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Special FX: The Future of Low-end Macs

Chris Lozaga - 2001.12.03

Over decade ago, the Mac IIfx out muscled all but the most expensive workstations and supercomputers. The letters "FX" will forever symbolize the pinnacle of Apple's engineering to that generation of Mac enthusiasts. The FX moniker is about to be reborn from an unlikely source.

IBM announced the PowerPC 750fx processor at the Microprocessor Forum and recently published extensive specifications for the newest member of the G3 family. As a G3 processor, the 750fx is destined to be used in value-oriented Macs; however, just because it is an inexpensive processor does not mean that it isn't worthy of the fx moniker. IBM has thoroughly tweaked the G3 design and created quite a cool little microprocessor (literally and figuratively).

What's the Cache?

Mac users have known since the days of installing cache cards in the IIci that the Mac OS truly enjoys a fast, large cache. This is equally true of OS X - perhaps more so. IBM has made some impressive changes to the cache of the 750fx that will speed performance considerably. First, they have doubled the secondary cache size from 256 KB in the previous 750 processors to 512 KB in the 750fx. That means more data is available precisely when you need it.

The cache is on-die (not on a card or external to the processor) as in the current G3, but it is four times as wide. Moving from a 64-bit data path to a 256-bit data path allows a 1 GHz 750fx to move 32 GB/second versus 8 GB/second with the older designs. Also, the 256-bit data path can fill an entire level one (L1) cache line in transfer, whereas the 64-bit wide cache of the previous G3 took four or more transfers to fill a line. These modifications are akin to improving the plumbing, allowing data to be moved more efficiently from memory to the processor core.

The 750fx should outperform the G4+ on most applications not optimized for the Velocity Engine.
The G4+ has a 256-bit secondary (L2) cache and a 64-bit third level (L3) cache. However, the G4+ only has 256 KB of L2 cache. In real world scenarios, these two cache configurations will perform very similarly, so the 750fx gets an improved cache but does not have to suffer the increased pipeline length of the G4+. The 750fx should outperform the G4+ on most applications not optimized for the Velocity Engine.

Get on the Bus

The G3 had been limited to a 66 MHz or 100 MHz system bus. IBM modified the bus of the 750fx to allow for operation at 200 MHz. The doubling of the bus frequency not only increases bandwidth from 800 MB/sec. to 1600 MB/sec., but also decreases latency, which has become a critical performance factor in many applications. Any application that moves the large quantities of data around will benefit from this enhancement - from Quake III to Final Cut Pro.

Smaller, Faster, Better

The 750fx is produced on a 0.13 micron Copper process with Low-K dielectrics and Silicon-on-Insulator technology (SOI). This is perhaps the most advanced process for a commercial grade chip, bar none. The 750fx is under 40mm2, making it smaller than today's G3s even though it has twice the catch. IBM states that it will dissipate 3.6 W at 800 MHz, which is half what the 600 MHz G3 dissipates today. In comparison, the G4+ is a power monster coming in at well over 20 W at 800 MHz and measuring over 100mm2. The cooler, smaller package and finer process allows the 750fx to start at 800 MHz and scale past 1 GHz.

The Big Secret

If Apple attached the 750fx microprocessor to a capable memory subsystem and gave it topnotch components, it would perform at least on par with a G4+ (and possibly better) on any application that wasn't thoroughly optimized for the Velocity Engine. Even on those applications only certain filters and effects would be faster. It is unlikely, however, that Apple would turn the 750fx into a flagship processor for three reasons. First, the 750fx is optimized for single processor machines, barring it from the high end. Second, Apple has spent a lot of cash promoting the Velocity Engine. Third, both the 750fx and the G4+ are hopefully to be eclipsed by the G5 sometime in 2002.

The 750fx will most likely appear in the iMac and iBook, running on a slower than 200 MHz system bus and coupled with a 5400 RPM hard drive, just as the G3 is now. However, this is a true boon to the low-end Mac user, who will get an extremely fast processor for a bargain price. Dissipating such little power, Apple could be truly creative. The 750fx should require only passive cooling, allowing engineers to craft a truly quiet and sleek Mac.

This time around the fx moniker will not be restricted to the costliest of systems or tower over the rest of the Mac product line, but it will continue to stand for quality and speed for some time to come. LEM

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