LDDQU—Load Unaligned Integer 128 Bits

Opcode/Instruction	Op/En	64/32-bit Mode	CPUID Feature Flag	Description
F2 0F F0 /r LDDQU xmm1, mem	RM	V/V	SSE3	Load unaligned data from mem and return double quadword in xmm1.
VEX.128.F2.0F.WIG F0 /r VLDDQU xmm1, m128	RM	V/V	AVX	Load unaligned packed integer values from mem to xmm1.
VEX.256.F2.0F.WIG F0 /r VLDDQU ymm1, m256	RM	V/V	AVX	Load unaligned packed integer values from mem to ymm1.

Instruction Operand Encoding

Op/En	Operand 1	Operand 2	Operand 3	Operand 4
RM	ModRM:reg (w)	ModRM:r/m (r)	NA	NA

Description

The instruction is functionally similar to (V)MOVDQU ymm/xmm, m256/m128 for loading from memory. That is: 32/16 bytes of data starting at an address specified by the source memory operand (second operand) are fetched from memory and placed in a destination register (first operand). The source operand need not be aligned on a 32/16-byte boundary. Up to 64/32 bytes may be loaded from memory; this is implementation dependent.

This instruction may improve performance relative to (V)MOVDQU if the source operand crosses a cache line boundary. In situations that require the data loaded by (V)LDDQU be modified and stored to the same location, use (V)MOVDQU or (V)MOVDQA instead of (V)LDDQU. To move a double quadword to or from memory locations that are known to be aligned on 16-byte boundaries, use the (V)MOVDQA instruction.

Implementation Notes

If the source is aligned to a 32/16-byte boundary, based on the implementation, the 32/16 bytes may be loaded more than once. For that reason, the usage of (V)LDDQU should be avoided when using uncached or write-combining (WC) memory regions. For uncached or WC memory regions, keep using (V)MOVDQU.
This instruction is a replacement for (V)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 (V)MOVDQA store-load pairs when data is 256/128-bit aligned or (V)MOVDQU store-load pairs when data is 256/128-bit unaligned.
If the memory address is not aligned on 32/16-byte boundary, some implementations may load up to 64/32 bytes and return 32/16 bytes in the destination. Some processor implementations may issue multiple loads to access the appropriate 32/16 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.
If alignment checking is enabled (CR0.AM = 1, RFLAGS.AC = 1, and CPL = 3), an alignment-check exception (#AC) may or may not be generated (depending on processor implementation) when the memory address is not aligned on an 8-byte boundary.

In 64-bit mode, use of the REX.R prefix permits this instruction to access additional registers (XMM8-XMM15). Note: In VEX-encoded versions, VEX.vvvv is reserved and must be 1111b otherwise instructions will #UD.

Operation

LDDQU (128-bit Legacy SSE version)
DEST[127:0] ← SRC[127:0]
DEST[VLMAX-1:128] (Unmodified)
VLDDQU (VEX.128 encoded version)
DEST[127:0] ← SRC[127:0]
DEST[VLMAX-1:128] ← 0
VLDDQU (VEX.256 encoded version)
DEST[255:0] ← SRC[255:0]

Intel C/C++ Compiler Intrinsic Equivalent

LDDQU:	__m128i _mm_lddqu_si128 (__m128i * p);
LDDQU:	__m256i _mm256_lddqu_si256 (__m256i * p);

Numeric Exceptions

None.

Other Exceptions

See Exceptions Type 4; Note treatment of #AC varies.