Q. If my PC hardware is 64-bit, why can it address only 4GB RAM? Getty Images

Q. If my PC hardware is 64-bit, why can it address only 4GB RAM?

Q. If my PC hardware is 64-bit, why can it address only 4GB RAM?

A. In theory, a 64-bit system is capable of directly accessing 264 bytes of memory — that’s almost 18 million terabytes or 18 billion gigabytes!

But there’s a world of difference between what’s mathematically possible and what’s commercially viable for desktop systems.

Most PCs are built to meet a target price, and that places hard limits on what’s in the box. For example, many full-size PCs come with four RAM slots; in lower-end systems the OEM usually populates these with inexpensive 1GB RAM sticks, one per slot.

Although you can’t add more slots to a mainboard, you can usually replace the existing memory sticks with higher-capacity versions.

Larger RAM sticks are almost always sold in even-numbered quantities, typically 2GB, 4GB, 8GB, and 16GB. The largest-size RAM sticks in common use are 128GB each; but they’re quite expensive, probably costing more than what you paid for your PC!

Besides price, there are other factors that determine what RAM a PC can use: power and heat, mainboard and BIOS/UEFI abilities, required memory-chip speed and allowed latency, and so on.

So here’s the bottom line: If you need more RAM, check your PC maker’s support site to see what your specific brand and model of PC allows; then buy RAM that’s compatible with your system’s constraints.

By adding compatible RAM, you’ll likely be able to bump up your 4GB system to whatever reasonable size you have in mind — short of the 18 billion gigabytes of RAM a 64-bit architecture is theoretically capable of supporting!

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Editor's note: We feature an abridged Q&A from Fred Langa's LANGALIST, a column available exclusively to paid subscribers of the Windows Secrets newsletter,. What you see here is just a small sampling of what Langa's writing for the newsletter — go here for more information on how to subscribe.

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