Intel ARCHITECTURE IA-32 User Manual

Coding for SIMD Architectures 3
3-25
The __declspec(align(16)) specifications can be placed before data
declarations to force 16-byte alignment. This is particularly useful for
local or global data declarations that are assigned to 128-bit data types.
The syntax for it is
__declspec(align(integer-constant))
where the integer-constant is an integral power of two but no greater
than 32. For example, the following increases the alignment to 16-bytes:
__declspec(align(16)) float buffer[400];
The variable buffer could then be used as if it contained 100 objects of
type
__m128 or F32vec4. In the code below, the construction of the
F32vec4 object, x, will occur with aligned data.
void foo() {
F32vec4 x = *(__m128 *) buffer;
...
}
Without the declaration of __declspec(align(16)), a fault may occur.
Alignment by Using a
union Structure. Preferably, when feasible, a
union can be used with 128-bit data types to allow the compiler to align
the data structure by default. Doing so is preferred to forcing alignment
with
__declspec(align(16)) because it exposes the true program
intent to the compiler in that
__m128 data is being used. For example:
union {
float f[400];
__
m128 m[100];
} buffer;
The 16-byte alignment is used by default due to the __m128 type in the
union; it is not necessary to use __declspec(align(16)) to force it.

Intel ARCHITECTURE IA-32 Specifications

General

Instruction Set	x86
Instruction Set Type	CISC
Memory Segmentation	Supported
Operating Modes	Real mode, Protected mode, Virtual 8086 mode
Max Physical Address Size	36 bits (with PAE)
Max Virtual Address Size	32 bits
Architecture	IA-32 (Intel Architecture 32-bit)
Addressable Memory	4 GB (with Physical Address Extension up to 64 GB)
Floating Point Registers	8 x 80-bit
MMX Registers	8 x 64-bit
SSE Registers	8 x 128-bit
Registers	General-purpose registers (EAX, EBX, ECX, EDX, ESI, EDI, ESP, EBP), Segment registers (CS, DS, SS, ES, FS, GS), Instruction pointer (EIP), Flags register (EFLAGS)
Floating Point Unit	Yes (x87)