Overview of Computer 15
We may come across a workstation in some designer’s of ce (or in residence), which might have
a larger display unit of higher resolution. There may be some additional peripheral devices attached
with the workstation, like a scanner or a digitizer or an XY-plotter . As we know, a scanner is capable
of generating a digital image of any drawing/photograph/text. Digitizer, on the other hand, echoes the
co-ordinate of the present position of the stylus in digital form. An XY-plotter is capable of producing
drawing on paper, using colour pens as per the commands transferred to it.
When one visits a chemical factory or any other factory, we may nd that the PC is attached with a suspi-
cious looking box from which plenty of wires are distributed to some unknown destinations. After a polite
query, we may be enlightened that the box is a multi-channel analog to digital converter ( ADC ), necessary to
monitor various operations of the plant or a digital to analog converter ( DAC ) for some control operations.
If we get a chance to see a mainframe of some bank (during its working hours and by prior permis-
sion from the authority), inevitably, our immediate attention would be drawn to the array of large, circu-
lar magnetic tape drives, which are mostly stationary but sometimes rotating in irregular fashion. The
appearance of the mainframe may not be as impressive as these dynamic units.
ADC/DAC
Figure 2.1 A computer system with some peripheral devices
In this so-called conducted tour , we may come across many external devices, which are interfaced with
the computer to cater to various demands ( Figure 2.1 ). Computer is a tool for general purpose, which may
be upgraded to suit some particular purpose of the user. If we are interested to know how this computer
functions, we need to locate and study its fundamental units, which we are about to discuss now.
2.2 FUNDAMENTAL UNITS
Let us assume that we have opened the chassis of an IBM PC. What do we nd inside? A big printed
circuit board (PCB) called the motherboard , and a few individual boxes ( Figure 2.2 ). These boxes are
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16 Computer Architecture and Organization
Opening the chassis of a working PC is not recommended, in general. It should be done by a
professional expert. Any discarded old PC may be opened. But remember to disconnect its
power supply, if it is operational and wait for at least 15 minutes. Its display unit must be
disconnected before any such investigations are carried out. Never try to open the display unit
of a computer. Neither should you venture to open an SMPS. This might be lethal.
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switching mode power supply ( SMPS ), hard disc drive ( HDD ) and optical disc drive (ODD) . We also
discover a lot of wires and cable connections connecting from one unexpected corner with another.
2.2.1 HDD and SMPS
The HDD, as we know, is meant for storing data or program. They are called secondary memory and is
generally used as mass storage devices. The SMPS can be ignored at present, as its only duty is to cater
to various power requirements for the computer. Therefore, what remains is the motherboard, which
carries out the real functions of the computer.
2.2.2 Motherboard
At a rst glance, the motherboard may be taken as an extremely complicated hardware assembly. De nitely,
it contains plenty of discrete components of various sizes and shapes ( Figure 2.3 ). However, its heart is
the processor or central processing unit ( CPU) , which, depending upon the model of the PC, may be of
various types. If it is the rst IBM PC, you may expect Intel 8088 in its 40 pin dual in-line package (DIP).
For an advanced one, like PC/AT, you would get a 386 processor. In more advanced models, you would
be able to locate a Pentium processor. In Chapter 5, we shall have a detailed discussion about this CPU.
Figure 2.2 Some internal features of a computer
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Overview of Computer 17
Next important item is the main memory . Within the motherboard, we may locate two types of
memory, RAM and ROM ; both belonging to the class of primary memory or main memory. In Chapter 7,
we shall have a detailed discussion about the memory system. However, at this stage, we shall only
point out that the ROM, which is a non-volatile memory, is necessary to accommodate the starting-up
software for the computer. The RAM, generally in the form of a RAM-card in present generation PCs
and in the form of array of ICs on the motherboard itself for old IBM PCs, is provided for all other
temporary storage during its operational stage.
We have encountered two terms, main memory (primary memory) and secondary memory. The
most important point is, program residing in the main memory may be directly executed by the
processor. However, program residing in secondary memory must be loaded to main memory for its
execution. No processor is able to directly execute any program residing in secondary memory.
Another point we should keep in mind is that the time required by the CPU for data com-
munication with secondary memory is much more than that required for communication with
main memory.
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Other important devices, scattered around the motherboard are interrupt controller, universal syn-
chronous asynchronous receiver transmitter (USART), programmable peripheral interface (PPI), timers
and counters, to name a few. The remaining major devices are hard disc controller, bus buffers and bus
drivers, graphics adopters and so on. Finally, we must mention the array of large plug-in sockets, where
extra service cards may be plugged-in to enhance the capability of the machine. Essentially, these sock-
ets offer the system bus, which we shall discuss shortly. The major question to be answered at this stage
Figure 2.3 Some details of a computer’s motherboard
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