Page 136 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 8: Memory

Notes
Figure 8.1 Read Only Memory (ROM)

ROM is useful for binary storage of cryptographic data, as it makes them
difficult to replace, which may be desirable in order to enhance information
security.

Draw the block diagram of CPU.

8.3 Programmable Read Only Memory (PROM)

A programmable read only memory (PROM) or field programmable read-only memory (FPROM)
or one-time programmable non-volatile memory (OTP NVM) is a form of digital memory where
the setting of each bit is locked by a fuse or antifuse.
There are two types of read only memory (ROM) manufacturer-programmed and user-
programmed. A manufacturer-programmed ROM is one in which data is burnt in by the
manufacturer of the electronic equipment in which it is used. For example, a personal computer
manufacturer may store the system boot program permanently in a ROM chip located on the
motherboard of each PC manufactured by it. Similarly, a printer manufacturer may store the
printer controller software in a ROM chip located on the circuit board of each manufactured by
it. Manufacturer-programmed ROMs are used mainly in those cases where the demand for such
programmed ROMs is large. Note that manufacturer-programmed ROM chips are supplied by the
manufacturers of electronic equipment and it is not possible for a user to modify the programs or
data stored in a ROM chip. On the other hand, a user-programmed ROM is one in which a user
can load and store “read-only” programs and data. That is, it is possible for a user to “customize” a
system by converting his/her programs to microprograms and storing them in a user-programmed
ROM chip. Such a ROM is commonly known as Programmable Read-Only Memory(PROM)
because a user can program it. Once the user programs are stored in a PROM chip they can be
executed usually in a fraction of the time previously required. PROMs are programmed to record
information using a special device known as PROM-programmer. However, once the chip has
been programmed, the PROM becomes a ROM. That is, the information recorded in it can only
be read (it cannot be changed). PROM is also non-volatile storage, i.e., the stored information
remains intact even if power is switched off or interrupted.

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