Page 104 - Digital Electronics by harish

UNIT - IV
MEMORY DEVICES
Memory devices

Modern digital systems require the capability of storing and retrieving large amounts
of information at high speeds. Semiconductor memories are digital circuits that store digital
information (binary data) in large quantity.

4.1 Classification of Semiconductor memories

The various classifications of semiconductor memories are shown in figure.

Figure : Classification of Semiconductor memory

Basically, Semiconductor memory is classified into Random Access Memory and
Sequential memory. In Random Access Memory, the data can be stored and retrieved in any
order but in Sequential Memory the data is stored and retrieved in serial order.

The Random Access Memory is further divided into Read and Write memory (RAM)
and Read Only Memory (ROM). Data can be read and written in RAM but in ROM the user
can only read the data and cannot write or change the data. RAM can retain the memory only
when power is present but ROM can retain the data even when the power is absent. So, RAM
is called volatile memory and ROM is called non-volatile memory.

RAM is further divided into Static RAM and Dynamic RAM. Static RAM can retain
the data indefinite time as long as the power is present. But, in Dynamic RAM the stored data
will gradually disappear even when the power is present. Hence, we need to refresh the
Dynamic RAM periodically.

ROM is further classified into Mask ROM, PROM, EPROM, EEPROM and Flash
ROM according to the process of entering the data into the memory (programming) and
erasing the data.

Shift registers and Queues are classified under Sequential Access Memory. There are
two types of shift registers namely Serial In Parallel Out (SIPO) and Parallel In Serial Out
(PISO). Queues are classified into First In Last Out (FILO) and Last In First Out (LIFO).

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