Vol. 3A 8-9

MULTIPLE-PROCESSOR MANAGEMENT

8.2.3.2  

Neither Loads Nor Stores Are Reordered with Like Operations

The Intel-64 memory-ordering model allows neither loads nor stores to be reordered with the same kind of opera-
tion. That is, it ensures that loads are seen in program order and that stores are seen in program order. This is illus-
trated by the following example:

The disallowed return values could be exhibited only if processor 0’s two stores are reordered (with the two loads 
occurring between them) or if processor 1’s two loads are reordered (with the two stores occurring between them).

If r1 = 1, the store to y occurs before the load from y. Because the Intel-64 memory-ordering model does not allow 
stores to be reordered, the earlier store to x occurs before the load from y. Because the Intel-64 memory-ordering 
model does not allow loads to be reordered, the store to x also occurs before the later load from x. This r2 = 1.

8.2.3.3  

Stores Are Not Reordered With Earlier Loads

The Intel-64 memory-ordering model ensures that a store by a processor may not occur before a previous load by 
the same processor. This is illustrated by the following example:

Assume r1 = 1.

•

Because r1 = 1, processor 1’s store to x occurs before processor 0’s load from x.

•

Because the Intel-64 memory-ordering model prevents each store from being reordered with the earlier load 
by the same processor, processor 1’s load from y occurs before its store to x.

•

Similarly, processor 0’s load from x occurs before its store to y.

•

Thus, processor 1’s load from y occurs before processor 0’s store to y, implying r2 = 0.

8.2.3.4  

Loads May Be Reordered with Earlier Stores to Different Locations

The Intel-64 memory-ordering model allows a load to be reordered with an earlier store to a different location. 
However, loads are not reordered with stores to the same location.
The fact that a load may be reordered with an earlier store to a different location is illustrated by the following 
example:

Example 8-1.  Stores Are Not Reordered with Other Stores

Processor 0

Processor 1

mov [ _x], 1

mov r1, [ _y]

mov [ _y], 1

mov r2, [ _x]

Initially x = y = 0
r1 = 1 and r2 = 0 is not allowed

Example 8-2.  Stores Are Not Reordered with Older Loads

Processor 0

Processor 1

mov r1, [ _x]

mov r2, [ _y]

mov [ _y], 1

mov [ _x], 1

Initially x = y = 0
r1 = 1 and r2 = 1 is not allowed

Example 8-3.  Loads May be Reordered with Older Stores

Processor 0

Processor 1

mov [ _x], 1

mov [ _y], 1

mov r1, [ _y]

mov r2, [ _x]

Initially x = y = 0
r1 = 0 and r2 = 0 is allowed