posted by Nicholas Blachford on Wed 13th Oct 2004 06:30 UTC
IconApple and PC users alike criticised Motorola when the G4 CPUs failed to keep pace with the aggressive clock speed ramping of processors from AMD and Intel. The newly announced 8641D with dual cores, dual DDR-II memory controllers and more is set to change all that.

The latest 7447A G4 runs at up to 1.5GHz and can be found in Apple's 15" and 17" PowerBooks. Freescale (previously Motorola Semiconductor) have concentrated on the embedded market where low power consumption rules rather than absolute maximum performance, thus the clock speed has not soared like the Pentium 4 but then neither has pipeline length or the power consumption. The highest performing Pentium 4s already use in excess of 100 Watts, the next generation G4 7448 is expected to require less than 10 Watts at 1.4 GHz. Of course when performance is needed there's always the Altivec unit which was never been lacking, to encourage it's use an increasing number of routines are being made available by Freescale for use by their customers.

The G4 however has been at a disadvantage as regards it's memory bandwidth. While it's bus has increased to 166MHz (200MHz on the 7448), it does not use the DDR or QDR signalling technology used in PCs thus limiting it's memory bandwidth. This impacts the performance of some applications but in general usage this doesn't seem to be much of a problem as applications are more often latency bound and PCs generally have no advantage over the G4 in this respect (apart from AMD64 systems).

All the CPU manufacturers have hit a wall though, clock speeds have rising rapidly but power consumption has been rising faster, the most visible sign of this was the cancellation of Intel's Tejas processor and their new found focus on dual core processors.

Intel however have also introduced the low power Pentium-M processors which give the majority (if not more) of the performance of a Pentium 4 at a fraction of the power consumption and a considerably lower clock rate. The Pentium-M CPU has been praised for this achievement, a rather odd situation considering the criticism the G4 got; with the Pentium-M Intel is following the exact same strategy the G4 designers have been implementing all along - with much the same results.

New Chips
It's been known for some time that Freescale were working on new PowerPC processors. Now they've been announced and they are substantially different from the previous G4 designs, they still follow the same philosophy of low power consumption but this time doing so at the same time as increasing overall system performance.

Freescale could have increased performance by upping the clock speed but in doing so it would would have sent the power consumption rocketing to PC CPU levels, something unacceptable in their primary market. Instead, they took another path and added a second core, the move to a 90nm SOI (Silicon on Insulator) process cuts power consumption considerably so the second core doubles performance without doubling power consumption. A larger cache (1MB cache per core) and some architectural changes will boost performance further.

They not only added a second CPU core and increased the CPU performance but integrated dual 667MHz DDR-II memory controllers sending aggregate memory bandwidth up by a factor of 8 to 10 GBytes / Second in a single leap, of course placing the memory controllers on die also reduces memory access latency.

As with the Pentium-M the G4 delivers much of the power of a Pentium 4 even given it's low clock frequency, consequently I fully expect a pair of these cores will outgun any current single core PC CPU though obviously not on single threaded operations. That alone would have brought Freescale back into the reckoning but they didn't stop there, they have added a slew of on board peripherals not only reducing costs but increasing overall system performance whilst simultaneously reducing power consumption.

AMD's Opteron moved a Hypertransport connection and a memory controller on board the CPU die, Transmeta's Efficeon went further adding an AGP X4 controller into the mix. The 8641 family goes much further than either of these by also adding dual PCI Express interfaces, a RapidIO bus (similar idea to HyperTransport) and 4 Gigabit Ethernet MACs with hardware TCP/IP acceleration and other interfaces all onto the same die as the CPU cores.

Despite all the interfaces, controllers and dual CPU cores power consumption is currently estimated at 15 - 25 Watts in typical usage.

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