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Definition: memory types


Following are the different types of RAM and storage technologies used in electronic devices. They fall into two categories: volatile and non-volatile. Volatile chips lose their content the instant power is removed, while non-volatile memories retain their content. Although all the following technologies have the word "memory" in their names, non-volatile memories are really storage chips (see storage vs. memory). For a blast from the past, see early memory.

Volatile Memory

Rewritable - Byte Addressable (DRAM and SRAM)
Dynamic RAM (DRAM) and static RAM (SRAM) chips are the "working storage" in every computer. DRAM is the main memory in a computer and SRAM is used for high-speed caches and buffers. Both types are "byte addressable," which means that data can be read and written one byte at a time. Their major drawback is that DRAM and SRAM require power to hold their content. See dynamic RAM, static RAM and byte addressable.

Future Memories
The Holy Grail for future computer memories is to create a byte addressable RAM chip with the speed of static RAM, the density of dynamic RAM and the non-volatility of flash. See future memory chips.





Non-Volatile Storage

Rewritable Flash - Block Writes - Asymmetric
Flash memory is the most widely used non-volatile storage chip in both computers and consumer electronics (CE) devices. Although reads are random access to the byte level, writing is done at the block level similar to writing a disk sector. Speeds are asymmetric; it takes longer to write than read. See flash memory.

Rewritable - Byte Addressable - Symmetric
EEPROM chips and F-RAM, which have been on the market for some years, along with newer memory technologies, offer the byte addressability of DRAM and SRAM, but do not lose their content when the power is turned off. In addition, read and write speeds are equal or nearly the same. See EEPROM, F-RAM and future memory chips.

Rewritable When Removed (EPROM)
EPROM chips were initially written in an external "programmer" device and had to be removed from the circuit board and placed back in the device for reprogramming. See EPROM.

Permanent (ROM and PROM)
Data and instructions in ROM and PROM chips can never be changed. ROMs are manufactured, while PROMs are programmed in an external device like EPROMs. See ROM and PROM.