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Review: XLR8's MACh Speed G4zTM
G4 CPU Upgrade for AIO/Beige/B&W G3s
By Mike
Published: 11/22/99
Software Cache Control Features
Intro | Benchmarks | Appl. Tests | Software Controls | Installation | Specs/Design | Summary
Cache Control Software
XLR8 continues to update their cache control software and a new version is soon to be released as this is going to press. Their control features automatic cache speed testing (which can be a bit optimistic as I'll explain later), 'Virtual Firmware' to address compatibility issues with older 60x CPU based Mac ROMs via an option to disable 'Speculative Processing' of the G3 & G4 CPU which can cause problems with some applications software and hardware when G3/G4 CPU upgrades are used in older Macs. This is not an issue with Apple G3 (AIO, Beige, B&W) systems (or even Kansas motherboard Macs like the 8600/250 and up, 9600/300 and up models).

"Speculative Processing" is often called code 'Branch Prediction' where the CPU makes educated guesses on future code execution. See XLR8's white paper for more details since this is not a concern for the Apple G3 systems that are the target of this review.

What's On The CD:

The XLR8 CD contains installers for all of the CPU upgrades (I've only shown the G4Z related ones for space reasons) and also features additional software such as the Power Pack (Fractal program and excellent Power Control utility that can perform system tests on the CPU, RAM, and SCSI devices. PowerControl can also report RAM configuration (interleaving status and installed dimm sizes) and much more. It's a great bonus and its RAM test has detected suspect DIMMs where other free utilities didn't in my experience.

For those of you that have never used it, I highly recommend you read my Power Control page for a summary of what this utility can do.

Installing the Software:
Although XLR8 recommends installing their software before installing the upgrade in older Macs, this is not a requirement for Apple G3 systems that don't have the issues of Speculative Processing (an issue in older ROM version Macs).

The XLR8 software installer automatically places the cache control and extension in the system folder. During the first boot with the G3 card (or whenever you change speed settings on the card), the XLR8 software tests the backside cache automatically and reports what it suggests is the maximum reliable cache speed (but offering you the option to manually set the speed to another setting). Regardless of suggested speeds from this test, I highly recommend keeping the cache at 1/2 the CPU speed or near the cache rating. Most cache used on today's upgrades cannot run reliably at speeds near 300MHz and cache speed is not a major factor in real-world application performance (CPU speed is).

I kept the review's card backside cache at a 2:1 ratio (half the CPU speed) throughout my tests. This particular card ran perfectly at speeds of up to 262.9Mhz with this particular 500/250 rated sample (results with another cards may vary). Again, backside cache speed is not a primary factor in real world applications performance so don't push the bleeding edge on cache speed as it can affect reliability.

MacBench's CPU test is highly influenced by cache speeds, but real world applications are far less so. Remember - a fast unreliable computer is far less useful (and hazardous to your data) than a slow reliable one. Don't sacrifice reliability for a few % faster CPU or Cache speeds. It's just plain foolish.

XLR8 Cache Control Settings Page
Cache Control
MACh Speed G4/400Z in B&W w/Default settings

Maximum Reliable Speed during limited testing:
450/225 (NOT RECOMMENDED)

Overclocking may void your warranty and lead to data loss
and/or affect hardware reliability
.

The main page of the control panel lists the amount of installed RAM and system bus speed, CPU type, speed and CPU junction (internal) temperature.

About Reported Temperatures:
Although the 36C temperatures indicated with this upgrade are closer to reality than some I've seen, as I've mentioned many times in the past in the front page news, and in the CPU ratings database; I don't believe the reported temperatures are 100% accurate. XLR8 commented they needed to do some sort of software adjustment for temperature readings on the early steppings of G4 CPUs.

Advanced Settings Page:
The image below shows the 'Advanced' settings tab of the control.

Advanced Settings Tab
Advanced settings page

The Advanced settings page will allow you to set the backside cache speed to manual (you select a speed) or automatic (the default based on the card type and bootup testing). Again I suggest you do not run the cache at speeds of more than 1/2 the CPU speed, despite what the boot testing may suggest as reliable. Cache speed doesn't really add a lot of real-world applications gain despite what MacBench 5.0 may report from faster cache speeds. Even pure memory bandwidth tests show little to no gain from 2:1 vs 1.5:1 cache speeds.

The Power Savings option is enabled by default and allows the CPU to go into low-power mode (perhaps even the L2 cache as I remember) during extended periods of idle time.


Copy-Back (default) vs Write-Through Cache Modes:
I'm not sure about the Apple G4 systems, but the previous G3 systems used 'Write-Through' cache modes vs. the faster 'Copy-Back' mode supported by some cache types. 'Copy-back' cache mode is faster as writes are written to the backside cache (and later copied back to RAM if data has been changed). Write-through mode means that each CPU write is written directly to much slower main system RAM. In my tests last year with a utility that would toggle these modes on a B&W G3s, I saw little real world gain from Copy-Back mode. Perhaps with the 100MHz memory bus speeds it's less of a performance issue than with older Macs with much slower memory bus speeds.


About Speculative Processing:
This is not an issue for Apple Beige G3 and later systems which have 'G3 clean' ROMs. The XLR8 software senses the machine its run on and only shows the option to disable Speculative Processing when run on older Macs. (In fact, due to the G4's more serious potential problems with Speculative Processing (often called Speculative Accesses) XLR8 is now revising their software to not allow enabling it on older Macs where it would be a problem. (V1.4.3 final release has this feature.) There is a small (4% or so) performance hit in many applications with Spec. Processing disabled, but Photoshop 5 tests actually showed higher filter performance with it disabled in my tests in the past. All tests in this review had Spec. Processing enabled as there was no option (or need) to disable it.

The 'Performance Tips' tab contains notes on faster cache speed settings and tuning. The 'Support' tab lists XLR8's contact information including toll-free tech support line and web site address (a nice feature).

Easter Egg:
Holding down the Option key and selecting the 'About' menu option (under the Apple menu) while the XLR8 control is active will show the G4 CPU revision.





Summary: Software controls were easy to install, use and worked well other than the puzzling negative effect seen in the memory bandwidth benchmarks shown on the Benchmarks page.


The next page describes installation of this upgrade in a B&W G3 including patching the firmware of that model. Or you may use the links below to jump to a specific page.

Index of XLR8 MACh Speed G4 400MHz Review

Intro | Benchmarks | Appl. Tests | Software Controls | Installation | Specs/Design | Summary

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