Apple Computer Inc’s use of the Motorola Inc MPC7400 PowerPC means that it is once again relying on Motorola as the sole supplier of chips for its computers – as it did for the 68000 series of chips, pre PowerPC. Although IBM Corp is currently supplying some copper PowerPC 750 chips for the iBook, it has opted not to support Motorola’s AltiVec vector processing extensions to the PowerPC, as used in the latest Macs, effectively cutting itself off from future business with Apple.
While IBM and Motorola continue to collaborate on the Book E architecture, designed to provide more consistency between the two companies’ embedded PowerPC implementations, IBM has not shown any inclinations to support AltiVec. IBM contributed to the development of the extensions under their original name of VMX (Video and Multimedia Extensions), the first specification of which came out in 1997. But the G4 design work reverted back to Motorola after IBM pulled out of the joint Somerset design facility in mid 1998. IBM is still thought to be considering support for AltiVec however. With its multimedia focus, it wouldn’t be of great interest to IBM on a CPU for its own server products, and IBM’s embedded systems business has primarily been at the low-end, where the cost of AltiVec would be prohibitive. But with Apple now showing some signs of health, IBM’s Semiconductor division might in the future stop regarding that market as a trivial one. IBM’s next upgrade of the PowerPC, the Pulsar, is due to be unveiled with new RS/6000 servers and workstations on September 12.
AltiVec, or the Velocity Engine as Apple calls it, offers a far greater leap in performance than Intel Corp’s MMX, even in its second generation Katmai SSE implementation. According to Keith Diefendorff, editor in chief of the Microprocessor Report, although the two architectures are both described as using a SIMD single-instruction multiple data architecture, the similarity ends there. AltiVec is the more powerful architecture and has far more silicon invested in it. For multimedia applications it will outperform the Pentium at any speed, he said. That includes the forthcoming Coppermine processor due next month at 700MHz clock speeds.
Part of Intel’s problem was that it offered no programming tools support with the first generation of MMX. Motorola has made sure that AltiVec instructions can be accessed via standard programming languages and optimized from existing compilers, and Apple has optimized Mac OS 9 to support the new instructions and register files with 32 entries of 128 bits each. Metrowerks Inc, the compiler company acquired by Motorola last month for $95m, already supports the AltiVec instructions from its CodeWarrior development toolset for the Macintosh.
The MPC7400 supplements AltiVec with a more powerful floating point unit, which is fully pipelined for double-precision operations, not just the single precision pipelined by the G3, and there is also an internal memory bus that is 128 bits wide, twice that of the G3. But the short pipeline of the 750 G3 is still in place on the G4, and this makes it harder for Motorola to implement the chip at higher clock frequencies. It’s a fair bet that Motorola engineers are at this moment working on a re- implementation of the 7400 using a larger pipeline. That’s one of the things that could emerge at the Microprocessor Forum in San Jose next month, when Motorola’s Naras Iyengar is set to discuss the follow-on part to the G4 7400.
