# PREFETCHWT1 — Prefetch Vector Data Into Caches With Intent to Write and T1 Hint

Opcode/Instruction | Op/En | 64/32 bit Mode Support | CPUID Feature Flag | Description
--- | --- | --- | --- | ---
0F 0D /2 PREFETCHWT1 m8 | M | V/V | PREFETCHWT1 | Move data from m8 closer to the processor using T1 hint with intent to write.

## Instruction Operand Encoding ¶

Op/En Operand 1 Operand 2 Operand 3 Operand 4 |  |  |  |
--- | --- | --- | --- | ---
M ModRM:r/m (r) N/A N/A N/A |  |  |  |

## Description ¶

Fetches the line of data from memory that contains the byte specified with the source operand to a location in the cache hierarchy specified by an intent to write hint (so that data is brought into ‘Exclusive’ state via a request for ownership) and a locality hint:

- T1 (temporal data with respect to first level cache)—prefetch data into the second level cache.

The source operand is a byte memory location. (The locality hints are encoded into the machine level instruction using bits 3 through 5 of the ModR/M byte. Use of any ModR/M value other than the specified ones will lead to unpredictable behavior.)

If the line selected is already present in the cache hierarchy at a level closer to the processor, no data movement occurs. Prefetches from uncacheable or WC memory are ignored.

The PREFETCHWT1 instruction is merely a hint and does not affect program behavior. If executed, this instruction moves data closer to the processor in anticipation of future use.

The implementation of prefetch locality hints is implementation-dependent, and can be overloaded or ignored by a processor implementation. The amount of data prefetched is also processor implementation-dependent. It will, however, be a minimum of 32 bytes. Additional details of the implementation-dependent locality hints are described in Section 9.5, “Memory Optimization Using Prefetch” of the Intel® 64 and IA-32 Architectures Optimization Reference Manual.

It should be noted that processors are free to speculatively fetch and cache data from system memory regions that are assigned a memory-type that permits speculative reads (that is, the WB, WC, and WT memory types). A PREFETCHWT1 instruction is considered a hint to this speculative behavior. Because this speculative fetching can occur at any time and is not tied to instruction execution, a PREFETCHWT1 instruction is not ordered with respect to the fence instructions (MFENCE, SFENCE, and LFENCE) or locked memory references. A PREFETCHWT1 instruction is also unordered with respect to CLFLUSH and CLFLUSHOPT instructions, other PREFETCHWT1 instructions, or any other general instruction. It is ordered with respect to serializing instructions such as CPUID, WRMSR, OUT, and MOV CR.

This instruction’s operation is the same in non-64-bit modes and 64-bit mode.

## Operation ¶

    PREFETCH(mem, Level, State) Prefetches a byte memory location pointed by ‘mem’ into the cache level specified by ‘Level’; a request
    for exclusive/ownership is done if ‘State’ is 1. Note that the memory location ignore cache line splits. This operation is considered a
    hint for the processor and may be skipped depending on implementation.
    Prefetch (m8, Level = 1, EXCLUSIVE=1);

## Flags Affected ¶

All flags are affected.

## C/C++ Compiler Intrinsic Equivalent ¶

    void _mm_prefetch( char const *, int hint= _MM_HINT_ET1);

## Protected Mode Exceptions ¶

#UD | If the LOCK prefix is used.

## Real-Address Mode Exceptions ¶

#UD | If the LOCK prefix is used.

## Virtual-8086 Mode Exceptions ¶

#UD | If the LOCK prefix is used.

## Compatibility Mode Exceptions ¶

#UD | If the LOCK prefix is used.

## 64-Bit Mode Exceptions ¶

#UD | If the LOCK prefix is used.