A lesson about interpreting the contents of stored memory data.

1. How to interpret
the contents of
stored memory
data

2. Memory cells store voltages - Interpreted as 0 or 1
Combinations REPRESENT numeric data ie. Letters, numbers,
punctuation marks, graphics characters
Memory is not restricted to using numeric coding
Bytes can represent:
● CPU Instruction codes
● alpha numeric characters (word processors)
● pure binary No's
It's all down to interpretation with the software application.

3. ASCII
AMERICAN STANDARD CODE FOR INFORMATION
INTERCHANGE
Original ASCII, 128 codes (0 to127) represented.
ASCII is a coding standard which means that data is stored
according to the ascii standards rules.
• English alphabet
• punctuation
• control codes
Each keyboard key has an ASCII code assigned

4. A.S.C.I.I. Coding
There are 255 characters available to 8 bits. 0 isn't used.
Only the numbers from 32 to 126 (20 to 7E hex) are defined as
*printable* characters (the others are defined as control codes)
0 1 2 3 4 5 6 7 8 9 A B C D E F
---------------------------------
2 | ! " # $ % & ' ( ) * + , - . /
3 | 0 1 2 3 4 5 6 7 8 9 : ; < = > ?
4 | @ A B C D E F G H I J K L M N O
5 | P Q R S T U V W X Y Z [ ] ^ _
6 | ` a b c d e f g h i j k l m n o
7 | p q r s t u v w x y z { | } ~

5. ASCII has been extensively enhanced.
Modern systems use Unicode, which is a 2-
byte 16-bit system.

6. Further reading
http://en.wikipedia.org/wiki/ASCII
http://www.asciitable.com/
http://www.unicode.org/
http://unicode­table.com/en/
http://en.wikipedia.org/wiki/Unicode
http://www.unicode.org/standard/WhatIsUnicode.html
http://en.wikipedia.org/wiki/List_of_Unicode_characters

7. HOW DATA IS
MANIPULATED AND
CONTROLLED
Disk Operating Systems

8. DOS provides the prime means of interaction between the user
and the computer hardware.
The command interface (DOS prompt) converts keyboard
commands into instructions understood by the machine hardware.
Without DOS there is no means of interpreting input commands.
The GUI "sits" on top of DOS and provides a graphical equivalent of
keyboard commands.
Windows - written by Microsoft for PC's.
UNIX and UNIX clones, eg. Linux, Apple
http://en.wikipedia.org/wiki/Operating_system
http://upload.wikimedia.org/wikipedia/commons/7/77/Unix_history-simple.svg

9. HOW THE OPERATING SYSTEM IS STARTED
THE BOOT PROCESS
Turning on the POWER switch ("Cold start")
Simultaneously pressing CTRL, ALT and DELETE ("Soft start")
The boot process is controlled by the Boot ROM, located on the
motherboard, which contains part of the BIOS.
Functions of Boot ROM:
● Checks hardware
● RAM check
● Looks for a boot disk

10. The Boot Loader
• The BIOS loads into memory the program residing
in the first sector of the boot device,
called the Master Boot Record or MBR.
• The MBR is only 512 bytes in size and contains machine code
instructions for booting the machine, called a boot loader, along with
the partition table which holds the information on how the partitions,
containing file systems, are organised on the drive.
• Once the BIOS finds and loads the boot loader program into
memory, it yields control of the boot process to it, from which the
operating system can be located and run up.
http://en.wikipedia.org/wiki/Master_Boot_Record
http://en.wikipedia.org/wiki/Booting#Modern_boot_loaders

11. The File System
A file system controls how data is stored and retrieved.
The data is stored individually and named, called a "file".
The structure and logic of the file storage is called a "file system".
There are dozens of different systems available, and unassociated
operating systems use unrelated file systems.
For instance,
• MS Windows uses NTFS
• Linux uses ext4
Among the most important aspects of filesystems is understanding
how filenames and access permissions are treated on the two
most common OS's, ie. MS and UNIX / Linux

12. Comparison of filenaming conventions
UNIX Files | Windows files
-------------------------|----------------------------
256 characters filename | 256 character filename
filename.ext.ext.ext | 3 character extension
sensitive to case | not sensitive to case
Windows: k:drivespace
Unix: /home/user/webspace/images
WWW: http://news.bbc.co.uk/1/hi/sci/tech/default.stm
Note: Internet web URLs follow Unix filename conventions

13. Microsoft file names
Older MS-DOS formatted filenames can contain no more than 8
characters, and these must not contain any command characters.
MS Windows now allows long file names and control characters to
be used.

14. File extensions
● The file name is followed by a dot . which separates it from its
extension.
● The file extension is 3 characters long and describes the type of
file.
*.COM - Command file. Runs from the DOS prompt. Small file =<64 KB
*.EXE - Executable file. Larger file >= 64 KB
*.SYS - System file. Installed in memory using an installation program.
*.BAT - Batch File. Contains series of text commands batched together.
Other extensions are produced typically by application software for their data
files, eg .doc, .html, etc.
(The * is used here to represent any filename)

15. It is bad practice to use control characters in a file name..
When transferring files to INTERNET systems, the filenames should
be re-formatted to comply with UNIX standards.
No character is allowed which the system uses for commands or
other functions, such as , /, ., etc.
The underscore character _ is accepted as a separator as a space
must not be used.
URL encoding is needed if a filename and path contains control
characters.

16. Web URLs use hexadecimal encoding
• All URLs address filespace on a web server.
• All URLs obey Unix filenaming rules.
• All Windows web servers obey Unix filenaming rules.
• There are no exceptions at all.
• All URLs that use Windows filenames have to be 'repaired' using URL encoding before
they work on the web.
'What are those "%20" codes in URLs?'
http://www.blooberry.com/indexdot/html/topics/urlencoding.htm
"...Only alphanumerics [0-9a-zA-Z], the special characters "$-_.+!*'()," [not including the
quotes], and reserved characters used for their reserved purposes may be used unencoded
within a URL."
http://www.w3schools.com/tags/ref_urlencode.asp
Spaces are converted into plus (+) signs or %20 to avoid semantic ambiguities.
Special characters or 8-bit values are converted into their hexadecimal
equivalents and prefaced with a percent sign (%).

17. Windows drive storage
The DOS system uses drive letters, such as
a: and b: for floppy disk drives
c: for the hard disk drive
d: or e: for a CD-ROM drive
f: onwards for network drives
|_ a:____
|_ b: |_ subdirectories
|_ c: |_
|_ d: |_
|_ e:
|_ f:____
|_
|_ subdirectories