Page 137 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes
Figure 8.2: Programmable Read Only Memory (PROM)

8.4 Erasable Programmable Read Only Memory (EPROM)

It is an array of floating-gate transistors individually programmed by an electronic device that
supplies higher voltages than those normally used in digital circuits.
Once information is stored in a ROM or PROM chip it cannot be altered. Erasable Programmable
Read Only Memory (EPROM) overcomes this problem. As the name implies, it is possible to
erase information stored in an EPROM chip and the chip can be reprogrammed to store new
information. EPROMs are often used by R&D personnel (experimenters) who frequently change
the microprograms to test the efficiency of a computer system with new programs. EPROMs are
also useful for those applications in which one may like to store a program in a ROM that would
normally not change but under some unforeseen conditions, one may like to alter it. When an
EPROM is in use, information stored in it can only be “read” and the information remains in the
chip until it is erased.
EPROM chips are of two types – one in which the stored information is erased by exposing the chip
for some time of ultraviolet light and the other one in which the stored information is erased by
using high voltage electric pulses is known as Ultra Violet EPROM ((UVEPROAM) and the latter
is known as Electrically EPROM (EEPROM). It is easier to alter information stored in an EEPROM
chip as compared to an UVEPROM chip. EPROM is also known as flash memory because of the
ease with which programs stored in it can be altered. Flash memory is used in many new 1/O
and storage devices like USB (Universal Serial Bus) pen drive and MP3 music player.
Figure 8.3: Erasable Programmable Read Only Memory (EPROM)

8.4.1 Cache Memory
Use of main memory helps in minimizing disk-processor speed mismatch to a large extent because
the rate of data fetching by a computer’s CPU from its main memory is about 100 times faster
than that from a high-speed secondary storage like disk. However, even with the use of main
memory, memory-processor speed mismatch becomes a bottleneck in the speed with which the
CPU can process instructions because there is a 1 to 10 speed mismatch between the processor

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