Why can't 32-bit Windows access 4GB of RAM

Reprint:http://www.pcworld.com/article/2013751/why-cant-32-bit-windows-access-4gb-of-ram.html

Every byte of RAM requires its own address, and the processor limits the length of those addresses. A 32-bit processor uses addresses that are 32 bits long. There are only 4,294,967,296, or 4GB, possible 32-bit addresses.

There are workarounds to these limitations, but they don't really apply to most PCs. I discuss them briefly below.

Okay, so if the processor, and the operating system designed for that processor, can only handle 4GB, why can't Qasim's PC (and yours if it's 32-bit) see that much?

Because not all of those addresses are available for RAM. There are other pieces of hardware inside your computer that need addresses, such as the PCI bus and the USB host adapter.

Your graphics card is probably the biggest address hog. Today's graphics adapters often contain a gigabyte or more of RAM, and every one of those bytes needs an address. To be fair, I doubt that many of those multi-gigabyte graphics cards are in 32-bit PCs, but even a 512mb video card will take a sizeable bite out of 4GB.