A lesson about computer memory.

1. MEMORY
128 64 32 16 8 4 2 1
+-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+
| | | | | | | | | | | | | | | |
+-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+
Each 'box' represents a single binary digit and its position in a
storage location, known as 'memory'. The 'value' of a binary digit in
any position is shown.
A 32-bit computer processes four bytes at a time

2. BITS & BYTES
1 BIT - stands for Binary digIT
1 Byte = 8 bits
½ byte = 1 nibble = 4 bits
2x byte = 1 word = 16 bits
2x word = 1 long word = 32 bits
1KByte = 1,024 bytes = 210
1MByte = 1,048,576 bytes = 220
1GByte = 230
bytes
1TByte = 240
bytes

3. Memory Addressing
The ability of a computer to access any particular area of its memory
is referred to as "addressing".
Addressing is a function of the number of microprocessor address
lines on its "BUS", and is therefore, hardware dependant.
● 8088/86 cpu can address up to 1MB with its 20 address lines
● 80286 cpu can address up to 16MB with its 24 address lines
● 80386 cpu can address up to 4GB with its 32 address lines
Intel® Desktop Processor Comparison Chart
http://compare.intel.com/PCC/default.aspx?familyid=1&culture=en-US

4. EXTENDED MEMORY MAP
2^32 4GB +---------+
| | This shows a 'grid' of
| | memory 'cells', 8-bits
| | wide by 4GB long.
2^24 16MB| |
| |
2^20 1MB |---------| FFFFF
|---------| E0000
|---------| C0000
640KB |---------| A0000
| |
| |
0KB +---------+ 00000

5. INTERPRETATION OF MEMORY CONTENTS
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.

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

7. Now 256 codes (0---->255)
= orig ASCII + 128 codes called extended character set
European characters
Graphic ''
Scientific '‘
Each keyboard key has an ASCII code assigned eg: 128 is stored
as 3 BYTES OF ASCII DATA
1-------> 49
2-------> 50
8-------> 56
whereas (pure) binary data would be stored as 1 byte

8. 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 { | } ~
http://en.wikipedia.org/wiki/ASCII
http://www.asciitable.com/

9. Microsoft file names
Windows 9x onwards allows long file names and control characters
to be used.
When transferring files to INTERNET systems, the filenames have
to 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 cannot be used.
Older MS-DOS formatted filenames can contain no more than 8
characters, and these must not contain any of the command
characters.

10. 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)

11. 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

12. UNIX drive storage
● There are no drive letters.
● The directory structure starts from the 'root' where
everything is seen as a file.
● Directories are simply 'mounted' into another directory.
● Internet URLs follow Unix filename conventions
Windows: k:drivespace
Unix: /home/user/webspace/images
Internet:
http://news.bbc.co.uk/1/hi/sci/tech/default.stm

13. +-----+
| / |
+--+--+
|
+--------+---+-----+--------+
| | | |
+--+--+ +--+--+ +---+--+ +--+--+
| bin | | usr | | home | | etc |
+-----+ +-----+ +--+---+ +-----+
|
+-------------+-----+
| |
+---+--+ +---+--+
| FRED | | DAVE |
+---+--+ +---+--+
| |
+-----+---+ +----+-----+
| | | |
+---+--+ +---+--+ +--+---+ +---+--+
| work | | web | | work | | web |
+------+ +------+ +------+ +------+

14. Commercial Web filespace
• ISP webspace hosting your own web domain, the industry standard access tool is FTP
• The web root, eg http://www.somenet.com will be kept in a subdirectory
• Every ISP puts this subdirectory in different places, eg.
http://splicehost.com/faq.html
PROGRAMS' LOCATION
- What is the path to PERL? /usr/bin/perl
- What is the path to SENDMAIL? /usr/sbin/sendmail
- What is the path to DATE? /bin/date
- What is the path of my files? /home/yourusername
- Web read-able files are in /home/username/public_html
However, you are only allowed to work from /public_html, and /home/username
is kept invisible to you.
NOTE: DRIVE LETTERS NEVER APPEAR - ONLY UNIX DIRECTORIES

15. Internet file
naming rules are
UNIX