Apple appears to have underclocked the ATI Radeon Mobility X1600 graphics chip in its MacBook Pro laptop. According to one online report, the GPU's core runs 35 per cent slower than the clock speed recommended by ATI. The memory clock is 41 per cent below par.

The speed differential was spotted by a poster on French-language site MacBidouille, the site reports. The correspondent ran ATI's ATI Tools utility - version 0.25, a beta release - running under Windows XP. The software revealed his MacBook Pro's X1600 was clocked at 310MHz and the memory at 278MHz (556MHz effective).

MacBidouille repeated the process and confirmed the numbers, adding that ATI recommends system vendors run the GPU with a core frequency of 475MHz and setting the memory clock to 470MHz (940MHz effective).

Apple's motivation would seem to be to keep the MacBook Pro's overall thermal envelope down in order to keep the fan running as slowly as possible and the system as quiet as it can make it. There's a battery life benefit too - important, given Apple's general reticence to discuss the new notebook's power consumption characteristics.

The report warns that ATI's code is rather unstable - it is a pre-release version, after all - but it does indicate there's room to speed up the notebook's GPU at the cost of a much more noisy fan.

The gain that's possible? According to the forum poster, who followed up his initially finding by installing "ATI-optimised drivers" rather than the ones supplied by Apple, his Counter Strike benchmark test with all settings pushed to the max and the resolution set to 1,440 x 900 saw frame rates jump from 61 to 97.

Of course, that's under Windows XP - it remains to be seen how Mac OS X users can gain the same benefit. ®