IBM previews latest PowerPCs

Could be even faster than Motorola's more advanced version

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IBM yesterday previewed its upcoming gigahertz-targeted PowerPC CPUs: the PPC 750CX and PPC 750CXe.

Both chips are extend the PowerPC Generation 3 architecture (aka G3) which marks the final point at which Motorola's and IBM's PowerPC development efforts coincided. Since the split, some two years back, Motorola has forged ahead with the PowerPC 7400 (aka G4), while IBM has focused on evolving the PowerPC 750.

And evolving it to the stage where it could outperform the Motorola chip. When IBM announced the two new G3 CPUs earlier this month, it said the 750CXe will hit 700MHz. With the current G4 still sitting at an increasingly mediocre-looking 500MHz, but equivalent to an existing G3-based chip operating at 600MHz, both the architectural enhancements of the 750CXe and the higher clock speed could easily put the IBM chip in the lead.

The G4 isn't due to hit 700MHz or more until Motorola begins shipping the so-called G4 Plus, announced last October but with no ship date. Based on a ground-up redesign of the G4 architecture, the Plus should provide a significant performance improvement, but it's going to take some time to do. We don't expect it to sample, let alone ship in volume until later this year.

The key architectural improvement over existing G3-class CPUs is the inclusion of 256KB on-die L2 cache. The 750CX and 750CXe are fundamentally the same chip, but the latter will be fabbed at 0.18 micron to allow it to attain the high clock speeds, which IBM insiders have claimed could hit 1GHz later next year.

At 0.18 micron, the 750CXe takes up 42 square millimetres, IBM Senior PowerPC Processor Architect, Peter Sandon, told the Embedded Processor Forum audience, yesterday. By contrast, the original, cacheless G3 took up 40 square millimetres.

Sandon also discussed the chips' use of a copper layer on top of the processor to help it lose heat more efficiently. The chips also sport better FPU performance and wider L1 cache buses, Sandon said.

The 750CX is due to ship later this year, with the 750CXe following early 2001. ®


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