Intel® Architecture Instruction Set Extensions Programming Reference

APPLICATION PROGRAMMING MODEL
Upper bits of YMM registers (255:128) can be read and written by many instructions with a VEX.256 prefix.
XSAVE and XRSTOR may be used to save and restore the upper bits of the YMM registers.
2.5 MEMORY ALIGNMENT
Memory alignment requirements on VEX-encoded instruction differ from non-VEX-encoded instructions. Memory
alignment applies to non-VEX-encoded SIMD instructions in three categories:
• Explicitly-aligned SIMD load and store instructions accessing 16 bytes of memory (e.g. MOVAPD, MOVAPS,
MOVDQA, etc.). These instructions always require memory address to be aligned on 16-byte boundary.
• Explicitly-unaligned SIMD load and store instructions accessing 16 bytes or less of data from memory (e.g.
MOVUPD, MOVUPS, MOVDQU, MOVQ, MOVD, etc.). These instructions do not require memory address to be
aligned on 16-byte boundary.
• The vast majority of arithmetic and data processing instructions in legacy SSE instructions (non-VEX-encoded
SIMD instructions) support memory access semantics. When these instructions access 16 bytes of data from
memory, the memory address must be aligned on 16-byte boundary.
Most arithmetic and data processing instructions encoded using the VEX prefix and performing memory accesses
have more flexible memory alignment requirements than instructions that are encoded without the VEX prefix.
Specifically,
• With the exception of explicitly aligned 16 or 32 byte SIMD load/store instructions, most VEX-encoded,
arithmetic and data processing instructions operate in a flexible environment regarding memory address
alignment, i.e. VEX-encoded instruction with 32-byte or 16-byte load semantics will support unaligned load
operation by default. Memory arguments for most instructions with VEX prefix operate normally without
causing #GP(0) on any byte-granularity alignment (unlike Legacy SSE instructions). The instructions that
require explicit memory alignment requirements are listed in Table 2-4.
Software may see performance penalties when unaligned accesses cross cacheline boundaries, so reasonable
attempts to align commonly used data sets should continue to be pursued.
Atomic memory operation in Intel 64 and IA-32 architecture is guaranteed only for a subset of memory operand
sizes and alignment scenarios. The list of guaranteed atomic operations are described in Section 7.1.1 of IA-32
Intel® Architecture Software Developer’s Manual, Volumes 3A. AVX and FMA instructions do not introduce any new
guaranteed atomic memory operations.
AVX and FMA will generate an #AC(0) fault on misaligned 4 or 8-byte memory references in Ring-3 when
CR0.AM=1. 16 and 32-byte memory references will not generate #AC(0) fault. See Table 2-3 for details.
Certain AVX instructions always require 16- or 32-byte alignment (see the complete list of such instructions in
Table 2-4). These instructions will #GP(0) if not aligned to 16-byte boundaries (for 16-byte granularity loads and
stores) or 32-byte boundaries (for 32-byte loads and stores).
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