Page 135 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes needs to continually go to the hard disk to overlay old data in RAM with new slowing down the
computer’s operation. Unlike the hard disk which can become completely full of data so that it
would not accept any more, RAM ever runs out of memory. It keeps operating, but much more
slowly than you may want it to.
The RAM is small, both in physical size (it is stored in microchips) and in the amount of data
it can hold. It is much smaller than your hard disk. A typical computer may come with 256
million bytes of RAM and a hard disk that can hold 40 billion bytes. RAM comes in the form of
“discrete” (meaning separate) microchips and also in the form of modules that plug into holes
in the computer’s motherboard. These holes connect through a bus or set of electrical paths to
the processor. The hard drive, on the other hand, stores data on a magnetized surface that looks
like a phonograph record.

Most personal computers are designed, to allow you, to add additional RAM modules up to a
certain limit. Having more RAM in your computer reduces the number of times that the computer
processor has to read data in from your hard disk, an operation that takes much longer than reading
data from RAM. (RAM access time is in nanoseconds; hard disk access time is in milliseconds.)

The RAM is called “random access” because any storage location can be accessed directly.
Originally, the term distinguished regular core memory from offline memory, usually on magnetic
tape in which an item of data could only be accessed by starting from the beginning of the tape
and finding an address sequentially. Perhaps it should have been called “non sequential memory”
because RAM access is hardly random. RAM is organized and controlled in a way that enables
data to be stored and retrieved directly to specific locations. Note that other forms of storage such
as the hard disk and CD-ROM are also accessed directly (or “randomly”) but the term random
access is not applied to these forms of storage.

8.2 Read Only Memory (ROM)

A special type of RAM, called read only memory (ROM), is a non-volatile memory chip in
which data is stored permanently and cannot be altered by usual programs. In fact, storing
data permanently into this kind of memory is called “burning in the data” because data in such
memory is stored by using fuse-links. Once a fuse link is burnt, it is permanent. Data stored in a
ROM chip can only be read and used they cannot be changed. This is the reason why it is called
read-only memory (ROM). Since ROM chips are non-volatile, data stored in a ROM are not lost
when power is switched off or interrupted unlike in the case of a volatile RAM chip. ROM is also
known as field stores, permanent stores, or dead stores.
ROMs are mainly used to store programs and data that do not change and are frequently used.
Most basic computer operations are carried out by wired electronic circuits. However, several
higher-level and frequently used operations require very complicated electronic circuits for
their implementation. Hence, instead of building electronic circuits for these operations, special
programs are written to perform them. These programs are called microprograms because they
deal with low-level machine functions and are essentially substitutes for additional hardware.
ROMs are used by computer manufacturers to store these microprograms so that they cannot be
modified by the users.
A good example of a microprogram is the set of instructions needed to make a computer system
ready to use when it is powered on. This microprogram, called ”system boot program”, contains
a set of start-up instructions to check if the system hardware like memory, I/O devices, etc. are
functioning properly. It looks for an operating system and loads its core part in the volatile RAM
of the system to produce the initial display-screen prompt. Note that this microprogram is used
every time the computer is switched on and needs to be retained when the computer is switched
off. Hence, ROM is an ideal storage for storing it.

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