by Scott Colvey
Thinking of buying a top-of-the-range Windows Vista PC with loads of Ram? Then just be sure that “loads” is no more than 3GB: you could be throwing away anything from £50 upwards.
Buy a PC with 4GB of Ram, and you can wave goodbye the last gigabyte. The reason is the memory-mapped input-output (MMIO) method - the system by which PCs communicate with installed devices, such as graphics cards.
A computer’s internal devices may require their own allocation of Ram to be mapped into the main system Ram. But 32-bit computers, most of which run XP or Vista, are limited to a maximum addressable memory of 4GB.
A machine fitted with, say, a 512MB graphics card must find mapping space for that half-gigabyte in the computer’s memory address book - even though the device will use its own memory during operation. However, Ram allocated for device-mapping is not available to applications. This is not a Microsoft-specific quirk; it affects any 32-bit system, including those running Linux.
The problem is that memory-mapping of devices takes place in the addressable area above 2GB, traditionally considered a “safe zone” because consumer PCs tended not to have more memory than this. With the arrival of Vista, though, computer buyers are ramping up their Ram requests.
Paul, a retired senior police officer, says: “I consider myself fairly knowledgeable about computers, but I didn’t know about Ram limitations. I bought a 2GB PC and later added more memory. Before doing so I checked the supplier’s website and the Ram options went to 4GB - so there appeared no reason not to purchase a further two 1GB sticks. That’s where the fun started.” Thanks to MMIO, his PC’s 512MB graphics card and other devices lopped a gigabyte off what he expected to be 4GB of memory available for applications.
The Guardian has found many PC manufacturers offering upgrades to 4GB. Dell, for example, sells PCs with 32-bit editions of Windows pre-configured with up to 2GB of memory. But the option to go to 4GB is available and would-be buyers are told: “Upgrading your memory is one of the most cost-effective ways to supercharge performance.” True, so long as you stop at 3GB.
The Guardian asked Dell why it actively promotes 4GB in machines that can only provide 3GB of usable memory. Dell responded: “Whilst Dell does not call out this restriction on its website it does clearly state the reduction in capacity in its legal birdseed [smallprint] that appears on advertisements.”
But Gareth Odgen, editor of Custom PC magazine, says that shoppers should simply not bother with 4GB PCs: “There’s not much point putting 4GB in a 32-bit system.”
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