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Types of Memory

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There are several types of electronic memory, each with a slightly different job to do. We can divide them into two main groups:

Read Only Memory (ROM)

• These devices are normally only read during the running of a program.
• The contents are not volatile. (The data remains stored even when the power supply is switched off.)
• They are often used to store the basic programs, known as 'operating systems', needed by computers.
• The group includes:
  • PROM (Programmable Read Only Memory),
  • EPROM (Erasable Programmable Read Only Memory),
  • EEPROM (Electrically Erasable Programmable Read Only Memory)

A PROM is a one-shot device. It arrives blank, ready to receive data. Data can then be 'burned' into it, but only once. After that it behaves like a ROM chip that can be read many times but not altered. With an EPROM shining ultraviolet light through a window in the top of the chip erases the contents. New data can then be 'burned' into the memory. Some older PICmicro devices operate in this way. The EEPROM devices work in a similar way to an EPROM, except that the contents are erased by sending in a special sequence of electrical signals to selected pins. 'Flash' memory is a form of EEPROM, widely used as the storage medium in digital cameras, (the memory stick) and in home video games consoles.

Random Access Memory (RAM)

• RAM allows both read and write operations during the running of a program.
• The contents are volatile and disappear as soon as the power supply is removed. (The exception is NVRAM, Non-Volatile RAM, where the memory device may include a battery to retain the contents, or may include an EEPROM chip as part of the memory to store the contents during power loss.)
• They are often used for the temporary storage of data or application programs.

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