Notes from Glenn Anderson:
"After a week of checking DIMMCheck results and fixing DIMMs, I now know exactly what is going on and can offer some definitive answers on this whole matter.First off, I would like to explain RAM speeds, as there appears to be a lot of confusion about it. After reading the statements from some RAM vendors, it would appear they are just are as confused as everyone else.
"The SDRAM chips on DIMMs can run at different speeds, which is set by the CAS Latency (CL) the DIMM is running. Most SDRAM chips can run in CL3 mode and CL2 mode, although a few less common ones can only run in CL3 mode. So far I have not seen any SDRAM chips that only run in CL2 mode in any of the specs I have reviewed. Contrary to popular misunderstanding, CL3 mode is faster than CL2 mode. The reason for this confusion is misunderstanding the PC speed ratings on chips. A PC100-322 DIMM can run at 10ns (100MHz) speed in CL3 mode, that is what that the 3 in PC100-322 is, and it will typically run at 12ns (83MHz) or 15ns (66MHz) in CL2 mode. A PC100-222 DIMM can run at 10ns (100MHz) speed in CL2 mode, but will typically run at 8ns (125MHz) in CL3 mode.
"I have only looked at the RAM requirements in the Apple developer notes for the B&W G3 and the G4s (except the new G4s that need PC133 RAM). Apple has always required PC100 RAM that is 8ns 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).
"The new firmware is stricter about enforcing this. About 99% of the RAM that is not working after the firmware update is not working because the timing information in the Serial Presence Detect EEPROM on the DIMM does not have specifications for CL3 mode. The vast majority of these only have timing information for CL2 mode at 10ns (100MHz) speed, they do not have timing for the faster CL3 mode. Even if Apple did support these DIMMs, the DIMMs would not be running at their full speed.
"Apple has actually been reasonably lenient with the firmware update, I have seen RAM that will only run at 10ns in CL3 mode working fine in G4s after the update. I have even seen RAM that will run at 10ns with an access time of up to 8ns working after the update, which is only PC66 RAM. For RAM to be PC100 it needs to be 10ns with an access time of 6ns or less.
"Of the remaining RAM that is not working after the firmware update, some of it is because the timing information in the SPD EEPROM says it is slower than 10ns, and some of it is just failing to work when run at 8ns or faster. It does appear the new firmware update will take advantage of faster DIMMs.
"I am working on a fix for the lack of CL3 timing info in the SPD EEPROM, however it is just a quick fix, all this fix will do is copy the CL2 timing info, which is typically only 10ns.
"If you have a DIMM that is not working and is supposedly PC100-222, it will not be running at it's full speed so you are better off to get it exchanged for one that is properly programmed as it will run faster with the new firmware update. I recommend to RAM vendors that they not just guarantee that their DIMMs will work with the firmware update, they should guarantee that it will work at full speed with the firmware update."
Important: If you are buying a 256MB or 512MB DIMM and you are not sure if it will work on a Mac, do not buy DIMMs that have a single bank made from 16 chips, make sure it has two banks of 8 chips each, or for 256MB DIMMs one bank of 8 chips or two banks of 4 chips are also OK. Incompatible DIMMs may also be identified as having 32Mx4 chips (for a 256MB DIMM) or 64Mx4 chips (for a 512MB DIMM). DIMM First Aid will identify these DIMMs as having an incompatible number of row address bits. DIMM First Aid can not fix these DIMMs.
Adds support for the QuickSilver G4, and can now check DIMMs on the Beige G3. Reporting the speed of PC133-222 DIMMs has been fixed. DIMMs that are incom
If your RAM has been disabled then your best solution is to contact your RAM vendor and get your RAM replaced. A list of vendor return policies is here:
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If they will not replace your RAM then you might try this tool to recover your RAM.
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• DIMM First Aid may not work unless your Mac's firmware is at least version 2.4.
• DIMM First Aid is able to write to the RAM board's EEPROM (the tiny chip on the RAM board), because, according to the author's email to me, "the EEPROM is connected to the motherboard via an I2C interface, which allows reading and writing."
• TattleTech is also able to report a RAM board's specs, including some data that DIMM First Aid doesn't translate, but I don't know yet how well the two utilities agree with each other.
• When DIMM First Aid reports "DIMM checks out OK", all this means is that DIMM First Aid has determined that the things it checks for, primarily the RAM board's CL2/CL3 characteristics, are compatible with the Apple firmware updates--not much more. It doesn't mean the RAM board's other specs are OK for the Mac that the RAM is installed in--I've found that RAM which DIMM First Aid reports as being OK (having correct CL2/CL3 settings), often won't work reliably in the Mac that DIMM First Aid is testing it in, if that RAM board is spec'd at a different bus speed than the Mac's bus speed--for instance, if DIMM First Aid reports the RAM board is PC133, it may not run reliably in a Mac that has a 100 mhz bus speed (apparently running a PC133 RAM board at CL2 speed doesn't compensate enough for running it on a 100 mhz bus, or something), at least in Macs up to the G4 AGP--later G4 models that have a 100 mhz bus speed might fare better with PC133 RAM, but I personally wouldn't take the chance. Also, RAM that DIMM First Aid reports "Incompatible number of column address bits" can still have DIMM First Aid reporting "DIMM checks out OK", since the number of column address bits has nothing to do with the CL2/CL3 settings, but you don't want to use RAM whose number of column address bits doesn’t agree with the specs of the Mac it's installed in.
• Many slower Macs like the G4 AGP can run fine for several years with PC133 RAM, but as the parts inside the RAM board and the logic board age, and their specs drift, a PC133 board will often cause trouble in a 100 mhz bus Mac--crashing, etc.--requiring you to replace the PC133 RAM with PC100 RAM.
• There are other RAM board specs that must agree with the Mac model they're installed in, like the RAM board's internal architecture (8 x 32, etc.). Usually, if you put a RAM board into a Mac whose RAM architectural requirements are different from the RAM board, you'll usually get an immediate and obvious problem--the Mac won't start up, or only half the RAM will be available, etc. DIMM First Aid's readme states: "DIMM First Aid will identify these DIMMs as having an incompatible number of column address bits." However, DIMM First Aid will still report "DIMM checks out OK" if the RAM board's CL2/CL3 specs are OK with the Apple Firmware Update.
• Even when DIMM First Aid reports "DIMM checks out OK", it can report any number of miscellaneous problems with a RAM board, like incorrect checksum, "SPD data revision is old or incorrect" and " SPD data revision is not set" etc.; these miscellaneous problems won't affect the RAM board's performance. However, sometimes DIMM First Aid reports some RAM as OK even when it has a problem with its CL3/CL2 data, like "CL2 and CL32 not set in Intel specification for 100Mhz support". Apparently this type of CL2/CL3 problem is not a real problem, since DIMM First Aid won't follow this up by saying you shouldn't use the RAM, but the type of CL2/CL3 problem that requires replacing the RAM, as DIMM First Aid will report, is "CL3 not supported", and possibly errors like "Fastest cycle time is slower than 15ns, this Mac requires 15ns or faster!", even if it appears to work properly. I think RAM that doesn't support CL3 can run OK in Macs that have a 66 mhz bus speed, but I haven't determined this for sure.
• To be safe, especially if your Mac is acting up even with a clean system, etc., replace RAM that DIMM First Aid reports various odd problems with, even if DIMM First Aid says the RAM should be OK.