Mac Musings

Will the Dual-Core G4 Appear in Future Macs?

Dan Knight - 2005.03.08 - Tip Jar

The G5 may point to Apple's future, but at present it runs hot, draws a lot of power, is available in limited supply, and costs more than the G4. That's why the Mac mini, eMac, and iBook continue to use G4 processors.

With the PowerBook line, it's less a matter of cost and more a matter of practicality. People expect to pay more for PowerBooks, are lusting after G5 processors, but are probably unwilling to accept the compromises necessary to make a PowerBook G5 today - it could be big and fast or small and slow.

Small and fast is not yet an option with the G5 processor, so don't expect to see it in a laptop until power consumption and heat dissipation are greatly improved.

Today's G4

The Bus

The G4 processor has come a long ways since Apple introduce the Power Mac G4 - topping out at 450 MHz - in August 2000. Macs have moved from a 100 MHz system bus to 133 MHz, and most current G4 Macs have a 167 MHz system bus.

Bus speed matters because that's how fast system memory is. For instance, a 500 MHz CPU with a 500 MHz system bus would access system memory five times faster than one with a 100 MHz system bus (all else being equal). The ratio between the CPU speed and the system bus is called the multiplier, and on early G4s it ranged from 3.5 (350 MHz on a 100 MHz bus) to 4.5 (450 MHz).

When Apple moved to a 133 MHz system bus, Power Mac speeds initially ranged from 466 MHz to 733 MHz - bus ratios of 3.5 to 5.5. The fastest Power Mac on a 133 MHz bus ran at 1 GHz and had a multiplier of 7.5.

As Apple wrapped up the Power Mac G4 line, models faster than 1 GHz used a 167 MHz system bus and had 1.25 GHz and 1.42 GHz CPUs. (Multipliers of 7.5 and 8.5 respectively.) That meant the CPU had to wait a greater number of clock cycles to read system memory.

The system bus is the G4's biggest bottleneck. Apple can sell a 1.67 GHz PowerBook G4, but it's still stuck with a 167 MHz system bus with a multiplier of 10. That's huge - but it's necessary because the current G4 doesn't support higher bus speeds and can't take advantage of the "double data" aspect of DDR (double data rate) memory.

The Cache

To help offset the difference between CPU speed and bus speed, modern processors have access to higher speed memory called a cache. The 1 MB "level 2" cache on early Power Mac G4s ran at half CPU speed - about twice the speed of system RAM.

When Apple introduced the Power Mac G4/667 and G4/733, it adopted a new version of the G4 processor with a 256 KB cache built into the CPU - and thus running at full CPU speed. The faster speed of the onboard cache helped offset its smaller size.

More is better when it comes to caches, and over time the G4 moved to 512 KB and 1 MB. Here the larger caches also helped offset the growing ratio between CPU speed and system memory.

Heat and Power

As CPUs get smaller, they can get faster and often draw less power and produce less heat. (A bigger cache means a bigger CPU.) That's crucial in laptops and small computers like the Mac mini. The 7447A version of the G4 typically draws about 10 watts running at 1.2 GHz (12" iBook), 16 at 1.25 GHz (eMac, Mac mini), 17 at 1.33 GHz (14" iBook, 12" PowerBook), 18 at 1.42 GHz (Mac mini), and 21 at 1.5 GHz (15" PowerBook G4). I'd guess the 15" and 17" PowerBooks running at 1.67 GHz average somewhere in the 27-28 watt range.

Compare that with a G5 (PowerPC 970fx), which averages 12.3W running at 1.4 GHz and 24.5W at 2.0 GHz. I wouldn't expect Apple to sell a PowerBook G5 slower than 1.6 GHz, which is probably 18W or so.

Yes, Apple could dissipate that heat - but there's one other factor. The memory used with the G5 runs at one-half or one-third of CPU speed. Let's say Apple chooses the higher multiplier. That still means 533 MHz memory, which contributes to higher power consumption and greater heat buildup than 167 MHz memory.

The Next G4s

Semiconductor (formerly Motorola's chip unit) is developing a next generation G4 that will work with a faster system bus, eliminating the up-to-10x multiplier found in today's PowerBooks.

The 8641D is a dual-core design, which means it's equivalent to having to G4 CPUs, and draws 15+ watts at 1.5 GHz. It has 1 MB of onboard cache memory for each core and supports a 667 MHz memory bus - four times as fast as that on current PowerBooks.

Freescale is also developing the 8641 CPU, a single-core version of the 8641D.

I'd guess Apple is seriously looking at this as an alternative to the G5, perhaps as a stopgap measure for the PowerBook line and, later on, a way of differentiating consumer (iBook, mini, eMac) lines from "pro" models (iMac, Power Mac, PowerBook).

The Next G4 Macs?

Assuming Apple goes with the 8641 CPUs, expect them to design around a 667 MHz data bus in desktops (possibly slower in 'Books) and initially run CPUs from 1.4 GHz to 1.8 GHz, using the single-core processor on low-end models and the dual-core on PowerBooks.

Going with this scenario, we might see a 12" iBook and Mac mini at 1.4 GHz; 14" iBook and faster Mac mini at 1.5 GHz. These would be single-core models. Dual core models might include a 12" PowerBook at 1.6 GHz and a 17" PowerBook at 1.8 GHz. Following their current pattern, the 15" PowerBook would then ship in 1.6 GHz and 1.8 GHz models.

It's hard to project the impact of a 4x as fast system bus on overall performance, but between a faster bus and a 1 MB cache, the single-core 8641 should easily outperform the current 7447A at the same CPU speed - and the dual-core 8641D would provide up to double the performance.

Dual cores have some advantages over single cores. The biggest is the additional computing power. Two brains are better than one, and depending on the program a dual-core CPU can be up to twice as efficient as a single-core one using the same architecture.

The other advantage is that this increased efficiency means you don't need as much clock speed to get the same amount of work done, which also means less energy used and less heat buildup. The dual-core G4 will probably have similar characteristics to the single-core G5 at the same CPU speed, but with two "brains" it will easily outperform it.

(Everything said about dual-cores will apply equally to the dual-core G5 IBM is developing.)

And the single-core CPU will draw less power, create less heat, and give laptops better battery life.

Looking at all of these factors, the PowerBook line looks like the logical place to introduce the 8641D, perhaps as a step toward an eventual PowerBook G5 when IBM gets high enough yield and Apple designers and engineers find a way to meet what they call "the mother of all thermal challenges" - putting a G5 in a laptop. LEM

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Dan Knight has been using Macs since 1986, sold Macs for several years, supported them for many more years, and has been publishing Low End Mac since April 1997. If you find Dan's articles helpful, please consider making a donation to his tip jar.

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