The Performance Edge: How Much Difference Does Having a Faster "Backside" Cache Make - A Performance Review

A couple of years ago when the first G3's came out we took a limited look at the effects of backside cache speed. At that time there were upgrade cards whose backside cache speed ran at full processor speed. These cards were, of course, very expensive. Now most G3/G4 upgrade cards run at half the processor speed. Some, like the XLR8 card, allow you to over-clock the backside cache speed, if possible, from the card's recommended speed. How far you are able to over-clock really depends on the quality of each individual chip.

We decided to take a second, in depth look at cache speed to get a fuller perspective on the benefits of backside cache, and exactly what processes it speeds up.

To do this we used a XLR8 MAChSpeed ZIF G4/500/250 upgrade card installed in a Beige G3/266. We clocked the processor to 400 MHz and then used the control panel to clock-up and clock-down the speed of the backside cache. For one series of tests we turned the cache off completely.

The test results are interesting. While having an operational backside cache as part of the processing system results in a tremendous boost in performance (up to 300% in one test), the speed of the cache seems less important, in most tests. Tests affected by cache speed the most were our AppleWorks scroll and search & replace tests, which showed a 14% and 30% improvement respectively over the default setup . Most other processor intensive tasks showed only between a 0 - 6 percent improvement when cache speed was doubled.

This would seem to indicate that most light tasks, such as scrolling, word processing and finder functions, will see an increased responsiveness, with a high-clocked backside cache. But for most heavy duty processing, a faster cache makes minimal difference.

It will be interesting to see if these numbers hold up when the cache is running at full processor speed.

G3/G4 Machine Cache Bus Scheme

Backside Cache -- Unleashing Processor Performance
By far the biggest boost to performance that the PowerPC G3 offers can be credited to its incorporation of an approach to level 2 cache memory known as backside cache. This approach effectively bypasses limitations on the speed at which transactions between the processor and the level 2 cache can occur. Earlier PowerPC processors used the system bus to access both the level 2 cache memory and the main memory, which could result in conflicts. For example, under the previous approach, at processor clock speeds above 200 megahertz, the CPU would often stall as it waited for data to arrive from the level 2 cache. To prevent such slowdowns, the PowerPC G3 processor features a new dedicated bus that handles only the CPU/cache transactions. This bus can operate at higher speeds than the system bus -- speeds that relate incrementally to the clock speed of the processor. This enables the more effective use of level 2 cache, because even the relatively large amounts of data they can store can be accessed by the processor rapidly and efficiently. In fact, as clock speeds increase, so does the performance value offered by the backside cache design.

- Apple Computer

MacBench 5.0 Results

The scores below are from MacBench 5.0. MacBench 5.0 is a subsystem-level benchmark that measures the performance of a Mac's processor, disk, and graphics subsystems to name a few. MacBench normalizes all scores relative to the base machine, a Power Macintosh G3/300. The base machine receives a score of 1000. For all MacBench tests, higher numbers indicate better performance.

PowerBook G3/466 upgrade card - scroll test


PowerBook G3/466 upgrade card - scroll test

"Real World" Tests

The tests below are from our newly revised suite of real world application tests. These tests feature a diverse selection of applications commonly used by the Mac community. The test suite was designed to render an accurate and well rounded picture of an upgrade's performance. Click here for detailed information on each test and our machine's configuration. All of the tests below (with the exception of the Quake III tests) were timed with a stopwatch. The times are then converted to percentages with the base configuration set to 100%. Lower numbers indicate a better score.

PowerBook G3/466 upgrade card - scroll test


PowerBook G3/466 upgrade card - scroll test


PowerBook G3/466 upgrade card - scroll test


PowerBook G3/466 upgrade card - scroll test


PowerBook G3/466 upgrade card - scroll test


PowerBook G3/466 upgrade card - scroll test

PhotoShop 6.0 "Real World" Test results

PowerBook G3/466 upgrade card - scroll test


PowerBook G3/466 upgrade card - scroll test

Quake Tests

Higher scores are better

PowerBook G3/466 upgrade card - scroll test


Actual Scores - In Seconds


AppleWorks 6.0.4

AppleWorks 6.0.4
Find & Replace

Analog 4.1.1 Code Analysis

Cinema 4D XL

Quake III
Default Fastest Setting

XLR8 MAChSpeed G4z/400/200/1MB


14.6 45.34 541.94


- with backside cache set to 267 MHz


11.93 45.6 535.47


- with backside cache set to 133 MHz


15.77 46.52 554.36


- with backside cache set to 100 MHz


18.92 47.61 569.81


- with backside cache disabled


45.96 58.74 719.34



SoundJam 2.5.2
MP3 Encode

QuickTime 4.1.2
(Sorenson 2X CD)

StuffIt 6

Photoshop 6.0
AltiVec Filters

Photoshop 6.0
Non-AltiVec Filters

XLR8 MAChSpeed G4z/400/200/1MB 74.78





- with backside cache set to 267 MHz 74.63





- with backside cache set to 133 MHz 76.97





- with backside cache set to 100 MHz 80.31





- with backside cache disabled 104.02





The Performance Edge Index

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