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.

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 in the newer CPU.

External L2 Cache

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

IBM has provided some comparisons for the G3 with various cache sizes. The numbers are rounded off, but a 400 MHz G3 with a 256 KB 200 MHz L2 cache* and a 100 MHz system bus (the configuration of the 400 MHz Blue & White Power Mac G3) 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 400 MHz G3 with no L2 cache could 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. Its internal cache is 256 KB in size and runs at full CPU speed, giving it a distinct advantage over an external 256 KB L2 cache running at half of CPU speed. In fact, IBM has benchmarked the 400 MHz 750CX at 16 using SPECint95. That’s one-third faster than a cacheless 400 MHz G3, about 6.5% faster than the old G3 with a 256 KB external cache, and about 6% slower than the current G3 with a 512 KB cache.

Of course, the 400 MHz G3 came with a 1 MB L2 cache, so a 400 MHz 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 400 MHz G3 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 the 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 would have 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 a power draining external, cache.
• Including a G3 CPU 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, an iBook with a 400 MHz 750CX would be about 25% faster than the current 300 MHz model and roughly equal to the 366 MHz iBook SE in overall performance while using a less expensive processor that draws less power. Likewise, a PowerBook with a 550 MHz 750CX would offer comparable performance to the 500 MHz PowerBook Pismo while reducing power consumption.

In the end, Apple would benefit from moving to the 750CX for its 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 worth 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.

Starting with the 500 MHz and 600 MHz Early 2001 iMacs, iMacs also adopted the 750cx.

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

Further Reading

• Hands on the FireWire iBook, the first Mac to use the 750CX
• The Truth About the New G3
• 466 MHz FireWire iBook vs. 400 MHz Pismo PowerBook, Bare Feats, 2000.10.23. Real world tests with iMovie, Cinema 4D, and Bryce.

Keywords: #powerpc750cx #level2cache

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