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.
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
random access memory (RAM), must be
readable-from and writable-to. Two types of RAM are used on modern
- Dynamic RAM (DRAM)
Dynamic RAMstores data for only a tiny fraction of a second before losing it. To maintain stored data, the system must constantly
refreshDRAM, 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 RAMautomatically maintains ...