Chapter 5. Memory

A few years ago, this book could have covered memory in a page or two. All memory came as discrete chips, and there were only a few sizes and speeds available. You bought however many chips you needed, installed them, and that was that.

Dramatic CPU speed increases over the last few years have brought designers up against one hard fact. It’s difficult to design faster CPUs, but it’s even harder to build faster memory. Building affordable fast memory is harder still. Faster processors require faster memory, so engineers have come up with various methods to increase memory speeds. One result is that memory isn’t a simple issue any more. This chapter tells you what you need to know to make good decisions about buying memory when you build or upgrade a system.

Understanding Memory

This chapter focuses on general-purpose memory, where PCs store programs and data that are currently in use, the pipeline that supplies data to and receives results from the processor. General-purpose memory, called read-write memory or random access memory (RAM), must be readable-from and writable-to. Two types of RAM are used on modern PCs:

Dynamic RAM (DRAM)

Dynamic RAM stores data for only a tiny fraction of a second before losing it. To maintain stored data, the system must constantly refresh DRAM, which exacts a performance penalty and limits its speed. Typical DRAM provides 60 ns access, but costs only a dollar per megabyte.

Static RAM (SRAM)

Static RAM automatically maintains ...

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