32-bit PC : Public Service Announcement

Jeff

If you are thinking of a romping 32-bit PC maxed out with 4Gb of memory for running CFX, be careful. The end results here were not obvious to me, so I'll share my experience.

I loaded a 2.8GHz machine (HP) with a smashing graphics card and loaded it up with 4Gb of RAM. I only have 3.25Gb available under both windows and Linux. Almost 1/4 of the memory is missing!!

After some research, here's what I found. Everything on the PCI bus needs an address. Being a 32-bit machine, this address has to be "below" 4Gb. On a machine with 2Gb of RAM, all these PCI addresses get allocated between the top RAM address and 4Gb. No problem.

On a 4 Gb machine, the PCI devices have no choice but to disable some of your RAM and steal the addresses below 4Gb. How much space can a PCI device take up? In most cases not much. Network, media, and controller devices are all chewing up around 100 to 100Kb each. My AGP controller, however, is sucking up 256Mb, and my dual display graphics card another 256Mb. Added to all the other PCI devices, I'm losing a total of 0.75Gb of RAM out of the 4Gb installed in the machine.

A similarly loaded DELL machine with much lighter graphics capability is seeing 3.75Gb of the RAM. Be carful spending the cash for RAM that may be useless. You may be better off paying for just 3Gb and living with it.

If you really want all that RAM for doing CFD calcluations, go to a 64-bit Itanium machine running Linux.

Jeff