Text b. Computer System

A computer system is made up of five fundamental units. The central part of a computer is the Central Processing Unit (CPU). The CPU includes 2 units: the Control Unit (CU), and the Arithmetic-Logical Unit (ALU).

The Arithmetic-Logical Unit is the "brain" of the computer system. It performs the most important operations of a computer system - arithmetic and logical operations on the data.

On the other hand, the Control Unit functions more like a "house master". It decides the sequence of operations for the system. It generates and manages all control signals to streamlining the operations and flow of data within the ALU.

Memory module is the next part of a computer system. 'The memory of the computer is used to store information. Several types of memories are normally present in a computer system. These two basic types of memories are respectively known as ROM (Read Only Memory) or RAM (Random Access Memory). Secondary memories can reside externally to the computer. A typical example would be diskettes.

The last part of a computer system is the input /output - module. The input - module supplies data to the ALU. It can be, for example, a keyboard. The output - module on the other hand displays to the outside world the data exiting from the ALU. A familiar example is the video display unit (VDU).

Personal computer systems are used now on a wide scale.

The basic components of a personal computer consist of the system unit, the keyboard, the video display unit, the printer and optionally the mouse. They can be easily connected and ready to be used in a matter of minutes.

The system unit contains a 16 / 24 / 32 - bit microprocessor, the two diskette drives housed in a single table - top unit. The system unit is powered from a standard 230 - volt ac grounded wall outlet.

The computer system has two modes: alphanumeric (A / N) or text mode and graphics mode.

A few words about A / N mode.

In text mode, it supports either a 40 / 25 or an 80 / 25 display format. Each character is format in an 8 / 8 character box. Characters can be black or white or displayed in colour with 8 background colours and 16 foreground colours. Multiple pages of text may also be stored in the adapter.

The display adapter card contains ROM character generator that provides 256 characters for displaying on the screen. In addition to the standard 96 ASCII characters, there are special characters for supporting games and text processing, plus international character and symbols, line graphics, scientific notations and Greek characters.

Each display character position is defined by two bytes in the adapter 16K RAM. One byte is for the display character code byte, which is the ASCII character code and the second byte is the attribute.

Thus, for the 80 column per 25 rows display, it's required 4000 bytes.

The second possible mode is a graphics mode. The adapter card has two graphics modes - a 320 / 200 four colour mode and a 640 / 200 monochrome mode. In the 320 / 200 mode, each dot on the screen can be programmed with any of the four colours.

The 320 / 200 medium resolution mode uses 16000 bytes of memory, organised in 4 pixels horizontally per byte. The screen is divided into two buffers, the first 8K buffer holding data for the even lines and the second hold data for the odd lines. Each pixel on the screen is defined by two bits

The 640 / 200 high resolution mode requires 16Kb. Every bit is mapped to a pixel on the screen. So, one byte will represent 8 pixels on the screen.

The personal computer can be connected to either 84-keys or 101-keys keyboard.

The 84-keys keyboard is a AT / XT compatible keyboard. It is changeable to the AT or XT by selective switch at the back of the keyboard.

Various languages keyboards are available: English, German, Russian and others.

The 101-keys keyboard with an outside is to select XT or AT mode. It is basically an intelligent and detachable keyboard.