• ROM (Read Only Memory): contents set at fabricationand unchangeable.
  • PROM (Programable ROM): contents written once electronically.
  • EPROM (Erasable PROM): contents may be erased using UV light, then written once.
  • EEPROM (Electronically EPROM): contents may be erased electronically a few hundred times.
  • RAM (Random Access Memory): contents may be read and changed with `equal’ ease.
  • DRAM (Dynamic RAM): the cheap and common flavour. Contents lost if power lost.
  • SRAM (Static RAM): contents may be retained with a few µW.


An EPROM with no UV window is equivalent to a PROM. A PROM once written is equivalent to a ROM. SRAM with a battery is equivalent to EEPROM.
Most `ROMs’ are some form of EEPROM so they can have their contents upgraded without
physical replacement: also called Flash RAM, as the writing of an EEPROM is sometimes
called ashing.