Files

Both program code and user data are saved as files on the hard disk. Often, you can rec

Files

Both program code and user data are saved as files on the hard disk. Often, you can recognize the type of file by its suffix. Here are some examples:

Text and Code

Now we have seen the PCs user data, which are always digitized. But there are many different kinds of data in the PC. You can differentiate between 2 fundamental types of data:

• Program code, which is data, that allows the PC to function.

• User data, like text, graphics, sound.

The fact is, that the CPU must have instructions to function. You can read more about this in the CPU module. An instruction is a string of data, of 0s and 1s. The CPU is designed to recognize these instructions, which arrive together with the user input data to be processed.

The program code is thus a collection of instructions, which are executed one by one, when the program runs. Each time you click the mouse, or hit a key on the keyboard, instructions are sent from your software (program) to the CPU, telling it what to do next.

User data are those data, which tells the software how to respond. The letters, illustrations, home pages, etc., which you and I produce, are created with appropriate software.

Example

Let us imagine a stream of bits sent from the keyboard to the computer. When you type, streams of 8 bits are sent to the computer. Let us look at a series of bits:

001100010011001000110011

Bits are combined into bytes (each 8 bits). These 24 bits are interpreted as three bytes. Let us read them as bytes: 00110001, 00110010, and 00110011.

When we convert these byte binary numbers to decimal numbers, you will see that they read as 49, 50, and 51 in decimal numbers. To interpret these numbers, we have to look at the ASCII table. You will find that you have typed the numbers 1, 2, and 3.

Example

Let us imagine a stream of bits sent from the keyboard to the computer. When you type, streams of 8 bits are sent to the computer. Let us look at a series of bits:

001100010011001000110011

Bits are combined into bytes (each 8 bits). These 24 bits are interpreted as three bytes. Let us read them as bytes: 00110001, 00110010, and 00110011.

When we convert these byte binary numbers to decimal numbers, you will see that they read as 49, 50, and 51 in decimal numbers. To interpret these numbers, we have to look at the ASCII table. You will find that you have typed the numbers 1, 2, and 3.

When you write the word "summer", you write 6 letters. If the computer has to process that word, it will be digitized to 6 bytes. In other words, the word summer occupies 6 bytes in the PC RAM, when you type it, and 6 bytes on the hard disk, if you save it.

Content	File name
Program code	START.EXE, WIN.COM, HELP.DLL, VMM32.VXD
User data	LETTER.DOC, HOUSE.BMP, INDEX.HTM

This is written as an introduction to naming files. The file name suffix determines how the PC will handle the file.

We can calculate that you can make 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 different patterns, since each of the 8 bits can have 2 values.

• 2⁸ (two in the power of eight) is 256.

  Character	Bit pattern	Byte number		Character	Bit pattern	Byte number
  A	01000001	65		¼	10111100	188
  B	01000010	66		.	00101110	46
  C	01000011	67		:	00111010	58
  a	01100001	97		$	00100100	36
  b	01100010	98		\	01011100	92
  o	01101111	111		~	01111110	126
  p	01110000	112		1	00110001	49
  q	01110001	113		2	00110010	50
  r	01110010	114		9	00111001	57
  x	01111000	120		©	10101001	169
  y	01111001	121		>	00111110	62
  z	01111010	122		‰	10001001	137

  Then there are 256 different bytes!

Introduction

The most basic data processing is word processing. Let us use that as an example. When we do word processing, we work at a keyboard similar to a typewriter. There are 101 keys, where we find the entire alphabet A, B, C, etc. We also find the digits from 0 to 9 and all the other characters we need:,.-;():_?!"#*%&etc..

ASCII

ASCII means American Standard Code for Information Interchange. It is an industry standard, which assigns letters, numbers, and other characters within the 256 slots available in the 8 bit code.

The ASCII table is divided in 3 sections:

• Non printable system codes between 0 and 31.

• "Lower ASCII" between 32 and 127. This part of the table originates from older, American systems, which worked on 7 bit character tables. Foreign letters, like Ø and Ü were not available then.

• "Higher ASCII" between 128 and 255. This part is programmable, in that you can exchange characters, based on which language you want to write in. Foreign letters are placed in this part.