As a cursory examination of the benchmarks will tell you, unless you're running an application optimized to take advantage of the AltiVec processor extension, there is very little difference in speed between a G3- and G4-based Macintosh.
That's because without AltiVec, or as Apple calls it, the Velocity Engine, a G4 essentially is a G3. So, under Mac OS 9.X, unless you're running Adobe Photoshop, SoundJam MP, iMovie, or some other application that's been modified to specifically take advantage of AltiVec, you won't notice any difference running them on a G3 or G4 of the same megahertz, in the same speed logic board. This became especially apparent with the recent release of the Titanium PowerBook, a G4-based portable that had the same logic board specifications as the G3-based PowerBook it replaced (see "PowerBook G4 Diary: The Lab Test" and "PowerBook G4 Diary: 400MHz Results" ).
But with Mac OS X, an OS Apple has programmed to be AltiVec-savvy from the ground up, all of that will change.
Or will it?
AltiVec is a coprocessor that gives your Mac the ability to process data in 128-bit chunks instead of 32-bit chunks, which is how the rest of the processor handles data. This is helpful for processing large amounts of data such as graphics, music encoding, and 3-D animation. In OS 9.1 we've seen Photoshop and SoundJam take advantage of AltiVec with excellent improvements in speed. However, in OS 9.1, the application needs to be specifically modified to take advantage of AltiVec, while the operating system and unmodified applications see no measurable performance increase.
Enter Mac OS X. OS X will take advantage of AltiVec at the lowest possible level in the operating system. However, this does not necessarily translate into increasing the performance of the entire user experience. Since OS X handles data differently than OS 9, you'll see a performance increase of 40 to 50 percent between OS 9 and OS X at the operating system level, regardless of the processor on which it runs. But, applications still need to be optimized to take advantage of OS X's efficiencies; being Carbonized , as Apple describes the process of taking an OS 9 application and compiling it to run natively on OS X, is not enough.
According to several developers Macworld talked to who are currently working on OS X applications, anytime the OS can take advantage of the AltiVec engine, it does. This ensures that the parts of the OS that can utilize AltiVec, such as working in the new user interface, experience a significant increase in performance. And applications that can take advantage of AltiVec will be able to do so with far less work, developers said, thereby increasing their own speed significantly. However, there are still two caveats: (1) developers will still need to make changes to their code to get this speed boost; there is no "free ride" for carbon-based applications trying to leverage AltiVec. (2) What the application is doing must be able to get some benefit from 128-bit processing. Although applications such as word processors and databases are unlikely to see much benefit from being made AltiVec-savvy, a huge spectrum of audio, video, and graphics applications stand to gain significantly on performance with an AltiVec kick.
And, one category of software that currently doesn't benefit from AltiVec on Mac OS 9 could get a big performance boost on Mac OS X -- games. Games have many processes that could be sped up with AltiVec acceleration.
And the developers who chose to use Mac OS X's Cocoa framework instead of the Carbon libraries to bring brand new applications to the new OS will essentially get AltiVec efficiencies without having to do additional work. Cocoa is a Mac OS X development environment that allows developers to quickly assemble applications using reusable code components. And, to the degree that those code components are able to leverage AltiVec, the applications built from those components will benefit from AltiVec, as well.
Some game developers have said they plan to use Cocoa to go to Mac OS X, because of the time saved in porting their code to the new OS.
We have just a few more hours to wait to find out if, indeed, the iMac turns G4. For iMac users who think a G4 is just for Photoshop jockeys, an OS X upgrade could be the cheapest accelerator they could buy for their computer.
Once enough developers ship AltiVec-savvy, Mac-OS X-savvy software, that is.
BRETT LARSON and JASON COX contributed to this report.