Where the hell is… the PowerPC 'G4 Plus'?

Motorola has of late been ominously quiet about the development of the PowerPC. Last October, the chip company unveiled its 'G4 Plus' project to rebuild the current PowerPC 7400 (aka G4) CPU. That chip slots in between the current G4 and G5, the next processor listed on Motorola's current (ish) PowerPC roadmap.

In the meantime, getting the PowerPC 7400 above 500MHz has proved difficult, to the extent that Apple has been forced to ship dual-processor machines to help convince buyers that its Power Mac isn't lagging way behind Wintel. It isn't, but persuading customers that 500MHz G4 is comparable to a 1GHz Pentium III has proved difficult, CEO Steve Jobs' MacWorld Expo Photoshop tests notwithstanding.

Last October's 'G4 Plus' announcement was made at the Microprocessor Forum. The chip is essentially about getting the PowerPC's clock speed up to counter the developments Intel and AMD have made - so much for the limitations of Cisc technology, ahem. So the new chip will feature a longer instruction pipeline to allow the chip's clock speed to be raised without adversely impacting the chip's efficiency. To counter the reduction in the number of instructions a processor can handle per second inherent in a longer pipeline, the Motorola has increased the number of instruction processing units in the chip, with three extra AltiVec engines and two more integer units.

The chip's L2 cache will be brought onto the die, to allow CPU and cache to communicate at the same speed. The L2 connects to the L1 across a new, 256-bit bus. The 'G4 Plus' will support external, backside configuration L3 cache, up to 2MB of it.

Motorola wouldn't say when the 'G4 Plus' would ship, but suggested we consider the timeline it had put in place for the original G4. By that logic, the 'G4 Plus' should be sampling real soon now, with volume production in the autumn.

The snag is that the G4 was released too soon, at a point at which Motorola could not guarantee supply of higher speed CPUs. That forced Apple to ship the 400MHz Power Mac G4 a month or so ahead of the 450MHz and 500MHz versions, and then to knock the entire line back 50MHz to 350, 400 and 450MHz, because Motorola effectively couldn't get the chip to run at 500MHz.

Motorola presumably wants to avoid a repetition of that incident, which is why sources are suggesting the 'G4 Plus' won't appear until year end. Motorola said last October it is aiming for a base speed of 700MHz.

That schedule suggests Apple will be pretty much stuck with its current range of dual-CPU Power Macs for some time to come. Short of upping the clock speed to 600MHz, there's little Apple can do until the 'G4 Plus' arrives. And it doesn't want to be stuck with a dual 500MHz G4 that's faster than a single-CPU 'G4 Plus' machine - remember, in the computer biz, newest is always best.

The schedule may also affect Motorola's G5 chip, its first multi-core CPU, which is believed to contain four G4s operating in close harmony to generate four times the performance of a single chip at a given clock speed. With the cores so tightly coupled, users should get all the benefit of four-way multiprocessing without the usual CPU management overhead - what, in other words, stops the new dual-CPU Power Mac G4s from delivering double the performance of a single-CPU Mac.

Presumably, the G5 will ultimately contain four 'G4 Plus' cores, in place of the existing G4. Either way, it really won't be much use to Apple until MacOS X, with its superior multi-processing support, is widely available and Apple's default OS. ®

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