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IBM: Cell-like CPU yields 10-20 per cent

But it is any less lucky than any other maker of modern CPUs?

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Cell processor producers Sony, Toshiba and IBM are getting rather low-sounding yields of the broadband-oriented CPU it seems - at least if comments made by Tom Reeves, VP of semiconductor and technology services at IBM, are anything to go by.

We quoted an interview with Tom on Monday, pointing out his words on Sony's keenness that the PlayStation 3 support as many old PS2 and PSOne games as possible. The interview was published by Electronic Times.

Tom said a number of other interesting things, one of which was: "With a chip like the Cell processor, you’re lucky to get 10 or 20 per cent [yields]." He compared that will the yields achieved with smaller, more basic chips, which can reach 95 per cent.

In other words, Cell yields are tiny, right? That's certainly what a number of sites have concluded, and while we'd pause before suggesting Cell yields are high - it's a new processor architecture, so they're not likely to be - it's worth waiting before you sell all your Sony shares and buying Microsoft stock.

Cell comprises a 64-bit PowerPC core along with eight subsidiary Synergistic Processing Elements (SPEs), which are essentially quasi-independent vector processors. Each of these nine elements has to operate for the Cell to be a top-grade part, but unlike other multi-core processors, the architecture means the chip can operate with the PPC plus at least one SPE. According to Reeves, Sony wants chips with at least seven SPEs functional.

Reeves' figure is likely to centre on the top-grade chips, so the number of suitably operational PS3-oriented parts coming off the production line is likely to be higher - and will continue to rise, in all probability, as more wafers are completed and the Cell partners continue to refine the process.

Sony has said it expects to sell 6m PS3s by March 2007, roughly four and a half months after the mid-November 2007. Given the size of a Cell die - approximately 221mm² - and the size of a 300mm wafer, IBM and co. are going to get no more than 320 Cells per wafer, ignoring the fact that the wafer's round and the die is rectangular.

At 20 per cent yield, that's 64 Cells per wafer, so Sony needs 93,7500 wafers to in 4.5 month to get enough Cells. That's 20,833 wafer starts per month, which isn't entirely out of order, given not only IBM but Sony will be producing Cells destined for PS3s.

But the issue here is not entirely one of numbers - it's about cost too. The lower the yield the more wafers needed, and the higher the cost of each suitable processor. But since Sony doesn't expect the PS3 hardware to be profitable for some years, it should be able to cope with that. That leaves it banking on not only improving yields - to help reduce manufacturing costs - but also buoyant games sales, which is where it will make its money in the early days, to pull the system into profitability. These trends are a given in the console business, and there are plenty of other factors than Cell yield that Sony can tweak on its PS3 profit/loss spreadsheet to help turn the Total cell from red to black. ®

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