GNU Info

(nasm.info)Section B.4.288

---

Next: Section B.4.289 Prev: Section B.4.287 Up: Section B.4

---

Enter node , (file) or (file)node

B.4.288. `SFENCE': Store Fence
------------------------------

     SFENCE                 ; 0F AE /7               [KATMAI]

   `SFENCE' performs a serialising operation on all writes to memory
that were issued before the `SFENCE' instruction. This guarantees that
all memory writes before the `SFENCE' instruction are visible before any
writes after the `SFENCE' instruction.

   `SFENCE' is ordered respective to other `SFENCE' instruction,
`MFENCE', any memory write 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,
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
`SFENCE' instruction provides a performance-efficient way of insuring
store ordering between routines that produce weakly-ordered results and
routines that consume this data.

   `SFENCE' uses the following ModRM encoding:

               Mod (7:6)        = 11B
               Reg/Opcode (5:3) = 111B
               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 `MFENCE' (*Note
Section B.4.151::).