Types of ROM - part 2

Introduction

The types of ROM listed in part 1 are all non-volatile, in that the data is preserved within the chip when power is switched off. They are typically used to store the basic machine code software layer which controls the input / output functions of the computer, and above which layer the operating system resides.

The advantages to computer system makers of using the various types of ROM are :

Mask ROM`s

Mask ROM`s provide the cheapest way to obtain a large quantity of a ROM with a particular set of data built into it.

They could be suitable for holding some basic instructions, or some fundamental look up tables.

PROM`s

Mask ROMS require long production runs, since the data is built into the chip. Using PROM`s means that the system maker can change the software used by changing the data programmed into the chip.

EPROM`s

Although the system maker can update software by changing the data programmed into the PROM`s, if any bugs show up after the PROM`s have been programmed, the chips already programmed have to be scrapped, and new ones used. By using EPROM`s, the system maker can reprogram his stock of chips, and hence reduce waste.

Although the system maker can reprogram his stock of chips if neccessary, this becomes more complicated if the chips have already been installed into computers, as they must be removed for reprogramming.

So as an alternative,

EEPROM`s

By using EEPROM`s, this problem can be avoided.

Provided the computer allows it, EEPROM`s can be reprogrammed cell by cell whilst installed in the computer.

The computer system maker can therefore have every machine leaving the production line with the most up to date software available.

An additional advantage of EEPROM`s is that by using EEPROM`s in computers on a multi-user system, the system installer can programme into each machine particular data about its position within the system, its identity or address, which machine it hands tokens over to, its priority level within the system.

The system installer or supervisor can also reprogram machines just by accessing software - the computer does not have to be taken apart.

© 1998 Ron Turner

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