Information Technology Reference
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is a binary code that defines the operation to be carried out, the addresses of its
operands (i.e., the codes of the numbers on which the operation is to be performed),
and the address of the cell in which the result of the operation will be recorded.
Storage devices of modern computers have a hierarchical structure (Fig. 2.5 ). The
main memory is a solid state random access storage device with the read/write
speed comparable with the processing speed of the arithmetic logic unit. In order to
neutralize the difference in these speeds, an additional, small capacity high-speed
memory can be used known as cache memory. In Pentium processors it contains
8,000 cells for code and another 8,000 cells for data. For technical and economic
reasons the storage capacity of memory devices is limited. Today it reaches several
gigabytes. Storage capacity can be extended by using slower storage on magnetic
disks (up to hundreds of terabytes) and magnetic tapes with practically unlimited
capacity.
All operations in a computer are controlled by the signals generated by the
control unit. The control unit generates the address of the next command to be
executed and sends a control signal for the contents of an appropriate memory cell
to be read. The command readout is transmitted to the control unit. According to the
information contained in the address fields of the command, the control unit
generates the addresses of operands and control signals for reading the operands
from the storage and transmitting them into the arithmetic logic unit. Subsequently
the control unit sends signals for executing the operation to the arithmetic logic
unit. The result is stored in the machine memory. Result attributes (sign, overflow
flag, zero flag, etc.) are delivered to the controller where they are written in a status
register. This information can be used while carrying out subsequent commands,
e.g., conditional jump instructions.
2.2 Semiconductor Devices: A Revolution in Electronics
Tremendous progress made by computer technology is due to the development of
the component base of computing devices. Over the past half century it underwent
revolutionary changes that led to modern means of information processing utilized
in virtually all areas of human life and activity.
The first major shift in digital computers was the transition from mechanical and
relay systems to vacuum tubes. It is now even difficult to remember what vacuum
tubes looked like and how radios, amplifiers, and control devices on their basis
functioned. In the simplest case three electrodes—cathode, anode, and an inter-
mediate electrode called the grid—were sealed into a glass vacuum tube. The cathode
was heated by electric current and emitted electrons that were accelerated toward the
anode by voltage passed through the grid. An electron tube, even in this simplest
three-electrode implementation, is a natural embodiment of a switching element
required for logical circuits. Depending on the potential on the grid, it either lets
the current flow through or not, thus constituting an element with two stable states.
Therefore, starting with ENIAC tube-based computing systems were created in
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