G5 iMac memory

The material on this page was provided by Daryl Thomas.
This item was taken from Kibbles and Bytes #386, 15 Oct 2004 published by by SmallDog.com and written by Jon Spaulding (Jon@smalldog.com) (used with permission)

In past issues of Tech Tails, I have discussed DDR RAM, the U3H system controller, and the difference between the evolving Xserve G5 and the two generations of the Power Mac G5 Tower. This week, let's talk about RAM expansion in the G5 iMac, and optimization of the iMac's RAM capabilities.

Unlike the G5 Towers and Xserves, you need not add RAM in pairs to the new iMac. It is kind of hard to do so because of the total of two RAM slots. The stock configuration of the 1.6GHz iMac includes a single PC3200 DIMM, leaving one slot open for RAM expansion. Recognizing just one chip, the U3 Lite controller opens up a single 64-bit 400MHz bus.

When you add a second DIMM of a different size, the U3 opens up a second 64-bit channel for addressing the two DIMMs separately, addressing each DIMM individually to maximize the total data path to the RAM.

The real fun is when you add matching RAM values. When you match the PC3200 DDR DIMMs in each slot, the U3 Lite opens up a single 128-bit path to service and address the two DIMMs as a single block of RAM.

The U3 Lite controller is finicky, though. The memory controller can recognize DIMMs with values of 256MB, 512MB, and 1GB. Apple says that the iMac will only recognize up to 2 GB of total RAM. The controller will recognized PC3200 DIMMs with CAS Latency of 2, 2.5,3, 4, and 5. Your "matching" DIMMs must have the same latency.

As in the G5 towers, you must use non-ECC DIMMs in the G5 iMac.

The standard precautions should be taken to control the chance of ESD (electrostatic discharge) when installing RAM in the machine. Always install RAM with the machine shut off.