We just got our new 27" i7 iMac in today. We ordered it with the standard 4GB of memory. I am wondering if I have to upgrade the memory in pairs, or can I do it one stick at a time?
iMac i7 2.8Ghz Quad-Core 4GB 1TB,
Mac OS X (10.6.1),
It's BIG! 8GB iPhone 3G 1GB iPod Shuffle 2GB iPod Nano
To get maximum memory performance via the dual channel access in the i7 and i5 processors, you need to install memory in pairs. However you can install another memory stick if you wish.
For some time now the more recent Intel processors/chip sets allowed you to install memory that is not in matched pairs. But before the i5 and i7, all memory access would degrade if memory was not installed in pairs i.e. it would run effectively in single channel access mode. With the i7 and i5 processors, which include memory access right in the processor, no longer using the chipset/front side bus, the memory that is installed in pairs will continue to run at dual channel access while the additional unmatched memory stick will run at single channel access speeds. You can do it, and if your memory requirements are such that all your memory is needed, the additional slower memory may still give faster performance than less memory installed forcing virtual memory to be used on a hard disk. As you know memory is hundreds of times faster than a hard disk.
Still it is highly recommended you install in pairs and get the benefits of dual channel access for all your memory and reduced virtual memory paging. If you, for example, have two 4GB RAM sticks already installed and want to add another 4 GB of RAM, it is better to add two 2GB sticks so the memory is paired rather than add a single 4GB RAM stick.
Be sure all memory sticks have the same performance characteristics. If you add a memory stick that is rated at a different speed than your other memory, the memory access to all memory will be at the speed of the lowest rated installed memory stick - so cheating a little by adding a stick of faster memory isn't going to help.
If this seems hard to believe, it is described in Intel's own datasheets on the i7/i5 processors.
To get maximum memory performance via the dual channel access in the i7 and i5 processors, you need to install memory in pairs. However you can install another memory stick if you wish.
For some time now the more recent Intel processors/chip sets allowed you to install memory that is not in matched pairs. But before the i5 and i7, all memory access would degrade if memory was not installed in pairs i.e. it would run effectively in single channel access mode. With the i7 and i5 processors, which include memory access right in the processor, no longer using the chipset/front side bus, the memory that is installed in pairs will continue to run at dual channel access while the additional unmatched memory stick will run at single channel access speeds. You can do it, and if your memory requirements are such that all your memory is needed, the additional slower memory may still give faster performance than less memory installed forcing virtual memory to be used on a hard disk. As you know memory is hundreds of times faster than a hard disk.
Still it is highly recommended you install in pairs and get the benefits of dual channel access for all your memory and reduced virtual memory paging. If you, for example, have two 4GB RAM sticks already installed and want to add another 4 GB of RAM, it is better to add two 2GB sticks so the memory is paired rather than add a single 4GB RAM stick.
Be sure all memory sticks have the same performance characteristics. If you add a memory stick that is rated at a different speed than your other memory, the memory access to all memory will be at the speed of the lowest rated installed memory stick - so cheating a little by adding a stick of faster memory isn't going to help.
If this seems hard to believe, it is described in Intel's own datasheets on the i7/i5 processors.
You could upgrade one stick at a time, so for example you wanted to take it to 6 GB you can order a 2 GB stick and that would give you your third 2 GB stick. When adding memory just make sure you get exactly what Apple recommends and follow the installation instructions.
