AltiVec
From Wikipedia, the free encyclopedia
AltiVec is a floating point and integer SIMD instruction set designed and owned by Apple Computer, IBM and Freescale Semiconductor, formerly the Semiconductor Products Sector of Motorola, (the AIM alliance), and implemented on versions of the PowerPC including Motorola's G4, IBM's G5 and future POWER6 processors. AltiVec is a tradename owned solely by Freescale, so the system was also referred to as Velocity Engine by Apple and VMX by IBM, though IBM is using AltiVec nowadays too.
It should be noted that while AltiVec refers to an instruction set, the implementations in CPUs produced by IBM and Motorola are separate in terms of logic design. To date, no IBM core has included an AltiVec logic design licensed from Motorola and vice-versa.
AltiVec is a standard part of the new Power ISA v.2.03[1] specification. It was never formally a part of the PowerPC architecture until this specification although it used PowerPC instruction formats and syntax and occupied the opcode space expressly allocated for such purposes.
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[edit] Features and similarities
Both AltiVec and SSE feature 128-bit vector registers that can represent sixteen 8-bit signed or unsigned chars, eight 16-bit signed or unsigned shorts, four 32-bit ints or four 32-bit floating point variables. Both provide cache-control instructions intended to minimize cache pollution when working on streams of data.
They also exhibit important differences. Unlike SSE2, AltiVec supports a special RGB "pixel" data type, but it does not operate on 64-bit double precision floats, and there is no way to move data directly between scalar and vector registers. In keeping with the "load/store" model of the PowerPC's RISC design, the vector registers, like the scalar registers, can only be loaded from and stored to memory. However, AltiVec provides a much more complete set of "horizontal" operations that work across all the elements of a vector; the allowable combinations of data type and operations are much more complete. 32 128-bit vector registers are provided, compared to 8 for SSE and SSE2, and most AltiVec instructions take three register operands compared to only two register/register or register/memory operands on IA-32.
AltiVec is also unique in its support for a flexible vector permute instruction, in which each byte of a resulting vector value can be taken from any byte of either of two other vectors, parametrized by yet another vector. This allows for sophisticated manipulations in a single instruction.
Recent versions of the GNU Compiler Collection, IBM Visual Age Compiler and other compilers provide intrinsics to access AltiVec instructions directly from C and C++ programs. The "vector" storage class is introduced to permit the declaration of native vector types, e.g., "vector unsigned char foo;" declares a 128-bit vector variable named "foo" containing sixteen 8-bit unsigned chars. Overloaded intrinsic functions such as "vec_add" emit the appropriate op code based on the type of the elements within the vector, and very strong type checking is enforced. In contrast, the Intel-defined data types for IA-32 SIMD registers declare only the size of the vector register (128 or 64 bits) and in the case of a 128-bit register, whether it contains integers or floating point values. The programmer must select the appropriate intrinsic for the data types in use, e.g., "_mm_add_epi16(x,y)" for adding two vectors containing eight 16-bit integers.
[edit] Development history
AltiVec was developed between 1996 and 1998 by Keith Diefendorff, the distinguished scientist and director of microprocessor architecture at Apple Computer.
Apple was the primary customer for AltiVec (Apple has switched to Intel made, x86-based CPUs as of 2006), and uses it to accelerate multimedia applications such as QuickTime, iTunes and key parts of Apple's Mac OS X including in the Quartz graphics compositor. Other companies such as Adobe use it for optimization of their image-processing programs such as Adobe Photoshop. Motorola was the first to supply AltiVec enabled processors starting with their G4 line (Motorola has since spun off its processor division into the separate company Freescale). AltiVec is also used in some embedded systems that are used for high-performance digital signal processing.
IBM has consistently left VMX out of their POWER systems, which are intended for mainframe and server applications where it is not very useful. However, the most recent desktop CPU from IBM, PowerPC 970 (dubbed the G5 by Apple) does include the AltiVec unit similar to the original PowerPC 7400. The core includes a multiplier/adder unit and a full VMX unit.
The forthcoming POWER6 also includes AltiVec, but the exact specifications of the implementation is unknown at this point.
AltiVec is the standard Category.VEC part of the new Power ISA v.2.03[1] specification.
[edit] VMX128
IBM enhanced VMX for use in Xenon and Cell (Xbox 360 and PlayStation 3) and called this enhancement VMX128. The enhancements comprise of new routines targeted at gaming (accelerating 3D graphics and game physics)[2] and a total of 128 registers. VMX128 is not entirely compatible with VMX/Altivec.
