Buses: Front-side and backside

In just the same way, savvy computer users know that merely plugging a top-end microprocessor into an untuned computer system doesn't guarantee a satisfying improvement in overall performance. And venturing farther under the hood, the speed and efficiency of the CPU itself depends to a considerable degree on the front-side bus that engineers have designed into the processing chip set, as the CPU and other chips associated with it are known.

An essential aspect of the CPU's actual performance is the speed of the front-side bus, the main pipeline a CPU uses to communicate with the rest of the system. Today's front-side buses, like the 400-MHz conduit in the Pentium 4, shuttle data back and forth at a rate more than three times faster than the Pentium III's 133-MHz front-side bus.

By contrast, the backside bus, which confines itself to handling cache data, actually runs at the clock speed of the CPU. In ancient times (circa the mid-1990s), the backside bus was an important way to keep data on the move. Intel Corp.'s Pentium II and Pentium Pro both used a so-called off-chip cache, which held frequently used data nearer (in both distance and the time needed to access it) to the main processing unit than data that was held in conventional memory. A wire bonding linked the CPU to this Level 2 (L2) cache resource and shuttled data between the two destinations at the clock rate of the CPU. Intel's rivals, like Advanced Micro Devices Inc. in Sunnyvale, Calif., soon began using the same tactic.

On- and Off-Chip

There were trade-offs in an off-chip cache design, however. The cost of producing a two-chip set was higher than single-chip designs, and the two separate elements took up precious real estate on the motherboard. In addition, the first Pentium systems to use the backside bus arrangement came with custom -- and very expensive -- static RAM for the cache.

More recently, microprocessor engineers have taken the next logical step in CPU-to-cache communications: they have integrated the L2 cache into the CPU's own silicon substrate. This shrinks the real estate requirements of the processing unit, cuts packaging costs and allows designers to move to lower-priced pipeline burst static RAM. Rather than needing an external wire to connect CPU and memory, chip designers could now incorporate the backside bus in silicon.

"Almost all mainstream processors have now put the second-levvel cache on the chip," says Kevin Krewell, an analyst at Micro Design Resources, a publisher and consulting firm in Sunnyvale, Calif., that specializes in chip design trends. "The backside bus is now on the chip die; it's not exactly a bus any longer."

But the days of the discrete backside bus aren't entirely over. The 400- and 500-MHz PowerPC G4 processors that power Apple Computer Inc.'s Power Mac G4, Cube and Titanium notebook, for example, continue to rely on a backside bus design. The G4 processing engine uses a 1MB backside L2 cache on the processor and a 64-bit backside bus that partners with a 100-MHz front-side bus to achieve a rated data throughput maximum of 800M bit/sec.

Intel and Compaq Computer Corp. haven't forsaken the backside bus, either. Advanced chips that provide a Level 3 cache include Intel's 64-bit Itanium processor and Compaq's Alpha EV8, both of which will continue to use this bus design to keep data flowing.

In addition, separate caches open the way for more efficient multiprocessing in PCs or servers that have more than one processor. If each processor didn't have its own cache reserve, it would have to share a central memory pool with its CPU mates, and that would reduce overall system performance as the processors contend to divvy up a precious resource.

"Everybody recognized this is a better solution than using a front-side bus," Krewell says. "To share bandwidth with system memory is nonoptimal."

Now if only that Yugo could get its backside in gear.