Download How modern computer is built?

How modern computer is built?
Modern computers have been perceived first of all as computing (calculating)
tools We will discuss digital computers, as the analog and hybrid machines are
very rarely used nowadays.
Construction of modern digital computers was described in memos written by
John von Neumann (American mathematician of Hungarian origin) in 1945. It is
still known as „von Neumann architecture”.
Memory is a device capable to store numbers
(at when it is on). It is built of numbered cells
and these latter numbers are known as their
addresses.
CPU is a device storing data (taken from
respective addresses in memory) temporarily
in registers and performing on them simple
arithmeric operations. Then – sending them
back to memory.
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Bus (in Polish) - magistrala
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How numbers can become commands?
It is quite understandable that data and addresses are numbers. The example
below illustrates how numbers can become commands. In a CPU it is a bit
more complex, but the robot below (a cube with six propellers capable of
moving in a 3D space under no gravity conditions) illustrates the principle rather
well.
Propeller No. (0-off,1-on)
direction of movement
5
4
3
2
1
0
0
0
0
0
0
0
stop
0
0
0
1
0
0
forward
0
0
1
0
0
0
backward
1
0
1
0
0
0
back/rightward
1
0
0
0
1
up/leftward, etc.
0
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How numbers are represented in
computers?
Today numbers are most frequently written using positional system with base
ten. Base ten means that ten digits are used to compose numbers using certain
rules. Digits are symbols (usually graphical) corresponding to certain, always
the same number of objects, without any regard to the number of these objects.
(describing the size of small sets). In childhood we were taught by imprinting
that two sweets, two bunnies, etc., we should associate with symbol 2.
In a positional system, however, the value represented by a digit may vary,
depending on its position in a number. This is best represented by the formula:
N = ∑ Di wi = ∑ Di B i
where: N is the value of a number, Di is absolute value of the digit at ith position,
wi is weight of this position, equal to base of the system B raised to power i.
When integers are expressed in this manner, position zero is the rightmost one,
while in the case of real numbers, position zero is that of digit with decimal
separator at its right side. Notice the role of zero in positional systems.
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How numbers are represented in
computers? (2)
N = ∑ Di wi = ∑ Di B i
Let’s see how it works:
117 = 1×102+1×101+7×100=100+10+7=117
23,75=2×101+1×100+7×10–1+5×10–2
In other positional systems (indicated in subscripts):
base 4 (quarternary)
1134=1×42+1×41+3×40=1610+410+310=2310
base 7
6027=6×72+0×71+2×70=29410+010+210=29610
base 2 (binary)
10010102=26+23+21=7410
base 16 (hexadecimal)
AF16=(10×161+15×160)10=17510
In the last system capital letters A through F serve as digits representing values
from 10 to 15.
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How numbers are represented in
computers? (3)
Usage of the binary system in digital computers was also suggested in von
Neumann’s memos. It has its advatages and disadvatages:
• numbers are very long (require much storage space and cause sending
transfer problems (–)
• only two digits are necessary (+). It was a decisive argument as „electronic
symbols” (electronic digits) had to be created.
• binary system is compatible with different information carriers (+): optical –
light 1, darkness 0; mechanical – hole 1, no hole 0 (punched cards and
perforated tapes); magnetic, etc.
The formula we used may be easily adopted to binary system, expressing
unsigned integers :
N = ∑ Di 2 i
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How numbers are represented in
computers? (4)
The smallest imaginable unit used in data storage and transfer transfer is
hence, a binary digit know as a BIT (binary digit, it is also the smallest piece of
information). Addressing such a small units would be difficult. Hence, a practical
unit is a byte, which consists of eight bits taken together.
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
One byte represents just a single unsigned integer from 0 (left) to 255 (right),
which means 256 different values.
If we want to include negative numbers, we must sacrifice one bit to bear the
sign (here leftmost one, normally weight 128, most significant bit):
1
1
1
1
1
1
1
1
0
1
1
1
1
1
1
1
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
–127
+127
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How numbers are represented in
computers? (5)
In the latter case one byte can carry only 255 different values, additionally
creating ambiguity:
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
–0 ??
+0 ??
To solve those issues, a modification was introduced to the binary system,
known as the two’s complement system. The formula is modified as follows:
N = −2 n + ∑ Di 2i
0 1 1
1
1 1 1
1
127
1 1 1
1
1 1 1
1
–1
0 1 1
1
1 0 0
0
120
1 1 1
1
1 0 0
0
–8
0 0 0
0
0 1 0
1
5
1 0 0
0
0 1 0
1
–123
0 0 0
0
0 0 0
0
0
1 0 0
0
0 0 0
0
–128
256 different values (from –128 to +127), no „zero” ambiguity:
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Two’s complement (in Polish) – uzupełnienie do dwójki (U2 – no kidding!).
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How numbers can become commands?
(2)
Returning to the example of our cube-robot, if we treat the propeller number as
the position of the byte, we can find numbers sent to the robot forcing it to
execute specific movements:
Propeller No. (0-off,1on)
direction of movement
number sent
(unsigned)
5
4
3
2
1
0
0
0
0
0
0
0
stop
0
0
0
0
1
0
0
forward
4
0
0
1
0
0
0
backward
8
1
0
1
0
0
0
back/rightward
40
0
1
0
0
0
1
up/leftward
17
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How numbers are represented in
computers? (6)
Obviously, we cannot be restricted to using only small integers (even signed).
Hence, all each variable used in computer programming is declared as being of
certain „type”, which automatically explains what kind of information it carries,
how many bytes it uses and how the bytes should be understood (combined) by
the computer. For example using 2 bytes, arranged as shown below, we can
cover values from 0 to 65535 in the unsigned system and from –32768 to
+32767 in the two’s complement system.
1
0
1
1
0
0
0
0
1
1
1
1
1
1
1
1
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
more significant byte (MSB)
less significant byte (LSB)
–20225
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How numbers are represented in
computers? (7)
For real numbers more bytes are needed. Part of them cover the significant
digits (the mantissa) and part of them the exponent.
When a variable is declared as being of certain type (e.g. Real – 4 bytes), only
limited number of significant digits may be stored. This may lead to numerical
instability in certain calculations. If we want more significant digits, we must
declare types using more bytes – requiring more space in memory and handled
longer by the CPU. For special purposes, like calculating milions of digits in Pi
number, quite different principles must be used.
Bytes are also used as a measure of storage devices capacity. They are very
small units, hence, multiple units are introduced:
1 kByte = 210 Bytes = 1024 Bytes
1 MByte = 1024 kBytes = 1048576 Bytes
1 Gbyte = 1024 Mbytes = 1048576 kBytes = 1073741824 Bytes
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Non-numerical information in digital
computers
Most important non-numerical information in computers are text data. It has
been achieved long ago by simple ascribing characters to numbers or viceversa. First such system is known as ASCII (American Standard Code for
Information Interchange). It has been developed for usage in teletypes and
originally used only 7 bits, permitting to carry only 128 characters, including 32
steering commands (like Line Feed 10 or Carriage Return 13).
N
C
32
N
C
N
C
N
C
N
C
N
C
N
C
38
&
44
,
50
2
56
8
62
>
68
D
33
!
39
‘
45
–
51
3
57
9
63
?
69
E
34
”
40
(
46
.
52
4
58
:
64
@
70
F
35
#
41
)
47
/
53
5
59
;
65
A
71
G
36
$
42
*
48
0
54
6
60
<
66
B
72
H
37
%
43
+
49
1
55
7
61
=
67
C
73
I
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This is only a part of the ASCII code.
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Non-numerical information in digital
computers (2)
Nowadays even an 8-bit ASCII (256 characters and seering commands) is too
small to cover all the symbols used in different fields of human activity. Hence a
2 byte system was adopted, permitting to embrace 65536 characters. Yet some
more exotic languages (using non Latin characters) must use special variants
of this system.
Variables carrying text data are known as strings (String type). Usually, the first
byte or bytes indicate the lenght of the string and successive bytes carry the
characters. Dynamic strings of no predefined length are also used.
Notice, that ASCII code is „constructed” in a way facilitating operations like
comparison of strings, ordering in alphabetical order: upper case letters are
represented by smaller numbers (codes) then the lower case letters. All these
operations are reduced to subtraction of consecutive characters in the string.
Other types of non-numerical information will be discussed later.
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Peripheral devices
Computer minimum – as described before – was a very user unfriendly device.
Input of programs and data required manual switching (0, 1) in binary. Reading
out the output also required writing down which bulbs are on (1) and which dark
(0). Then, another tedious job was translation of all this stuff into easily
readable decimal numbers.
For this and other reasons, computer minimum started growing, i.e., additional
devices were added to it to facilitate working with the computer. First,
permanent external memories were added to avoid loading the program
manually every time. Also, output could be stored for future use. Those
memories were magnetic, first tape, later disc (easier access to any point of the
storage space – surface).
Second, methods of input and output had to be improved. For the first purpose,
teletypes were employed directly or via pyunched tape (output was already in
binary – ASCII). For the second – printers – producing a hardcopy od the
output, preceded by binary to decimal translation. It was the situation with big
mainframe computers.
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Peripheral devices (2)
Since those days, especially with appearance of PCs, many other peripheral or
I/O devices were added. Let’s name some of them:
name
function
name
function
Monitor (screen)
O (I/O)
Scanners
I
keyboard
I
Modem or net card
I/O
mouse
I
Camera
I
loudspeakers
O
ADC
I
earphones
O
DAC
O
printers
O
Pen drives
I/O
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Operating systems
It is not enough to connect physically all these devices to the computer. It must
know how to use them. Hence, each device is accompanied by a driver, which
is a short program telling the computer how to use this specific device. For
example, if you buy a printer, you must usually install it, meaning to load the
driver into the computer operating system. If you use a new pen drive with your
computer, you must wait a minute to let the computer familarize with the new
device (the driver is on the pen drive).
All drivers for those I/O devices which are customary included in the basic
configuration of a computer (memory discs, screen, keyboard, mouse), along
with some features making the whole system capable to run several tasks
(applications) parallelly and user friendly (user-computer interface) constitute
this computer operating system. It is also a program, or even a set of programs,
which the computer loads automatically from the disc memory after switching it
on (booting).
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Operating systems (2)
Most popular operating systems are:
• UNIX and its family (big computers)
• OS 2 and successors (original IBM PC)
• Microsoft family: DOS, Windows (all PCs but Apple)
• Apple (Apple DOS, Mac DOS) concept of graphical interface
• Linux – open system, which may be improved by the users.
Main actions of operating systems are:
• planning time of CPU usage by different tasks (programs running at the same
time)
• controlling the usage of RAM by different tasks (assigning the space to them)
• controlling communication between the tasks
• controlling and assigning the access to the I/O devices by different tasks
• controlling the file system
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Operating systems (3)
Each operating system consists of:
• the kernel (the core of the system executing the control tasks)
• the shell (enabling communication between the user and the computer)
• the system of files.
File is a sequence of data (bytes) of finite length, having a name and some
attributes. Such data set is treated by the OS as a single entity.
Types of files (general):
• catalogues (folders, directories) – contain other folders or files
• symbolic links
• executable files
• queues.
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Files and their organization.
File is a sequence of data of finite length, having a name and some attributes.
Such data set is treated by the OS as a single entity.
Attributes of files:
• read – only,
• hidden
• system
• archive
• folder, directory
• volume
• coded (Windows only)
• compressed (Windows only)
• offline (Windows only)
• temporary (Windows only).
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Files and their organization (2)
File names are built of two parts: proper name and extension. They are
separated by a dot (point, period). Nowadays proper names may be up to 64,
128 or even 255 character long – which is impractical, though better than
before, when they were restricted to 8 characters (usage of some charcters is
forbidden). Extensions are up to 3 characters (rarely 4 in net applications).
Types of files (detailed, indicated by their extensions):
• *.exe (executable file – containing a program),
• *.sys; *.dll; *.ini; *.drv (system files used by the OS or by several programs –
common files)
• *.txt („pure” text or ASCII files)
• *.bmp; *.jpg; *.tif; *gif (graphics in different
• *.doc (MSWord documents)
• *.xls (MSExcel documents)
• *.ppt (MSPowerPoint documents)
• *.mp3 (sound containing documents)
• *.zip; *.rar (archive files – packed),
• *.pdf portable document format (Adobe Reader),
Changing the extensions is not
recommended (sometimes even forbidden).
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OSs still recognize the filetype by their extensions not the contents.
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Files and their organization (3)
Disks (partitions),
folders (directories)
and files are
organized in a treelike structure.
There is a
hierarchical order,
starting from the root
directory.
Full sequence of
directories leading to
a given file is known
as the access path.
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Files and their organization (4)
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Files and their organization (5)
All this tree-like structure reflects only a virtual or logical structure of data. They
are actually (physically) located on a disk (in the memory) in quite different
order. Two main file systems used by PCs are:
• FAT (File Allocation Table) used by MS-DOS and older versions of Windows,
newer version used FAT32 (32 bytes, older 12 and 16).
• NTFS (Windows NT File System), somewhat similar to HPFS (High Performance File System) by IBM. Used by all Windows systems from NT up. It
allows big files, long filenames, access control, more tree levels, etc.
Files are actually stored as a series of clusters. When file is removed, first –
only its name is marked as „removed” and shifted to a directory known as a bin.
When the bin is emptied, its filename is removed permanently from the system,
while its contents is still present in the respective clusters. When some new
information (file) is written over these clusters, recovery of the old files become
more difficult, yet still possible.
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This is the reason why so frequently hard disks containing sensitive information are
sought by police and other security services.
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Files and their organization (6)
After some time of using a disk, the file storage becomes a nightmare: there are
empty spaces, files are divided in many fragments stored in distant clusters,
etc. One can even observe certain slowing down of the computer operation. It
means that defragmentation is recommended. It may be executed by clicking
the icon of the respective volume (disk
or partition) with the right key of the
mouse, then choosing properties and
tools. There are three tools:
• volume check (for physical errors)
• volume backup
• volume defragmentation.
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Files and their organization (7)
• volume check (for physical errors)
• volume backup
• volume defragmentation.
If you wish flawless operation of your
PC, you should perform regular checks
and defragmentations. After starting
the latter, the system tells you if the
operation is really necessary.
Running defragmentation you will ba
able to observe how physical
placement of the files on disk is
changing (some files are unmovable –
these are system files, which
addresses on the disk are fixed.
What regards making backup copies of the volumes, it is also highly
recommended. These operations are appreciated in a moment your computer
crashes down.
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Files and their organization (8)
Disk formatting is a serious operation, in which all the contents of the disk is
erased and new sectors and clusters are formed. All the data are lost (special
services can still recover at least part of them).
Recently quite too frequently formatting becomes necessary. To avoid these
troubles, partitioning of the hard disk is strongly recommended.
Partitiong means dividing a physical disk into a number of volumes (partitions or
virtual disks). First partition will maintain main symbol C:, while the next will be
assigned with consecutive letters D:, E:, etc.
OS is always located in C: partition and must be reinstalled from the original
media after formatting. Also program files prefer being installed there. Your
data, however, which means fruits of your work and efforts, should be stored in
D:. Partitioning may be done using special programs like Partition Magic (and
others).
In a case of necessity of formatting, one must format only system partition (C:).
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Files and their organization (9)
In a case of necessity of formatting, one must format only system partition (C:).
All programs must be reinstalled (no big deal, though time consuming operation
for those who have legal programs) yet the data are not lost.
One should remember to avoid storing one’s data in folders provided by the
system (MyFiles, MyDocuments, MyPhotos, etc.) because these folders are
always created in volume C:. Hence, they will be removed when formatting this
volume.
Most practical way is to have three partitions:
C: main volume, containg the system and program files (large)
D: data containing volume (large)
E: mirror volume (small) – safety volume containing the mirror image of the
former ones (compressed)
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Booting sequence
When a computer is switched on, we say it is booted or rebooted ...
BIOS
ROM (EPROM)
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Computer viruses
Computer viruses are small computer programs that can copy and distribute
themselves without permission of the computer user. They can attach
themselves to other files and be distributed with them worldwide using e-mail
and internet system. One should not forget, though, spreading of viruses via
external media (pen drives, CD or DVD).
They can be divide into two main groups:
• benign, once installed (also by themselves) they perform relatively innocent,
practical jokes like drawing some pictures on the screen or writing some
unjustified warnings.
• malicious – these can do real harm like removing files from the storage
devices, interfering with the OS, even formatting disks.
Viruses usually infect the boot sector of the disk, to ensure that they will be run
when the user boots the computer.
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Computer viruses (2)
A wider term is malware. Besides true viruses, it includes also worms, trojan
horses, rootkits, spyware, adware (spam), even crimeware, and other malicious
and unwanted software (worms and trojans differ from true viruses technically).
Using suitable antivirus software is essential for secure and
successful operation of your computer!
One can observe slower operation of the system, though it is a
price paid for the „civilization progress”.
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Non-numerical information in digital
computers (3)
Graphics – semigraphics (using ready made blocks prepared as bitmaps).
Character matrix (the example comes from old toy computers like ZX Spectrum
or Commodore 64). Nowadays 12 bytes are used in height.
It may be seen that 8 bytes defines the
look (shape) of this character (ASCII code
97).
There are programs known as font editors,
permitting to define your own fonts by
selecting the squares to be made black in
the matrix.
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Non-numerical information in digital
computers (4)
Complete screen is also a bitmap. Resolution is: 800×600 pixels (old),
1024×768 (most common SVGA), 1152×864, 1280×768 (good for movies),
1600×1200, 1792×1344. The number of bytes used in the screen memory (part
of RAM reproducing the screen) depends also on the number of colors to be
reproduced.
Bitmap representation of graphics is also used in printers. Resolution here is
indicated not by the pixels but by dots (essentially the same but the technical
name is different) per inch: dpi. Typical resolutions are 150 dpi (poor), 300 dpi
(standard), 600 dpi (good quality, I prepare drawings for my lectures in 600 dpi),
1200 dpi (very high quality). Say, „letter” format sheet (8,5×11 inch) printed as
full page picture requires 300×300×8,5×11 (8415000) bits to be sent from the
PC to the printer (B&W only).
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Non-numerical information in digital
computers (5)
Bitmaps vs. Vector graphics
pixels visible
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Non-numerical information in digital
computers (6)
Bitmaps vs. Vector graphics
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