From Multimedia Futures, a sister publication
Intel Corp has brought its architecture into the multimedia age with its MMX MultiMedia eXtensions instruction set, which is claimed to speed up video and multimedia applications by between 50% and 400% (CI No 2,868). Intel demonstrated the 57 new MMX instructions to software developers at the Intermedia World show in San Francisco. This is the most important change in the Intel’s architecture for a decade: It’s been a long time since the bedrock of the PC industry has experienced such an upheaval. The first rumbles began early last year, when Intel announced plans to extend its basic architecture to better serve the needs of multimedia applications. The tremors intensified on March 5 this year, with the release of specific details of the MMX technology, (which, despite an obvious correlation with ‘multimedia extensions’, is not an acronym). But the quake proper won’t hit until the fourth quarter, when Intel launches its first MMX-equipped processor – the P55C. This will be a Pentium class chip, probably clocked at the design maximum of 200MHz. Subsequent chips in the Pentium line and above will get the extra MMX silicon as standard, avoiding the kind of confusion caused by the co-processor that came as an option for the 486. Intel has also confirmed that an overdrive chip will be offered to upgrade existing Pentium PCs.
By Lem Bingley
The P55C is being sold as a chip with epochal significance because it will be the first Intel CPU in a decade to offer an enlarged instruction set. Intel’s processors, from the 386 right up to the Pentium Pro, have evolved by adding new technology to the same basic tasks – but MMX adds new tasks. In 1985, the 386 stretched the original operational capacity from 170 instructions to 220. MMX adds a further 57, significantly expanding the range of basic building blocks available to programmers. The changes appear at the assembly language level and will be hidden away (even from most programmers) behind device drivers, libraries and APIs, but the end results will be apparent to everyone. It is claimed that multimedia applications will run between 50% and 300% faster on MMX chips. MMX achieves these gains through a technique known as SIMD (Single Instruction, Multiple Data). As Alex Peleg, senior computer architect at Intel Haifa explains: Programs carrying out tasks like image processing typically spend large amounts of time repeatedly cycling through small loops of code, applying the same series of computations to a long stream of data. In a lot of cases a parallel approach would be highly appropriate – because each calculation is independent of the others. Many such tasks in multimedia make a great fit for SIMD. It is also fortunate that most of the data types used by multimedia applications are small. Audio is normally encoded as 8-, 12- or 16-bit samples; pixels are stored using groups of four, six or eight bits. With a 64-bit internal architecture, Intel is able to lump several of these small data types together into a single 64-bit package (that is, the ‘multiple data’ part of SIMD) and process them all at once (the ‘single instruction’ part). This simple process provides most of the performance increase, since a 64-bit operation performs just as quick as an 8-bit one but this time operating on many more instructions at a time. It also provides all of that ‘parallelism’ that Intel has been making such a fuss about. The new MMX instructions recognize the internal boundaries between small data types and prevent cross-corruption. Whereas a normal 64-bit addition could easily cause a ‘carry’ from one pixel to the next, an addition using MMX’s ‘saturating arithmetic’ instructions will not. Attempts to increment an already full byte will have no effect on its neighbors. Although this can sound like an imprecise kludge, in most practical applications it’s actually beneficial. For example, when carrying out Gouraud shading to make 3D objects made up of flat polygons appear smooth, color values are modified across each
polygon. The progressive alteration of color can cause a byte to overflow (or underflow past zero), causing a sudden transition in color – so Gouraud shading routines usually check for this condition. With saturating arithmetic, no checks are needed and routines can be made simpler and quicker. Together with the new data types and new instructions come eight new registers. Registers are binary receptacles built into the CPU, into which data is put when calculations are to be executed. The MMX registers are described as ‘virtual’ – because they are actually the existing floating-point (FP) registers put to a different use. Where programmers need to use both FP and MMX – for 3D graphics, say, where geometry calculations and display routines rub shoulders – they must keep the two types separate and purge the registers in-between. Architectural steps have also been taken to avoid mix-ups. Intel maintains that it came up with the clever specifications for MMX after wholesale analysis of ‘bleeding-edge’ applications, like videoconferencing, 3D rendering, image processing and games. But MMX bears a striking similarity to VIS (Visual Instruction Set), the ‘New Media’ extensions to Sun’s SPARC architecture, launched last year. VIS uses packed data types, employs the same FP register trick, and also provides saturating arithmetic amongst its 30 special instructions.
AMD and Nexgen follow – Cyrix? probably
Original or otherwise, Intel has been puffing out its chest over MMX – with some justification. Competing chipmakers NexGen and Cyrix have been keenly developing their own independent multimedia extensions, but a unified approach would clearly be beneficial to the industry. Fortunately, NexGen purchaser AMD has committed to MMX, with a cross-licensing deal signed in January. Cyrix has been offered a license for MMX, but has made no formal commitment. In all probability we will support MMX, commented Cyrix European general manager Brendan Sherry. But I believe this is essentially a short-term and low-end solution. I don’t believe it addresses the core issue, which is that processors like the Pentium simply aren’t fast enough for multimedia. Intel, of course, is not blind to that fact. And since it has often repeated that home multimedia users are driving its chip development, MMX may well turn out to be just the first gentle warning of much bigger shifts to come.
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