Computer Hardware and Software
- The Processor
- Central Processing Unit (CPU)
- Primary Memory(Main Memory)
- Input Devices
- Output Devices
- Secondary Storage
See Figure 1.
It uses the Binary Number System
- Decimal
The decimal number system is positional numbering
system, i. e. depending upon the position a digit
occupies in the number, different values are represented,
e. g.\
as compared to
In the 
case, 6 represents the value of 600,
in the 
case 6 represents the value of 60.
Similar to the decimal system , the binary numbering
system is also a positional numbering
system.
- Binary
The binary numbering system has a base of
two, i. e. there are only two digits
to be used: 0 and 1.
Thus, the value thirty seven which is represented
as 37 in decimal would be represented in Binary (using
only 0 and 1) as:
A binary digit, which can take a value of 0 or 1 is
referred to as a bit.
There are two other numbering systems that are used
mainly by systems programmers:
- Octal and Hexadecimal
Octal has a base of 8, i. e. any value can be
represented by only 8 digits 0 - 7.
Hexadecimal has a base of 16, i. e. any
value can be represented by only 16 digits 0 - F.
Octal and Hexadecimal provide a shorthand method for reducing the long string
of 0's and 1's in the binary system.
- Binary, Octal, and Hexadecimal in relation to Decimal
It is easy to convert from decimal system to Binary,
Octal, or hexadecimal. The following are examples.
Decimal to Binary:
Fractions can also be represented in binary.
For example,
Decimal to Octal:
- Manipulation of Binary data by the Computer
- Using Binary to represent Non-numeric data
- ASCII
A - 1000001
- EBCDIC
A - 11000001
- The Central Processing Unit (CPU)
Is the heart of the computer.
(The cost of CPU may range anywhere between $10 and $1,000,000.)
- Arithmetic and Logic Unit (ALU)
ALU contains circuitry necessary to perform arithmetic and
logical operations.
- arithmetic operations - addition, subtraction, multiplication, and
division
- logical operations - greater than, less than, equal
to, not equal to, etc. The
logical operations include comparison of two quantities of
data. They also include the test and branch operations; the
computer takes a different course of action based on the
result of the logical operation.
- Control Unit
- directs the flow of operations and
maintains order in the computer.
- coordinates the flow
of data in and out of ALU, primary storage,
secondary storage and various I/O devices.
- Registers
- high-speed storage areas used to temporarily hold small
units of program instructions and data
- to temporarily store specific types of data (the current
program instruction, the address of the next instruction,
current data, etc.)
- There are general purpose registers, which are
available to applications programmers and
special- purpose registers such as the
instruction counter and
instruction register.
- The size of a register and the number of
registers on a computer varies from one to another.
Typically, the word size used on a given computer
matches the register size. Common word sizes include:
8, 16, and 32 bits.
- Primary Memory(Main Memory)
Main memory holds instructions of
the program(s) that are currently being executed and
the related data. It is made up of
on - off type of devices (representing a bit).
A group of bits, typically 8 is referred to as
byte and a collection of bytes is referred to as
word.
Memory size is measured in terms of kilobytes (K)
(1024 bytes), megabytes (MB), or gigabytes (G).
Each memory cell has an address.
- Types of main memory:
- RAM this is the part of the main memory that
holds the program and its data. The time it takes to access
any location in RAM is the same independent of
where it is. RAM is volatile, that is, the data will be lost if
the power is turned off.
The amount of RAM is one of the factors influencing the overall power
of a computer. It does not matter how fast or how powerful a CPU is if
the amount of RAM is limited.
- ROM its contents can only be read. It
includes instructions ( such as part of the OS)
that are meant to
remain in memory permanently, i.e., it is non-volatile.
- PROM its contents must be first programmed into the
chip. Later it behaves like a ROM.
- EPROM the contents can be erased and reprogrammed.
- Cache It is faster than RAM but more expensive.
Holds those instructions and data that
the CPU will likely request next.
- Word length and Bus Line Width
Data is moved around within the computer in groups of bits,
usually in multiples of bytes. Data Bus line width should match
the CPU wordlength for optimum performance. The size of the
address bus limits the size of the primary storage capacity.
( Common word sizes include:
8, 16, and 32 bits.)
- A program is a sequence of computer instructions
- A program is usually written in an English like language.
It is made up of statements (instructions).
The compiler translates the high level language instructions
into machine instructions ( instructions
which the computer is able to understand )
- Each operation a computer can perform has a corresponding
machine instruction.
The set of these machine instructions is the
instruction set of the machine.
A machine instruction - operation code (opcode), number
of characters in the
first field, number of characters in the second field,
location of the first field, and location of the second field.
Ex: A 44 7000 9000
- When the user issues the command
``run prog1'' from the keyboard
a part of the systems software (the loader) then
loads the executable module of this program
from the secondary storage to the
main memory.
The loader also
puts the
memory address of the first instruction in a special
register called instruction counter.
- To execute an intruction, the control unit
performs the following operations.
- fetching - obtaining the next program instruction
from the main memory
- decoding - translating the instruction into the
command
- executing - activating the ALU to execute the command
- storing - placing the result of the instruction back
in the main memory.
The first two steps are referred to as the instruction cycle.
The time taken to perform these two steps is called the
Instruction time (I-time).
The latter two steps are referred to as the execution cycle.
The time taken to perform these two steps is called the Execution
time (E-time).
These
four steps are called the machine cycle.
- Machine cycles are measured in
milliseconds (thousandth of a second),
microseconds (millionth of a second),
nanoseconds (billionth of a second), or
picoseconds (trillionth of a second).
- Processing Speed
- usually measured in MIPS
( one million instructions per second), Mflops
( one million floating-point operations per second) or
Gflops
( one billion floating-point operations per second).
- factors influencing the processing speed
- the clock frequency, measured in megahertz (MHz). (Clock
frequency of PC's range from 4MHz to 25MHz.)
A machine cycle may involve one or more clock cycles (clock
cycle = inverse of the frequency).
- bus width
- word size
- CPU speed is limited by physical constraints, such as speed and
power consumption of the semiconductor devices (Silicon, Gallium
Arsenide).
- Alternative Types of Processing
- RISC Processing
- Parallel Processing
- Optical Processing
- Distributed, Decentralized, and Centralized processing
- keyboard - is the most popular of all
It has four types of keys: function keys, alphanumeric keys,
numeric keys, additional cursor control keys
- mouse - it maneuvers a pointer on the monitor screen
when dragged across a flat surface.
Usually consists if a ball; the ball's rolling motions are
converted into electrical signals.
- Terminals. Examples include
CRT, and Point-of-Sale-Terminal.
- Optical Recognition. Examples are: Mark
Sensing (such as the cards used to automate test grading),
Optical Character Recognition, Bar Code (similar to
that used in grocery stores).
- Magnetic Character Recognition. typically used
in banks, e. g. used on checks. Characters are imprinted
in magnetic ink, which can be read by a special-purpose
reader/sorter, which reads the information on the check and sorts
them in account number order.
- Voice Recognition. The input device converts
speech to a digital code that the computer accepts.
Typically has a limited vocabulary.
Why?
- The main memory is volatile, limited in capacity, costly
and therefore is used to store program and data temporarily.
- Secondary (Auxiliary) storage, on the other hand,
has high capacity, is non-volatile, and inexpensive.
The most commonly used secondary storage devices are: magnetic
disks and magnetic tapes.
- Floppy Disks
- The most common microcomputer
secondary storage medium is diskette, where
data are recorded on one side (single sided diskette)
or two sides (double sided diskette)
- available in two sizes - 3 1/2'' and 5 1/4''
- Data are recorded on a series of concentric circles (
tracks).
The access mechanism steps from one track to another reading or
writing one at a time. Tracks are numbered starting from 0
on.
- Tracks are subdivided into sectors.
Each sector has a unique track/sector address.
- Several sectors on the first track are set aside to hold
an index or directory where the location of each
named file is kept. Thus, the system is able to determine the
location of files on a diskette.
- Disks serve as both sequential and direct access
storage
medium.
total storage = # of disk sides 
# of tracks
# of sectors sector size in bytes
each sector stores 512 bytes
|
Disk size | sides | tracks/side | sectors/track | total
sectors | capacity |
| | | | | |
|
double density 5 1/4'' | 2 | 40 | 9 | 720 | 360 KB |
|
High density 5 1/4'' | 2 | 80 | 15 | 2400 | 1.25 MB |
|
double density 3 1/2'' | 2 | 80 | 9 | 1440 | 720 KB |
|
High density 3 1/2'' | 2 | 80 | 18 | 2880 | 1.44 MB |
- Hard Disks
- Unlike a diskette (which spins only when data are to be read from
or written onto thus, it takes time to bring the drive up
to
the operating speed, a hard disk spins
constantly. (2400 to 3600 rpm)
Thus, reducing access time.
- Most large systems use disk packs.
A disk pack is a collection of recording surfaces stacked on a
common
spindle.
Each recording surface has its own read/write head.
- The heads are arranged on a single comb like access mechanism.
All heads move together.
- One position of the access mechanism corresponds to one-track
on each surface- this set of tracks
is called a cylinder.
- magnetic tapes
can only serve as sequential
storage devices.
- Optical Disks
- CD-ROM
- capacity is 550 MB (5 1/4''), approximately 150,000 printed pages
about 250 big books. Electronic Encyclopedia with 33,000
articles and a total of 9 million words is available on a
single CD-ROM.
- access time is large
- WORM (Write Once Read Many times)
- capacity is 2 GB
- Ex: DMV of California uses to store pictures, finger prints
and signatures
- Erasable Optical Disks (250 MB - 1 GB). These are very
slow.
Computers are classified according to characteristics, cost and
purpose.
|
Characteristics | Micro | Workstation | Mini | Mainframe |
super |
|
| | | | | |
|
Processor speed | 4MIPS | 8MIPS | 31MIPS | 102MIPS | 2.7Gflops |
|
Main Memory(MB) | 1-16 | 16-64 | 128-192 | 128-2048 | over 256 |
|
Price | 2-12K | 5-20K | over 700K | over 13M | 5M-20M |
Microcomputers
Microcomputers are built around a microprocessor. The microprocessors
manufactured by Intel Corporation have become the standard CPU's
around which IBM and its compatible microcomputers are built.
Evolution of Microprocessors:
Please see Table 1.
Hardware Multitasking: Allows the computer to work on more than
one program at a time. For example, if you need to move data from a
spreadsheet to a word processor, the multitasking capability allows you to move back and forth between the applications without completely backing out of either program.
Math Coprocessor: For heavy duty mathematical calculations such
as CAD operations, large spreadsheet, math or database calculations, a
math coprocessor chip is added to the system board. 486 has a built-in
math coprocessor.
Cache Controller: Cache memory is a special high -speed memory used to enable quick and easy access. Data and instructions are transferred from the main memory prior to their use by the processor. Cache controller directs the cache memory.
Consists of computer programs that control the operations of the
computer system
- Systems Software
manages the resources of a computer system and makes its
operations more efficient and effective.
- Operating System
- Language translators
- utility programs
Examples include assemblers, compilers, linkage editor, text
editors, and OS.
- Application Software
consists of programs written to solve particular user oriented
programs
An OS is a master control program which is loaded into
a computer's memory. Its purpose is two-fold:
- to provide a common set of computer functions
- controlling the operation of the overall system
- getting input from the input device
- reading data from and writing data into disks or other
secondary storage devices
- writing information to the computer screen
- monitoring loading of programs
- handling interruptions
- Scheduling I/O devices
- to provide hardware independence
- The application software is independent of the hardware
modifications
Some of the services offered by an OS include:
- resident loader. It loads programs
and data into core from a variety of devices assigned by
the programmer. As it loads data, it also checks
for transmission error.
- I/O services. Instead of including I/O
instructions in each applications program, they
are placed in the OS. When an applications program needs
to input or output data, it sends a call to the OS, which
then processes the data transfer and returns control
to the program.
- Memory management. Allocation of
main memory for use
by applications programs and protection of each
program's space from being accessed and/or destroyed,
by mistake or by intention, from other programs that are being
executed.
- Dispatching.
OS switches attention of the CPU from one program
to another as time and processing needs dictate.
- Terminator. Frees the main memory and
connections to input files when
applications program completes its work.
- Driver routines. Each physical device attached
to the CPU must have a software routine (driver program)
which controls the transfer of data between the device
and the main memory. It is common to place these driver
routines in the OS.
Multiprogramming
An extreme speed disparity between the CPU and its
I/O devices. Since the CPU cannot process data that it
does not have yet, processing must wait for the
completion of an input operation. In addition,
in order to avoid possible errors, the CPU waits
for the completion of an output operation.
Thus, the CPU might process instructions for few microseconds,
wait for hundred microseconds as an I/O
operation takes place.
Multiprogramming (or multitasking) is a technique for having a
number of programs placed in memory and the CPU switches its
attention from one program to another, taking advantage of the
time that it would otherwise spend waiting for I/O.
Although we may have more than one program in main memory,
the CPU works only on one program at a time.
Some of the popular OSs are:
- OS-MVS
- (OS-multiple virtual storage) Developed by IBM for
its 370 series machines. Handles multiprogramming in a
virtual memory environment.
Virtual memory is where the program to be executed is
divided into segments (pages).
Pages are brought into memory as needs arise.
Thus, only part of (not the whole) the program
executing reside in the main memory. this results in
having the CPU process more programs concurrently.
- MS-DOS
- (Micro soft-disk OS). Developed for use
on microcomputers. Supports one user at a time.
- Unix
- Developed at Bell Labs in 1969. It is
designed to handle multiusers and multitasking.
It is portable, i. e. can be moved from
one machine to another.
- OS/2
- Developed by IBM and Microsoft.
It is a multitasking system. Requires a larger memory as compared
to
MS-DOS.
A translator converts a statement from a high level
programming language into machine language.
Translators are of two types:
- interpreter
translates one instruction at a time
useful when small programs to be run only once or few times
- compiler translates complete program at a time
it is a two stage process
useful for larger programs that are to be run number of times
Programs used to merge and sort files, keep
track of computer jobs being run, keep track of number of users
and resource utilization etc.
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