A utility has been released to fix a bug in Apple's Firmware Update 4.1.7/4.1.8, which was reported to disable some third-party RAM modules.

DIMMFirstAid has been developed by Glenn Anderson, who last week released DIMMChecker, built to identify RAM that may be disabled by installing Apple's firmware update.

In a statement sent to MacCentral and Macworld, Anderson said: "After a week of checking through DIMMCheck results and fixing people's DIMMs, I now know exactly what's going on."

Speed the key The problem, says Anderson, lies with the speed of installed RAM, which is set by a software-based component called CAS Latency (CL). Most RAM is capable of running in CL3 and CL2 mode. CL3 mode is the fastest.

Apple has always specified the speed of RAM it requires in its machines but the firmware update is stricter about enforcing these specs. Anderson explained that nearly all RAM disabled by the firmware update lacks CL3 timing specifications – meaning the firmware-updated Mac believes the third-party RAM cannot run at the speed the Mac requires.

Anderson's fix works by copying new timing information to the RAM module. Anderson recommends only "properly programmed RAM" should be installed, as this will run faster with the new update – and that people should use his fix "only as a last resort".

Apple has always required PC100 RAM that is 8 nanoseconds (ns) or faster. The original iMac requires 10ns RAM, newer iMacs require 8ns, iBooks require 15ns, the PowerBook G3 FireWire and G4 require 10ns. PC100-222 RAM will only run at 8ns (125MHz) or faster when in CL3 mode, in CL2 mode it will only run at 10ns (100MHz).