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Apple hit by 'PowerPC G4 can't reach 500MHz' bug

Data cache corruption glitch not to be fixed until December

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Apple may have a tough time shipping its upcoming 500MHz Power Mac G4, thanks to Motorola's apparent inability to ship a PowerPC 7400 (aka G4) that works at that clock speed. According to Apple-watching Web site MacWeek, Motorola's list of bugs yet to be fixed ('errata', as they're called in the semiconductor business) in the 7400 include a rather worrying data cache corruption problem that manifests itself in all versions of the CPU running at 500MHz or more. While 400MHz and 450MHz chips are 'safe' (since they're not being run at the higher clock speed) any attempt to overclock them to 500MHz -- whether by users themselves or by unscrupulous upgrade merchants trying to save money by selling cheaper chips at unrated clock speeds -- will invoke the bug. Long-term, Motorola claims the glitch will be fixed in version 2.8 of the 7400 silicon -- the chip is currently at version 2.6 -- but that's not likely to happen before December. Given Apple is supposed to be shipping 500MHz G4s later this month, the ship date promised by interim CEO Steve Jobs at the Power Mac G4's launch in August, this is going to make it rather hard to ship a 500MHz machine on time. Or rather ship a 500MHz machine that actually clocks at 500MHz. Apple can ship a 500MHz G4 and claim that it's able to do so because the chip inside has been rated by Motorola for operation at 500MHz -- even though... er... it can't run at that speed. Motorola's short-term fix is to enable a register within the chip which effectively slows the chip down to below 500MHz. Of course, Apple could disable the register, but the data corruption problem will at the very least push performance down to a point way below 500MHz anyway -- with the cache corrupt, the machine will constantly have to refetch data from slow main memory -- or at worst cause the machine to crash. Either way, the 500MHz Power Mac G4 will not offer 500MHz performance -- until it contains a version 2.8 PowerPC 7400. According to comments from Motorola Tech Support seen by The Register, the chip company appears not to be rating CPUs for 500MHz, so Apple (or anyone else, for that matter) isn't likely to be receiving 500MHz parts yet anyway. Of course, all chips ship with bugs, and Motorola's parts are no exception -- a few years back the 604e was released to repair some serious performance-limiting glitches in the original 604 -- but the G4's problems come at a tricky time for Apple, already having a job convincing potential buyers that a 400MHz G4 is faster than a 500MHz Pentium III. Clock rates are no way to compare the performance of different CPUs, but megaHertz remain one of the key purchase criteria, and with 700MHz Athlons already available and 700MHz Pentium IIIs coming later this month, the 450MHz PowerPC 7400 is increasingly being perceived as underpowered. ® Related Story Motorola speeds PowerPC to compete on clock speed

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