Digital Memory

This month's treatment of digital electronics focuses on memory devices. There are basically two types of digital memory: volatile and nonvolatile. Volatile memory loses its contents when power is removed, and nonvolatile memory retains the stored data even when the power is turned off. Making a gross generalization, the most common volatile memory is RAM (random access memory), and the most common nonvolatile is ROM (read-only memory) or EPROM (erasable/programmable read-only memory). I'm not going to get into the intricate details of how the memory components work at a transistor level; instead I'll describe them from a functional standpoint.

RAM is what the majority of memory in your PC is. It actually comes in two flavors—static and dynamic. Static RAM is very fast, but fairly expensive. It is not used in great quantity because of its cost. Static RAM stores data for as long as power is applied, without any attention. It is frequently used where a small amount of memory is needed, such as in your HF or VHF transceivers. It is easy to manage. To store data, the "address" signal is placed on the RAM and then the data is stored in that location. The address is just like it sounds: It is a unique location on the chip, just like your house address specifies a unique location on your street. Retrieving data is done in the same way. You can read data from the memory as many times as you want, and it won't be lost as long as power is maintained.

Dynamic RAM is less expensive and is what the memory in your PC is. Dynamic RAM uses capacitive effects to store the data, and as such, the charge on the capacitor can bleed off. Unless it's recharged, the data will be lost. So, dynamic RAMs have to be refreshed periodically. This is done by the computer reading the data and then writing it back to the RAM. A tedious process, but it's worth it because of the cost differential.

ROM (and EPROM) are permanent memory, but they can't be rewritten. So, they are only used to store information that doesn't change. For example, part of the computer's

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