How to down-clock RAM

memoryupgrade

My Mac (Mid 2009- MacBookPro5,5) laptop installing OS 10.8.2.

Currently, it is using 2 x 1GB (1066 Mhz).

My friend gave me 2 x 2GB (1600 Mhz) of his 2012 Mac.

After installing 2 x 2GB (1600 Mhz) on my Mac, it can not boot but can not start OS. This is the error that show on boot start up screen:

enter image description here

But it is possible to install 1x 1GB (1066) + 1x 2GB (1600). The system can down-clock it to both 1066 Mhz. But what i still have 1 x 1GB (1066) + 1x 2GB (1600) left!

3GB @ 1066 Mhz

My question is: Any better solution than using 1x 1GB (1066) + 1x 2GB (1600)?

More certainly, is there any way to install 2 x 1600 Mhz? (even down-clocking to 1066 is accepted). (currently, 2×1600 Mhz can not startup OS).

Edit 1: 4 RAM are healthy.

Best Answer

I recently ran in to a similar issue when trying to squeeze a little more life out of an old iMac. I could only get 800mhz DIMM's but the specs say it needs 667mhz. I can interchange 1x 667mhz DIMM with 1x800mhz DIMM but a single 800mhz DIMM or two 800mhz DIMMs result in a failure to boot.

My hunch was that the motherboard thinks it can run at 800mhz so doesn't downclock it to 667mhz unless there is a slow DIMM in one of the slots forcing it to.

I did some research, and discovered the timing information is set in the SPD data stored on an EEPROM on the DIMM. I booted an Ubuntu 16.10 liveUSB on the iMac, apt-get installed i2c-tools, modprobeed the i2c-i801 module to enable the SMBus (which talks to the DIMM EEPROM) and read out the SPD of a working and a too-fast DIMM with i2cget. After a lot more research, I merged the timing information from the working DIMM in to the SPD of the too-fast (but larger) DIMM. decode-dimms can calculate the correct CRC. I then wrote the modified data back to the fast DIMM with i2cset.

I can now pair that down-clocked DIMM with an unmodified 800mhz DIMM, and the system correctly downclocks the memory bus to 667mhz and boots.

FULL DISCLOSURE: After about a month the upgraded machine started hard-crashing and was replaced by a 2017 iMac ;)

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