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8-20 Vol. 3
MULTIPLE-PROCESSOR MANAGEMENT
It is possible for processor 1 to perceive that the repeated string stores in processor
0 are happening out of order. We assume that fast string operations are enabled on
processor 0.
In Example 8-12, processor 0 does two separate rounds of rep stosd operation of 128
doubleword stores, writing the value 1 (value in EAX) into the first block of 512 bytes
from location _x (kept in ES:EDI) in ascending order. It then writes 1 into a second
block of memory from (_x+512) to (_x+1023). All of the memory locations initially
contain 0. The block of memory initially contained 0. Processor 1 performs two load
operations from the two blocks of memory.
It is not possible in the above example for processor 1 to perceive any of the stores
from the later string operation (to the second 512 block) in processor 0 before seeing
the stores from the earlier string operation to the first 512 block.
The above example assumes that writes to the second block (_x+512 to _x+1023)
does not get executed while processor 0’s string operation to the first block has been
interrupted. If the string operation to the first block by processor 0 is interrupted,
and a write to the second memory block is executed by the interrupt handler, then
that change in the second memory block will be visible before the string operation to
the first memory block resumes.
In Example 8-13, processor 0 does one round of (128 iterations) doubleword string
store operation via rep:stosd, writing the value 1 (value in EAX) into a block of 512
bytes from location _x (kept in ES:EDI) in ascending order. It then writes to a second
memory location outside the memory block of the previous string operation.
Initially on processor 0: EAX == 1, ECX==128, ES:EDI ==_x
Initially [_x] to 511[_x]== 0, _x <= _y < _z < _x+512
r1 == 1 and r2 == 0 is allowed
Example 8-12. Stores Across String Operations Are not Reordered
Processor 0 Processor 1
rep:stosd [ _x]
mov r1, [ _z]
mov ecx, $128
mov r2, [ _y]
rep:stosd 512[ _x]
Initially on processor 0: EAX == 1, ECX==128, ES:EDI ==_x
Initially [_x] to 1023[_x]== 0, _x <= _y < _x+512 < _z < _x+1024
r1 == 1 and r2 == 0 is not allowed
Example 8-11. Stores Within a String Operation May be Reordered
Processor 0 Processor 1