short fragment of code either in C or as pseudocode that illustrates the problem.

What will be an ideal response?

When executing strictly sequential code, the problem of memory disambiguation does not arise. For example,
suppose we execute (here we are using an absolute memory address X to illustrate a point) the following



LDR r0,X ;load r0 with the contents of memory location X

ADD r0,r0,#1

STR r0,X

This sequence performs the operation X = X + 1. Obviously, the load and store have to be performed in the
correct sequence to preserve the meaning of the code. If you change the order of the load and store, the
outcome will be different. Now, suppose we have the code



LDR r0,[r1]

ADD r0,r0,#1

STR r0,[r1]

We have the same situation as the previous example, except that the source and destination are provided by
pointers. Let’s go one step further. Now consider the code fragment



LDR r0,[r1]

ADD r0,r0,#1

STR r3,[r2]

Here we have two different pointer registers r1 and r2. Consequently, we can change the order of the load and
store operations. Or can we? If r1 and r2 point to different entities, there is no problem. If they point to the
same entity, there is a problem and we have to preserve order. The programmer doesn’t have a problem with
this code because he or she knows what objects are being referenced. However, the processor does not know
that and can’t alter the order of loads and stores.

Disambiguation refers to the ability of some processors to determine whether pointers point to the same or
different objects and therefore whether the order of loads and stores can be changed. You can say that memory
disambiguation removes spurious (or false) dependencies.