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Apple aims to boost bandwidth with 1.5GHz G4s

G5 on track

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Recent word from sources close to Motorola confirm that a significant speed bump will be timed for MacWorld Expo in July, raising current bus speeds, and seeing new 7470-based G4s raise frequencies to as high as 1.5Ghz.

The 7470 supports PC2100 DDR-SDRAM memory at 266MHz, as we told you back in February, (see New G4 roadmaps promise Apple harvest
), and PC2100 made its debut in Apple's product line with the launch of the Xserve rack last week. More importantly the new G4 supports front side bus speeds of 133MHz and 166MHz, which double to 266MHz and 333MHz.

The most dramatic bandwidth improvement would be a jump to 266Mhz, and although we don't know if that choice has been made, but a more modest increase to 166Mhz (2x) would be more in keeping with recent announcements.

And as we told you back in February, the 7470 has a little brother, the 7460, which differs by omitting support for a L3 cache. The 7470 supports up to 4MB of L3.

What of the G5? That's still on course for volume production early next year, in the form of what's codenamed the 7500. Public Motorola roadmaps describe this as a processor that conforms to the e500 Book E G5 spec, and it's been rated stable at 2.4GHz internally at Motorola. But to get a handle on this it's worth paying attention to the internals, as this indicates an important rethink in the Megahertz wars.

Like the grown-up big iron vendors, Apple is going to focus on bandwidth, rather than frequencies, and you'll notice that this marketing move started discreetly with the most recent bump in January.

With RapidIO ports and interleaved PC2700 memory, a humble G5 clocked to 1.5Ghz should be able to achieve memory throughput of 12GB/s.

Now this sensible approach - fill up the execution units as quickly as you can, rather than ratchet up the chip frequency, only to have the chip do nothing, or throw instructions away for much of the time - isn't new. Compaq's Alpha, IBM's POWER4 and AMD's Hammer have all adopted this approach. UCB's Nick Weaver explained it very eloquently in this piece, before he changed his mind. (See Miracle cures Berkeley man of Itanic wickedness for the explanation).

If Apple's marketing can rid the world of frequency comparisons, and focus on bandwidth, it will have done us all a favour. Check back on July 16... ®

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