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(nasm.info)Section B.4.151

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B.4.151. `MFENCE': Memory Fence
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     MFENCE                        ; 0F AE /6        [WILLAMETTE,SSE2]

   `MFENCE' performs a serialising operation on all loads from memory
and writes to memory that were issued before the `MFENCE' instruction.
This guarantees that all memory reads and writes before the `MFENCE'
instruction are completed before any reads and writes after the
`MFENCE' instruction.

   `MFENCE' is ordered respective to other `MFENCE' instructions,
`LFENCE', `SFENCE', any memory read and any other serialising
instruction (such as `CPUID').

   Weakly ordered memory types can be used to achieve higher processor
performance through such techniques as out-of-order issue, speculative
reads, write-combining, and write-collapsing. The degree to which a
consumer of data recognizes or knows that the data is weakly ordered
varies among applications and may be unknown to the producer of this
data. The `MFENCE' instruction provides a performance-efficient way of
ensuring load and store ordering between routines that produce
weakly-ordered results and routines that consume that data.

   `MFENCE' uses the following ModRM encoding:

               Mod (7:6)        = 11B
               Reg/Opcode (5:3) = 110B
               R/M (2:0)        = 000B

   All other ModRM encodings are defined to be reserved, and use of
these encodings risks incompatibility with future processors.

   See also `LFENCE' (*Note Section B.4.137::) and `SFENCE' (*Note
Section B.4.288::).