Intel ARCHITECTURE IA-32 User Manual

Optimizing for SIMD Floating-point Applications 5
5-3
• Is the data arranged for efficient utilization of the SIMD
floating-point registers?
• Is this application targeted for processors without SIMD
floating-point instructions?
For more details, see the section on “Considerations for Code
Conversion to SIMD Programming” in Chapter 3.
Using SIMD Floating-point with x87 Floating-point
Because the XMM registers used for SIMD floating-point computations
are separate registers and are not mapped onto the existing x87
floating-point stack, SIMD floating-point code can be mixed with either
x87 floating-point or 64-bit SIMD integer code.
Scalar Floating-point Code
There are SIMD floating-point instructions that operate only on the
least-significant operand in the SIMD register. These instructions are
known as scalar instructions. They allow the XMM registers to be used
for general-purpose floating-point computations.
In terms of performance, scalar floating-point code can be equivalent to
or exceed x87 floating-point code, and has the following advantages:
• SIMD floating-point code uses a flat register model, whereas x87
floating-point code uses a stack model. Using scalar floating-point
code eliminates the need to use
fxch instructions, which has some
performance limit on the Intel Pentium 4 processor.
• Mixing with MMX technology code without penalty.
• Flush-to-zero mode.
• Shorter latencies than x87 floating-point.

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)