Mac Musings

Should Apple Use the New G3?

Dan Knight - 16 June 2000 - Tip Jar

There's been some interesting discussion of IBM's new PowerPC 750CX and forthcoming 750CXe processors, especially related to IBM discusses new PowerPC chips on MacWeek. A lot of Mac users seem to think the 750CX would be a poor choice; I beg to differ.

PowerPC 750CX The biggest difference between the 750CX and the old 750 (commonly known as the G3 in Macintosh circles) is the integrated 256 KB level 2 (L2) cache.

External L2 Cache

The old 750 supports an external L2 cache in sizes of 256, 512, and 1024 KB. Apple has only produced one model, the "Road Apple" PowerBook G3 Series I at 233 MHz, which has used the G3 without a cache - and performance suffered significantly in comparison to the later PB G3 Series II 233 MHz model which did include a cache.

IBM has provided some comparisons for the G3 with various cache sizes. The numbers are rounded off, but a G3/400 with a 256 KB 200 MHz L2 cache* and a 100 MHz system bus (the configuration of the Yosemite G3/400) scores approximately 15 using the SPECint95 benchmark. Doubling the L2 cache to 512 KB increases that to roughly 17, and going to a 1 MB L2 cache brings the SPECint95 score to about 18.

With the old G3, the PowerBook G3 demonstrated that a 512 KB L2 cache improved performance by about 40%. Thus, a G3/400 with no L2 cache would be expected to score about 12 on the SPECint95 benchmark. Adding a 1 MB L2 cache improves that by about 50% - very impressive.

Internal L2 Cache

The new G3 (750CX) doesn't use an external L2 cache. The internal cache is 256 KB in size and runs at full CPU speed, giving it a distinct advantage over the external 256 KB L2 cache. In fact, IBM has benchmarked the 400 MHz 750CX at 16 using SPECint95. That's one-third faster than a cacheless G3, about 6.5% faster than the old G3 with a 256 KB cache, and about 6% slower than the current G3 with a 512 KB cache.

Of course, the G3/400 came with a 1 MB L2 cache, so the 750CX won't quite match it's performance. In fact, it will be about 12% slower - but a 450 MHz 750CX will match the performance of a G3/400 with a 1 MB external cache.

All of this closely matches IBM's theoretical analysis, which points to the 750CX being 6.5% faster than a G3 with a 256 KB cache. The same theoretical model points to a 512 KB internal L2 cache offering an insignificant (1.5%) improvement over a 256 KB cache.

The Benefits of 750CX

I believe Apple should adopt the new IBM processor for the iBook, iMac, and PowerBook. It may not match the performance of a G3 with a larger external cache, but it has the following benefits:

The 750CX uses less power than the 750, about 4W at 400 MHz vs. 7W.
An external L2 cache draws power, reducing battery life in a portable. The 750CX doesn't use an external cache.
Putting a G3 along with L2 cache chips and other support circuitry is an expensive process. With the 750CX, there's just one chip to plug in, reducing complexity.
It's also less expensive to buy the 750CX than a G3 plus cache chips plus the board they sit on.
Today's computers are plenty fast. Being 5-15% slower than some theoretical maximum is virtually unnoticeable.

Based on IBM's data, a iBook with a 750CX at 400 MHz would be about 25% faster than the current 300 PowerPC 750CX MHz model and roughly equal to the iBook SE in performance while using a less expensive processor that draws less power. Likewise, a PowerBook with a 750CX at 550 MHz would offer comparable power to the PowerBook 500 while reducing power consumption.

In the end, Apple would benefit from moving to the 750CX for their consumer line. It will reduce costs, making these machines even more affordable, increase battery life, and give Apple a boost in the megahertz wars.

That's something work discussing, especially in the engineering and marketing departments.

Update: On Sept. 13, 2000, Apple announced the second-generation iBook and iBook SE, both running the 750CX processor. Apple managed to trim US$100 from the iBook's price while boosting CPU speed and adding FireWire. The iBook SE received a DVD-ROM drive and faster processor with no change in price.

* Note that some less expensive Macs run the cache at only 40% of CPU speed. This provides lower performance than a cache running at 50% of CPU speed.

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