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Instruction Descriptions 4–145
4.11.10 Write Hint
Format:
Operation:
va ← Rbv
IF { va maps to memory space } THEN
Write UNPREDICTABLE data to the aligned 64-byte region
containing the addressed byte.
END
Exceptions:
Instruction mnemonics:
Qualifiers:
Description:
The WH64 instruction provides a hint that the current contents of the aligned 64-byte block
containing the addressed byte will never be read again but will be overwritten in the near
future.
The processor may allocate cache resources to hold the block without reading its previous con-
tents from memory; the contents of the block may be set to any value that does not introduce a
security hole, as described in Section 1.6.3.
The WH64 instruction does not generate exceptions; if it encounters data address translation
errors (access violation, translation not valid, and so forth), it is treated as a NOP.
If the address maps to non-memory-like (I/O) space, WH64 is treated as a NOP.
Software Note:
This instruction is a performance hint that should be used when writing a large continuous
region of memory. The intended code sequence consists of one WH64 instruction followed
by eight quadword stores for each aligned 64-byte region to be written.
Sometimes, the UNPREDICTABLE data will exactly match some or all of the previous
contents of the addressed block of memory.
WH64 (Rb.ab)
! Memory format
None
WH64 Write Hint - 64 Bytes
None