Read Only Memory (ROM):
ROM is a semiconductor memory which permits only read access. The ROM functions as a memory array whose contents, once programmed, are permanently fixed and cannot be altered by the microprocessor to which the memory is interfaced. Other names for this type of memory are dead memory, fixed memory, permanent memory, and Read-Only Store (ROS).
In ROM, the memory cell (storage unit) will have a MOS transistor either with an open gate or a closed gate. Transistors with closed gates represent 1’s and the ones with open gates represent 0’s. Since the configuration is fixed, they permanently store 1’s and 0’s.
The ROM is non-volatile memory, i.e., loss of power or system malfunction does not change the contents of the memory. Also, ROM memory has the feature of random access, which means that the access time for a given memory location is the same as that for all other locations. The process of storing information in ROM is called programming.
Types of ROM:
The technique employed for storing information in the ROM provides a convenient method for classifying ROMs into one of the following three categories. They are as follows:
The Read Only Memory (ROM) which has a reprogrammable feature is called EPROM (Erasable-Programmable Read Only Memory). The EPROM memory is non-volatile and also has the feature of random access. In an EPROM, the binary information is entered using electrical impulses, and the stored information is erased using ultraviolet rays. The typical erase time varies between 10 to 30 minutes.
In EPROM the memory cell (storage location of a bit) consists of a MOS transistor with an isolated gate. The isolated gate is located between the normal control gate and the source/drain region of a MOS transistor. This gate may be charged with electrons during the programming operation and when charged with electrons, the transistor is permanently turned OFF. The state of the floating gate, charged or uncharged, is permanent because the gate is isolated in an extremely pure oxide.