memory types

memory types

Following are the different types of memory technologies used in electronic devices. The two major categories are volatile and non-volatile. Volatile chips lose their content the instant power is removed, while non-volatile memories retain their content. See early memories for a blast from the past.

Volatile

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

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 Flash - Block Writes - Asymmetric
Flash memory is the most widely used non-volatile memory 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. Therefore, it takes longer to write than to read, and read and write speeds are asymmetric. See flash memory.

Rewritable When Removed (EPROM)
EPROM chips are initially written in an external "programmer" device and must 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.