Dual-processor machines have been a part of the Macintosh landscape since Apple put a pair of 500MHz G4 chips into a Power Mac back in 2000. But that landscape shifted this week when Apple introduced a refreshed Power Mac G5 line built around dual-core chips.

Apple's Power Macs now feature dual-core 2GHz and 2.3GHz PowerPC chips from IBM. There's also a dual-core, dual-processor 2.5GHz model, which Apple is touting as the Power Mac G5 Quad. The machines are powered by dual-core processors that IBM introduced in July, shortly after Apple announced that it would use Intel CPUs from 2006. And it's very likely that some future Intel-based Mac systems will also take advantage of dual-core technology.

So what's the difference between a dual-core system and one built around dual processors? And more important to Mac users used to hearing that two CPUs are better than one, how will dual-core improve the overall performance of their machines?

One chip, two processing units
First, the setup: instead of having two processors in two separate sockets on a computer's motherboard, a dual-core system has one physical processor in one single socket -- but with two independent processing units inside it. This can save space and money.

A dual-core chip can also allows the two CPUs to share memory cache and bus connections to the system, a cost and design advantage. Otherwise, the system's hardware looks pretty much the same, whether it's a dual-core or dual-processor setup.

Dual-core may be a new addition to the Mac platform. But on the PC side, Intel and AMD have been supplying PC makers with dual-core chips in recent years. It's been a way for chip makers to deliver high performance without high heat.

For that reason, a dual-core chip makes a lot of sense for Apple. Today's dual-processor setups pose several big problems, starting with the fact that the two chips throw off a lot of heat, requiring huge heat sinks and special cooling systems. Thus, a dual-processor setup limits Apple's design options while boosting costs.

A dual-core chip, on the other hand, potentially frees up Apple to create brawny machines that will be smaller than today's Power Mac G5 models, said Roger Kay, president of market research firm Endpoint Technologies Associates. "Most users would rather have the same power in a smaller system," he said. "Unless you need extra space for cooling, you can get rid of it."

Performance boosts
It will take some time for those size advantages to appear -- the Power Macs introduced by Apple Wednesday retain the dimensions of the previous model. However, Apple is already taking advantage of the fact that a dual-core chip fits two CPUs in the space of a single chip by announcing the Power Mac G5 Quad, which offers a total of four processing units.

By combining two processor cores on a single silicon chip, Apple said a dual-core processor can deliver double the computational power of a single-core CPU. In the case of the Power Mac G5 Quad, applications are able to take advantage of four 1MB L2 caches, four 128-bit Velocity Engines and eight double-precision floating-point units, boosting performance over a single-core, dual-processor system.

Still, today's dual-core chips aren't perfect yet. So chip makers are working on new generations of higher-performance, lower-power dual-cores -- including Intel.

One problem with many of the dual-core chips now available: Intel and AMD chips can't run as fast as a single-core processor (made using the same manufacturing process and the same design architecture as the dual-core chip). That's because they would run too hot and require too much power, said Kevin Krewell, editor-in-chief of the Microprocessor Report. Today's dual-cores aren't running at their top clock-speed potential.

So some applications, including many games, still run faster on a PC using a single-core chip. Macintosh users have to wait for testing of the new Power Mac G5s to see how IBM's dual-core processors fare in this regard -- but it's interesting to note that the new dual-core G5 processors top out at 2.5GHz, compared with 2.7GHz for the previous top-of-the-line single-core G5 chip.

With dual-core-powered PCs, users see the greatest performance gains when using applications that have been multi-threaded. That's a promising development for Mac users because many of their programs have already been rewritten to recognise and take advantage of multiple CPUs, thanks to the abundance of dual-processor Power Macs. And those Mac applications will recognise and tap into the benefits of the two CPUs whether you're using a dual-processor or a dual-core machine.

Because of Mac OS X's symmetric multi-processing, the operating system itself can also direct individual tasks to different cores. This means one process or application can be directed to one core and a different task to another core. Other programs that haven't been multi-threaded will run on a dual-core system just as they would a dual-processor machine; they just won't take full-advantage of the benefits.

Many mainstream PC applications haven't been released in multi-threaded versions yet -- but the ones that have been tend to be in the creative arts and science and engineering -- two markets where the Mac is extremely popular.

"Heavy-duty applications, such as those that use video, will benefit from a dual-core chip because the system can throw more horsepower at the application or throw one core at the application and devote the other core to maintaining system tasks," said Shane Rau, programme manager for PC semiconductors at market research firm IDC.

Looking ahead
It's fair to say dual-core represents the way of the future for the major chip makers, which are adopting the design strategy to keep making significant performance gains. "Just increasing clock frequency, which has driven much of the performance increases over the last decade, has become unattractive due to the extra power requirements," Krewell said. "Slower-clocked dual-core processors can still offer performance increases with multi-threaded applications."

Dual-core technology really gets interesting -- and really takes a big step ahead of dual-processor systems -- with Intel's next generation of processors, expected in the second half of 2006. They're designed from the ground up to consume less power and run much cooler. (These low-power dual-core chips are code-named "Merom" for notebooks and "Conroe" for desktops.) While the exact timing and details of Apple's transition to Intel chips remain under wraps, it's safe to assume that the changes to the Power Mac system signal a long-term interest in dual-core technology on the part of Apple.

And that's fortunate because Intel intends to make its product line (except for Celeron chips, designed for the lowest-cost PCs) completely dual-core by the end of 2006. AMD will likely follow a similar strategy, Krewell said.