0195136047.pdf

316 DIGITAL BUILDING BLOCKS AND COMPUTER SYSTEMS

Solution

The rate of the clock will be constant, independent of the analog input. It must be slow enough

to allow the counter to count up to the highest possible input voltage. This will require 255(=

27 + 26 + 25 + 24 + 23 + 22 + 21 + 20 )counts, which will take 255/( 40 × 106 )= 6. 375 μs.

Thus, the process can be repeated 10^6 / 6. 375 =156,863 times per second.

EXAMPLE 6.2.9

If it is desired to store English-language writing with 1 byte representing each letter, find the

minimum number of bits per byte that could be used.

Solution

For the 26 letters in the English alphabet, we must have 2N≥26, whereNis the number of bits

per byte.Nbeing an integer, the smallest possible value forNis 5. One can represent the letter

A by 00000, B by 00001, and so on up to Z by 11001; the remaining six combinations could be

used for representing punctuation marks or spaces. This is, in fact, the method by which letters

are represented in teletype systems.

If one wants to store capital English-language letters also, there will be 52 letters instead of

26; in such a case 6-bit bytes would have to be used.

6.3 Computer Systems

Digital computers, in general, are automatic machines that accept data and instructions, perform

predefined operations very quickly on the data, and have the results available to the user in

various forms. They can be classified asmicrocomputers, minicomputers, mainframes,and

supercomputers.

Microcomputershave become a common part of everyday life. The cost ranging from a

few hundred to about ten thousand dollars, today’s 16- and 32-bit microcomputer systems are

also dedicated for real-time applications in a distributed system. Amicroprocessoris an LSI

device, which is a realization of the computercentral-processor unit(CPU) in IC form. The

microprocessor is the CPU of the microcomputer system.

Minicomputers,developed in the early 1960s, are high-performance, general-purpose multi-

user computers. These are also designed for real-time dedicated applications. The PDP-11 series

from Digital Equipment Corporation (DEC) have been the most prominent 16-bit minicomputers,

and are now obsolete. The 32-bit minicomputers, known assuperminis,were developed in the

1970s, the most prominent one being the VAX 8600 from DEC, which was capable of executing

about 5 million instructions per second (MIPS). New VAX lines with larger MIPS have been

developed since.

Mainframes,capable of executing in excess of 53 MIPS, are high-performance, general-

purpose computers supporting very large databases, ranging in price from one to ten million

dollars. These are used by many universities, large businesses, and government agencies, and are

supplied mainly by IBM. Examples include IBM 360, CDC 7600 of Control Data Corporation,

and Texas Instrument Advanced Scientific Computer (TI–ASC).

Supercomputers, capable of executing in excess of one billion floating-point operations per

second (FLOPS), are very powerful, extremely high-performance computers for applications that