3-543
INSTRUCTION SET REFERENCE
PREFETCHPrefetch
Description
If there are no excepting conditions, the prefetch instruction fetches the line containing the
addresses byte to a location in the cache hierarchy specified by a locality hint. If the line is
already present in the cache hierarchy at a level closer to the processor, no data movement
occurs. The bits 5:3 of the ModR/M byte specify locality hints as follows:
temporal data(t0) - prefetch data into all cache levels.
temporal with respect to first level cache (t1) - prefetch data in all cache levels
except 0th cache level
temporal with respect to second level cache (t2) - prefetch data in all cache levels,
except 0th and 1st cache levels.
non temporal with respect to all cache levels (nta) - prefetch data into non-
temporal cache structure.
The architectural implementation of this instruction in no way effects the function of a program.
Locality hints are processor implementation-dependent, and can be overloaded or ignored by a
processor implementation. The prefetch instruction does not cause any exceptions (except for
code breakpoints), does not affect program behavior, and may be ignored by the processor
implementation. The amount of data prefetched is processor implementation-dependent. It will,
however, be a minimum of 32 bytes. Prefetches to uncacheable or WC memory (UC or WCF
memory types) will be ignored. Additional ModRM encodings, besides those specified above,
are defined to be reserved, and the use of reserved encodings risks future incompatibility. Use
of any ModRM value other than the specified ones will lead to unpredictable behavior.
Operation
FETCH (m8);
OpcodeInstruction
Description
0F,18,/1
0F,18,/2
0F,18,/3
0F,18,/0
PREFETCHT0 m8
PREFETCHT1 m8
PREFETCHT2 m8
PREFETCHNTA m8
Move data specified by address closer to the processor using
the t0 hint.
Move data specified by address closer to the processor using
the t1 hint.
Move data specified by address closer to the processor using
the t2 hint.
Move data specified by address closer to the processor using
the nta hint.
