Why Apple Can't Use IBM's PowerPC 970

The following article reflects many misconceptions about the IBM PowerPC 970 processor. It is followed immediately by Why Apple Can User IBM's PowerPC 970, which debunks those myths. dk

The sad truth of the matter is that Apple can not and will not use IBM's PowerPC 970 64-bit chip in any of its systems anytime in the foreseeable future. Why? Well, suffice to say it gets complicated and involves support and developers, in that order.

IBM's PPC 970 is a different kind of processor. It's 64-bit. Apple's current series of G3 and G4 processors are all 32-bit. Nowhere in IBM's technical documents does it state that the PPC 970 has a 32-bit compatibility mode. Hence, the existence of Problem Number One™. Apple would have to maintain a 64-bit version of OS X along with a native 32-bit version for compatibility with G4 and G3 processors, at least until all its systems were switched over to the PPC 970. At which time, Apple loses 1% (example only) of its customer base for being "left behind," and picks up 1.5% of former *nix users as the intrigue of another 64-bit market player sets in.

This scenario, of course, rules out the possibility that IBM won't build its own PPC 970 systems. IBM is a big company. IBM makes money. IBM does not pursue ventures that do not make money. Hence, making the PPC 970 specifically for Apple and upgrade market (Sonnet and the like) probably doesn't cover the development costs of the PPC 970, and that doesn't make IBM money.

Motorola has a long standing (albeit shaky) relationship with Apple. If Apple were to switch chip manufacturers, Motorola would not be releasing updated G4 chips, would they? All Apple would have left would be overstock, and I doubt Apple has overclock friendly G4 chips stockpiled for the next six months and just haven't been telling their customers.

OS X was released close to two years ago, and Apple had a long start with developers to make sure that applications were going to be developed for OS X. Without a compatibility mode, software would have to be rewritten for the PPC 970. Apple can not simply tell it's developers, "Um, we know you just rewrote all your software two years ago, can you do it again?" All that would be left would be Adobe (Photoshop and the like) and Microsoft (Office, otherwise Microsoft is a monopoly).

Even if the PPC 970 does include a 32-bit compatibility mode, optimizing code for the PPC 970 would take time. And that's time that Apple quite frankly does not have and can not afford.

IBM's PPC 970 is geared toward a different market, a market that does not include Apple Computer.

The truth of the matter is that Apple's need for a new and improved processor is the one and only reason IBM would ever have for adding the AltiVec velocity engine instructions to the PowerPC 970. Until the PPC 970 announcement, IBM had been intractably opposed to the idea, seeing it as diametrically opposed to the RISC philosophy. But with Apple insisting that it must have AltiVec, IBM apparently decided that IBM was a big enough market to pursue and licensed the AltiVec design from Motorola.

As Hannibal notes on ars technica, "Apple, of course, is the obvious customer for the 970, and as I'll discuss in a moment IBM's newly announced chip would fit well with their general needs and direction."

The PPC 970 is not a new chip cut from new cloth. It is an altered, simplified version of IBM's existing POWER4 architecture. But it's also designed with the past in mind. As IBM notes on its own website, "32-bit applications needed to be supported with the same level of performance" as 64-bit applications. The PPC 970 has "full native support for 32-bit applications."

That 32-bit compatibility mode you're so worried about exists. It is not a dream. It is not fiction. In fact, according to Details emerge on IBM's PowerPC 970 chip on MacCentral, the PPC 970 doesn't just handle 32-bit instructions, it can run as a 32-bit processor - and this is done natively in hardware, not through any kind of emulation.

From the beginning, the PowerPC has been designed as a 32-bit processor family that would eventually evolve into a 64-bit one. This is in stark contrast to the way Intel grew the 8080 into the 8086, the the 80286, 80386, and so on up through the Pentium 4. Or the way Motorola moved from the 24-bit 68000 to the 32-bit 68020 and later. A 64-bit path was a deliberate forward looking decision on the part of the AIM consortium.

The biggest obstacle I can see to Apple adopting the PPC 970 is the system bus. The PPC 970 wants to access the system bus at half CPU speed, which means a 600 MHz bus on the low end and 900 MHz on the high end. That's a big jump from where Apple is today - and also faster than any bus used by the Pentium 4 or AMD Athlon CPUs.

Okay, maybe it's not quite as bad as it sounds. According to another article on ars technica, "This bus physically runs at 450 MHz, but it's double-pumped." But it's still way faster than the 167 MHz bus in today's Power Mac G4.

The geek's are in love with this thing. Not only will a single PPC 970 at 1.8 GHz hold its own against a 3.0 GHz Pentium 4, it will also offer competitive performance to Intel's next generation Itanium 2 processor. And the PPC 970, being a POWER4 derivative, will readily support dual, quad, and other multiple processor configurations when even more horsepower is needed.

According to those in the know, Apple would need to recompile Mac OS X for the new CPU, much as they recompile it for each new piece of hardware they produce. (This is why, for instance, a commercial Jaguar distribution cannot be installed on January 2003 computers - the compile didn't take the new hardware into account since it didn't exist when Jaguar was finalized.)

But once OS X is recompiled and the new motherboards are ready, everything will be good to go. Old 32-bit software will run just fine according to every source I've read.

The only thing preventing Apple from selling a PPC 970 system today is that the chip hasn't gone into full scale production yet. You can bet dollars to donuts that if IBM has distributed preproduction samples to anyone, Apple has a few of these in their labs. It's more promising than any vaporware G5 from Motorola, which seems destined to ship three week's after Christ's return.

As for the size of the market, Apple is already selling 3 million computers a year. If the whole line went PPC 970 next year, that would assure IBM some nice sales volume. But that's not the whole picture. The PowerPC also supports Linux, the OS of choice among geeks and the OS that IBM is pinning its hopes on. IBM could sell its own Linux boxes with PPC 970s, further growing the market for their chip.

As for your speculation that Motorola would stop selling G4s to Apple if they went with IBM, realize that Motorola gets a royalty on every chip IBM makes with AltiVec - and that Motorola's failure to keep up with the rest of the industry in CPU speed is already reducing the size of the Mac market, which is the only major user of the G4 processor.

Motorola may not make as much by supplying low-end G4s to Apple while IBM takes over the high end, but the power of the PPC 970 will also be a very important tool in helping Apple grow its market while Intel tries to convince the world that their Itanium 2 processor, running at similar speeds to the PPC 970, is a viable alternative to the speed pumped Pentium 4.

Yes, Intel will have to contend with the MHz Myth in marketing its own 64-bit CPUs, which will not have the PPC 970's advantage of being backward compatible with an existing chip architecture.

All things considered, I can't see any reason for Apple not to embrace the PPC 970 when IBM begins shipping it.