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(nasm.info)Section B.4.137

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B.4.137. `LFENCE': Load Fence
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     LFENCE                        ; 0F AE /5        [WILLAMETTE,SSE2]

   `LFENCE' performs a serialising operation on all loads from memory
that were issued before the `LFENCE' instruction. This guarantees that
all memory reads before the `LFENCE' instruction are visible before any
reads after the `LFENCE' instruction.

   `LFENCE' is ordered respective to other `LFENCE' instruction,
`MFENCE', 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 and
speculative reads. 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 `LFENCE' instruction
provides a performance-efficient way of ensuring load ordering between
routines that produce weakly-ordered results and routines that consume
that data.

   `LFENCE' uses the following ModRM encoding:

               Mod (7:6)        = 11B
               Reg/Opcode (5:3) = 101B
               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 `SFENCE' (*Note Section B.4.288::) and `MFENCE' (*Note
Section B.4.151::).