Things are going from bad to worse for AMD's new quad-core CPU. The latest news is that the BIOS workaround for Phenom's hardware bug can reduce performance in some applications by up to 50 per cent.

As we revealed in our review last month, the Phenom quad-core processor suffers from a so-called hardware erratum or bug in the chip's circuitry. The problem involves part of the interface for Phenom's 2MB L3 cache memory pool. In rare instances, the bug can cause data corruption leading to total system lock ups.

BIOS fix

At the Phenom launch event, AMD said a BIOS-applied CPU microcode fix would be applied to all Phenom-compatible motherboards. AMD said the fix completely resolves stability issues. However, it also conceded that it reduced performance by up to 10 per cent.

Due to limited access to Phenom samples, Tech.co.uk has been unable to verify AMD's claims. However, US website TechReport has managed to get its hands on some retail Phenom samples and compared performance with and without the BIOS fix applied. The results look fairly ugly.

On average, TechReport found performance with the fix applied dropped off by around 14 per cent. Far more worrying are a handful of applications in which performance plummets much more dramatically.

Firefox fears

In the image analysis application picCOLOR, performance nose dives by 29.9 per cent. However, the really terrifying result was a 57.1 per cent fall off in the popular web browser Firefox. That's particularly worrying given that Firefox is a single-threaded app that can only take advantage of one CPU core.

In other words, browsing the web using Firefox on a Phenom processor could be a painful experience. The explanation for the more dramatic performance issues is most likely the way particular applications access memory. The patch appears to restrict bandwidth and increase access latency for certain types of memory calls.

Whatever the truth, one thing is for sure. Phenom's perfomance is already compromised by disappointing clockspeeds and a conservative microarchitecture. The last thing it needs is a hardware bug that reduces performance even further.

Opterons also affected

It's also worth noting that the bug is present in all AMD's quad-core chips including third generation Opteron server and workstation CPUs. Indeed, there have been reports that the bug has forced AMD to stop shipping quad-core Opteron chips to customers.

However, speaking to Tech.co.uk, Christoph Dobroschke, AMD's Server and Workstation product manager, flatly denied that suggestion. "I was at [AMD's Dresden] fab yesterday. Production is in full swing and chips are shipping to customers," Dobroschke told us on Thursday.

AMD has also clarified that the bug has not had any impact on the clock frequencies of early Phenom processors. AMD originally claimed that the 2.4GHz Phenom 9700 model had been pushed into 2008 due to the hardware erratum.