recent comments about its firmware updates disabling RAM in some Mac models have ignited a firestorm of debate in the Mac user community. Some users defend Apple’s decision to deactivate RAM that it says may be problematic, while others are indignant that Apple provides no way to check RAM before applying the firmware update. One independent Mac developer is trying to get to the bottom of the problem, and he thinks he has a handle on it.
According to Glenn Anderson, his early research shows that almost without exception, DIMMs deactivated by Macs with the new firmware update installed exhibit the same behavior — timing settings for the DIMMs don’t conform to Apple’s requirements — “CL3 mode” is not supported by these problem chips.
“The technical details of the ‘CL3 not supported’ problem are that the Serial Presence Detect EEPROM on the DIMM, which contains timing settings, doesn’t have settings for CL3 mode,” Anderson told MacCentral. “Checking through the developer notes for the models of Mac that are having the problem, CL3 mode is required, so the firmware is just being more strict about this.”
That’s not the only problem that DIMMcheck is turning up, said Anderson, but that seems to be the clincher that’s keeping most of the affected RAM from working. To check problem DIMMs, Anderson has created a small application aptly named
DIMMCheck [4K] , which reports the status of DIMMs installed in the Mac.
Anderson notes that it is possible to actually reprogram the SPD EEPROM on problem DIMMs with the settings for CL3 mode. He said he’s successfully reprogrammed a DIMM for himself, but doing so requires technical knowledge about the SDRAM chips on the DIMMs themselves — a daunting task for general consumers.
“At least now there is a way to check for this problem before updating firmware, and hopefully more DIMM vendors may be able to sort out this problem with their DIMMs or provide details for users to reprogram them,” Anderson told MacCentral.