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AHCI issues
A documented compatibility problem between Windows Vista and the 790FX chipset's SB600 south bridge component currently complicates running drives in AHCI mode. As Microsoft notes:

When you try to install Windows Vista on a portable computer that uses an ATI SB600 Series chipset, each driver may take a long time to install. Therefore, the Windows Vista installation process may take several hours.

That's if it works at all. One can install Vista by putting the SATA controller into IDE mode, but AHCI mode is necessary to take advantage of Native Command Queuing and SATA device hot-swapping, so it's not a trivial capability to lose.

With a hotfix from Microsoft, we were able to get drives running in AHCI mode on both boards. However, doing so quickly all but requires an auxiliary storage controller. Vista must be installed with the south bridge running in IDE mode. You then have to move the hard drive to a secondary controller, switch the south bridge to AHCI mode, and apply the hotfix and necessary drivers before swapping the drive back. This problem should be resolved by Vista Service Pack 1, but without slipstreaming, that won't necessarily make the installation process any easier.

The 790FX chipset's AHCI problems don't end with installation, though. There are also some performance oddities that we can illustrate nicely with IOMeter. In the results below, the 790FX is shown with the SB600 running in IDE mode, in AHCI mode with Windows Vista's drivers, and in AHCI mode with 2.5.1540.47 drivers provided by AMD. Incidentally, those drivers come from Silicon Image, which is apparently responsible for the SB600's Serial ATA controller.

With AMD's official drivers, performance doesn't scale at all as the load increases—even IDE mode offers higher transaction rates. Windows Vista's native drivers seem to have it right; SB600 performance ramps more aggressively than even the nForce 590 SLI.

AMD's official drivers also deliver much slower response times, suggesting that Vista's native drivers may be the way to go.

Except for one little problem: CPU utilization. Vista's native drivers may offer the AHCI performance we're used to seeing, but they exact a huge CPU utilization penalty in the process. Silicon Image's AMD's drivers offer competitive CPU utilization, at least, but we've already seen that their performance is lacking.

At the moment, running the SB600 in AHCI mode requires choosing between poor performance or high CPU utilization. Neither is acceptable in our opinion, especially since competing chipsets seem to have no problems getting it right. Let's hope AMD can address this issue with its upcoming SB700 south bridge.