The programming of ROM for selection of required paths may be done by two ways. The first is called mask programming and is done by the manufacturer during the last fabrication process of the device. The procedure for fabricating ROM is that the customer should provide the truth table for the ROM to the manufacturer in a prescribed format. The manufacturer makes the corresponding mask for the links according to the truth table provided by the customer. This procedure is costly as the manufacturer demands a special charge from the customer for custom masking of a ROM. This procedure is economic only for large production of the same type of ROM. It is also less flexible because once it is programmed the functions cannot be modified by any means. With the advent of technology development various types of ROM are available nowadays.

1. Programmable Read Only Memory (PROM). It is more economic in cases requiring small quantities. In this method the manufacturer provides the PROM with all 0s (or all 1s) in every bit of the stored words. The required links are broken by application of current pulses. This allows the user to program the device in his own laboratory to obtain the desired relationship between input addresses and stored words. Special equipments called PROM Programmers are commercially available to facilitate this procedure. In any case, all procedures for programming ROMs are hardware procedures even though the word programming is used.
   
   
2. Erasable PROM (EPROM). The hardware procedure for programming of ROMs or PROMs as described above is irreversible, and once programmed, the configuration is fixed and cannot be altered. The device must be discarded if the bit pattern is required to be changed or modified. A third type of unit is available to overcome this disadvantage which is called Erasable PROM or EPROM. This device can reconstruct the initial bit patterns of all 0s or all 1s, though it is already programmed for some bit configuration. In other words, this device can be erased. This is achieved by placing the Erasable PROM or EPROM under a special ultraviolet light for a given time. The short wave radiation discharges the internal gates that serve as links or contacts. After erasure, the device returns to its initial state and can be reprogrammed.
   
   
3. Electrically Erasable PROM (EEPROM). With the advancement of fabrication technology, further improvement of ROM has taken place, where ultraviolet light is not necessary to erase the programmed data. A new technique has been introduced to erase the bit pattern of ROM, where bit patterns are reset to their original state of all 0s or all 1s by applying a special electrical signal. Afterwards, the device can be reprogrammed with an alternate bit pattern. The equipment called EPROM Programmer serves the purpose of erasure as well as programming the bit patterns.

The function of a ROM may be interpreted two different ways. The first interpretation is of a device that realizes any combinational circuit. Each output terminal may be considered separately as the out of a Boolean function expressed in sum of the minterms. Secondly, it may be considered as a storage unit having a fixed pattern of bit strings called words. From this point of view, the inputs specify an address to a specific stored word which is then applied to the outputs. For example, the ROM in Figure 6.10 has three address lines specifying eight stored words, each of which is four bits long as given in the truth table. For this reason the device is called read only memory. Generally a storage device is called memory and the terminology read signifies the content in a specified location of a memory device, as addressed by the inputs available at the output. Thus, a ROM is a memory unit with a fixed word pattern that can be read out upon application of a given address. The bit pattern of ROM is permanent and cannot be altered during normal operation.