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Memory

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Desktop PCs or notebooks usually have between 128 to 512 Mbytes of memory available for users, whereas handheld devices typically have only 32 to 128 Mbytes. PDAs normally have more storage space than smart cellular phones. The former commonly have 64 Mbytes, and the latter may have a memory size as low as a few Mbytes. Three types of memory are usually employed by handheld devices:

• Random access memory (RAM): There are two basic types of RAM, dynamic RAM (DRAM) and static RAM (SRAM). Dynamic RAM, the more common type, needs to be refreshed thousands of times per second in order to hold data, whereas static RAM does not need to be refreshed, making it faster but also more expensive than dynamic RAM. Both types of RAM are volatile; i.e., they do not retain their contents when the power is switched off.


• Read-only memory (ROM): ROM is manufactured with fixed contents, and it is usually used to store the programs that boot the device and perform diagnostics. It is inherently non-volatile storage, in contrast to RAM.


• Flash memory: It functions like a combination of RAM and hard disk and it is a kind of non-volatile storage similar to EEPROM (electrically erasable programmable read-only memory), but updating can only be done either in blocks or for the entire chip, making it easy to update. Flash memory is not as useful as random access memory because RAM can be addressable down the byte (rather than the block) level.

It is expected that hard disks, which provide much more storage capacity, will be adopted by handheld devices in the near future.