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Memory Layout Overview
The system memory in the PC, despite the fact that it is often referred to as a single number ("my PC has 32 MB of memory") is in fact broken into several different areas. Even though every PC user today probably wishes it wasn't this way. :^) This design is the legacy of system limitations built into the earliest versions of the IBM PC and the versions of DOS that ran on them.
While it's easy--and fun! :^)--to damn those responsible for the PC's current warped way of organizing its memory, it's more realistic to remember the circumstances that led to this happening. In 1981, when the IBM PC was first released, 1 MB was a lot of memory. The most popular home computers at that time were probably the Apple ][ and the Commodore 64, both of which didn't go above 64 KB of memory. I remember at the time using a large minicomputer at school that had just 8 KB of memory! Memory was also very expensive. In short, the computer world was just a totally different animal then from what it is now. And hindsight, as they say, is 20-20.
As a result of the design decisions made in the earliest PCs, memory is broken into the following four basic pieces (with some of the pieces being divided further):
Next: Conventional Memory