Navigating the Memory Upgrade Jungle

Capacity And Compatibility

Density and capacity limits remain a problem for a decreasing number of systems, but yours may be among them. These limits include capacity per bank (with each bank representing one side of a memory module), total capacity supported by the chipset, BIOS limitations, and OS restrictions. Because these problems have been more prevalent in the past, they are most easily explained from a historical perspective.

The first time many system builders saw capacity become a compatibility issue was with Intel's woeful 430VX chipset, which supported a meager 2 MB per-chip capacity, meaning a 32 MB limit on 16-chip modules. Double-height 64 MB modules with 32 chips were available, but only at a fear-inducing price. Most technicians blamed the chipset for not being "PC100 compatible", though the few PC100 modules available with this low density worked flawlessly.

Intel was also responsible for the next major capacity limit, with its 430TX/440LX/440EX/440BX/440ZX chipsets supporting at most 16 MB per chip, for a maximum of 256 MB per 16-chip module. Most higher-density modules were PC133; again, uninformed technicians - even those with otherwise advanced educations - mistakenly blamed the issue on these chipsets being "not PC133 compatible". Adding to the confusion was Intel's documentation, which often sited a (false) 128 MB module limit, and IBM's refusal to incorporate support for 256 MB modules into its BIOS.

In modern systems, you'll usually encounter a memory capacity limit of 2 GB to 4 GB, depending on the chipset and BIOS. However, operating systems also have limits: Windows 9x and ME supported only 512 MB, and though workarounds were possible, they were usually more trouble than they were worth. Windows 2000 and XP support up to 4 GB, but will usually limit access to 3 GB, making 2 GB a practical goal for most purchasers.

Thomas Soderstrom
Thomas Soderstrom is a Senior Staff Editor at Tom's Hardware US. He tests and reviews cases, cooling, memory and motherboards.