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Will a New Bus Leave Your System in the Dust?

-- by Jonathan Blackwood

Let's face it, the PC business moves so quickly these days that there's little more than the blink of an eye between state of the art and obsolescence. Unless you bought your PC in the last few months, it's unlikely that's it's equipped for USB or FireWire. But there may still be hope for your older system.

Both of these bus technologies are fast, but their bandwidth still falls within the capacity of a PCI bus. If your PC doesn't have a PCI bus, then you're probably past due for a new system anyway. PCI-based expansion cards for USB and FireWire should show up in computer stores later this year.

If what you really want is the realistic 3-D video that Accelerated Graphics Port (AGP) technology promises, your options become much more limited. You can either buy a new system or replace the motherboard in your existing system. This is because AGP is basically a direct, high-speed bus, or connector, from the graphics controller chip to system RAM. An AGP-equipped system must be designed from the ground up, with the high-bandwidth connector and a supporting chipset on the system board itself.

Unlike USB and FireWire, neither of which produces bandwidth exceeding the capabilities of the PCI bus, AGP's throughput can range as high as 528MB per second (burst rate), several times the 100MBps throughput possible on the PCI bus. So even if it were somehow possible to construct some sort of "virtual" direct connection that tunneled through the PCI bus, it would still fall far short of the bandwidth required by the AGP specification. Add to that the difference in the "bus speed" of PCI vs. AGP-33MHz vs. 66MHz; add further the fact that chipset support is necessary to arbitrate the memory contention issues among PCI transfers, DMA transfers and AGP graphics, and you begin to see why it's quite impossible to accomplish all this sleight of hand with a simple expansion card. (For more on AGP, see the feature "3-D" in this issue.)

Windows Magazine, May 1997, page 230.

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