Intel 253666-024US Manuel D’Utilisation
3-524 Vol. 2A
LDDQU—Load Unaligned Integer 128 Bits
INSTRUCTION SET REFERENCE, A-M
LDDQU—Load Unaligned Integer 128 Bits
Description
The instruction is functionally similar to MOVDQU xmm, m128 for loading from
memory. That is: 16 bytes of data starting at an address specified by the source
memory operand (second operand) are fetched from memory and placed in a desti-
nation register (first operand). The source operand need not be aligned on a 16-byte
boundary. Up to 32 bytes may be loaded from memory; this is implementation
dependent.
This instruction may improve performance relative to MOVDQU if the source operand
crosses a cache line boundary. In situations that require the data loaded by LDDQU
be modified and stored to the same location, use MOVDQU or MOVDQA instead of
LDDQU. To move a double quadword to or from memory locations that are known to
be aligned on 16-byte boundaries, use the MOVDQA instruction.
memory. That is: 16 bytes of data starting at an address specified by the source
memory operand (second operand) are fetched from memory and placed in a desti-
nation register (first operand). The source operand need not be aligned on a 16-byte
boundary. Up to 32 bytes may be loaded from memory; this is implementation
dependent.
This instruction may improve performance relative to MOVDQU if the source operand
crosses a cache line boundary. In situations that require the data loaded by LDDQU
be modified and stored to the same location, use MOVDQU or MOVDQA instead of
LDDQU. To move a double quadword to or from memory locations that are known to
be aligned on 16-byte boundaries, use the MOVDQA instruction.
Implementation Notes
•
If the source is aligned to a 16-byte boundary, based on the implementation, the
16 bytes may be loaded more than once. For that reason, the usage of LDDQU
should be avoided when using uncached or write-combining (WC) memory
regions. For uncached or WC memory regions, keep using MOVDQU.
16 bytes may be loaded more than once. For that reason, the usage of LDDQU
should be avoided when using uncached or write-combining (WC) memory
regions. For uncached or WC memory regions, keep using MOVDQU.
•
This instruction is a replacement for MOVDQU (load) in situations where cache
line splits significantly affect performance. It should not be used in situations
where store-load forwarding is performance critical. If performance of store-load
forwarding is critical to the application, use MOVDQA store-load pairs when data
is 128-bit aligned or MOVDQU store-load pairs when data is 128-bit unaligned.
line splits significantly affect performance. It should not be used in situations
where store-load forwarding is performance critical. If performance of store-load
forwarding is critical to the application, use MOVDQA store-load pairs when data
is 128-bit aligned or MOVDQU store-load pairs when data is 128-bit unaligned.
•
If the memory address is not aligned on 16-byte boundary, some implementa-
tions may load up to 32 bytes and return 16 bytes in the destination. Some
processor implementations may issue multiple loads to access the appropriate 16
bytes. Developers of multi-threaded or multi-processor software should be aware
that on these processors the loads will be performed in a non-atomic way.
tions may load up to 32 bytes and return 16 bytes in the destination. Some
processor implementations may issue multiple loads to access the appropriate 16
bytes. Developers of multi-threaded or multi-processor software should be aware
that on these processors the loads will be performed in a non-atomic way.
In 64-bit mode, use of the REX.R prefix permits this instruction to access additional
registers (XMM8-XMM15).
registers (XMM8-XMM15).
Operation
xmm[127:0] = m128;
Opcode
Instruction
64-Bit
Mode
Compat/
Leg Mode
Description
F2 0F F0 /r
LDDQU xmm1, mem
Valid
Valid
Load unaligned data from mem
and return double quadword in
xmm1.