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Users track down Apple's firmware RAM parameters

It's all in the DIMM data

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Apple's firmware fix for dodgy memory now appears to target ill-configured DIMMs rather than the SDRAM chips themselves, it has emerged.

The Mac maker released its firmware upgrade last week, aimed at G4 Macs and some G3-based machines. Included in the upgrade is a check that disables memory that Apple considers to be incompatible with its RAM specifications.

Originally, it was thought that the incompatibility issue centred on the type of SDRAM chips used. Apple spec. memory is 3-2-2 configuration, while many users have added extra DIMMs containing 2-2-2-type SDRAM. It's increasingly looking like these early reports, which we covered yesterday, were wrong. Since we published out report, we've had a host of emails from readers whose 2-2-2 SDRAM has survived the firmware upgrade.

So what's going wrong? The problem centres on the configuration information recorded on each DIMM. The data is supposed to indicate the chips' timing setting. Apple's spec. requires that what's called CL3 mode - it's a reference to the chips' latency - is supported, and it appears that the blocked DIMMs only register as CL2 mode. CL3 mode is required under the terms of the PC-100 memory standard.

That said, many of the DIMMs out there are PC-100 compliant and will work, it's simply that the relevant data hasn't been recorded in the DIMMs Flash memory. Programmer Glenn Anderson, who uncovered the CL3 issue, has written DIMMCheck, which reports back which modes each installed DIMM claims to support. Anderson claims he successfully reprogrammed his own DIMMs, and that they are now accessible to his new firmware.

Incidentally, it's telling that Apple hasn't publicly said this is the issue - it has simply noted that "the new memory test disables memory DIMMs that are found to be out of specification and DIMMs that can not be determined to be compatible. As a result, some third party memory that was recognized by previous versions of firmware may no longer be recognized after the updates".

How then are users supposed to tell whether they have incompatible DIMMs before running the upgrade? You're not being exactly helpful here, Apple. ®

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