Types of read only memory

ROM

Sometimes referred to as mask ROM. During fabrication of the chip, a mask is used to build into the chip a particular set of data. This is a permanent part of the chip, and cannot be changed in any way.

PROM

This is a kind of read only memory chip in which each cell can be either a logical 1 or 0.

The data is loaded into each cell by a unit called a programmer, which injects a pulse of current into each cell in turn to fuse a link within the cell, thus setting the cell to thereafter act as a logical 1 or a logical 0.

This fusing is permanent, and the data cannot subsequently be changed in any way.

EPROM

This is a type of programmable read only memory in which each cell can be a logical 1 or 0.

The required data is programmed in by applying a higher than normal voltage to each cell, and the cell thereafter remembers the data.

However the cell is affected by ultra-violet light - a quartz window in the casing of the chip can be used to flood ultra-violet light into the chip, which clears the data in each cell by setting each cell to a logical 1.

The chip can then be reprogrammed as before with new data.

The process of clearing data and reprogramming with new data usually neccessitates the removal of the device from the computer, and loading it into a programmer.

Also, the chip is vulnerable to stray light getting in through a damaged cover, and the quartz window is vulnerable to damage caused by automatic insertion machines.

EEPROM

This is a type of ROM which is electrically re-programmable.

Provided the computer in which it is installed allows it, the device can be cleared and reprogrammed on a cell by cell basis within the computer - the device does not have to be removed from the computer.

However it does have a limitation in that the process of reprogramming shortens its life somewhat. Typical life is around 1,000,000 cycles of clearing / reprogramming.

This limitation means that, whilst the device is perfectly suitable as a type of ROM, it cannot be used as a type of non-volatile RAM.

However it would be suitable for programming an identification number or an address into equipment such as a network interface card.

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