3. Types of computer systems.

What is the difference between mainframe and microcomputer? Describe their characteristics and functions. What are two main types of PCs? Describe different kinds of portables (laptops, tablet PCs, PDAs, wearable computers).

Mainframe computers are the largest and most powerful. They usually fill a whole room and are sometimes referred to as mainframes or computer installations. Mainframes are large, powerful, expensive computers that are operated by a team of professionals and are used for large-scale computing purposes in banks, big companies and in large institutions like universities or government departments. The most powerful mainframes are called supercomputers.

The most common type of computer is the microcomputer, sometimes called personal computer.

A PC is a computer designed to meet the needs of a single person. It is used by an individual, usually in an interactive mode. There is a wide variety of PCs but two common types are desktop computers and portables. A desktop PC has its own processing unit (or CPU), monitor and keyboard. It is used as a personal computer in the home or as a workstation for group work.

A laptop (also called a notebook PC) is a lightweight computer that you can transport easily. It can work as fast as a desktop PC, with similar processors, memory capacity, and disk drives, but it is portable and has a smaller screen.

A tablet PC refers to a laptop or slate-shaped mobile computer, equipped with a touchscreen or graphics tablet/screen hybrid to operate the computer with a stylus or digital pen, or a fingertip, instead of a keyboard or mouse. It looks like a book, with an LCD screen which you can fold and rotate 180 degrees.

A personal digital assistant or PDA is a tiny pocket-sized computer which can be held in one hand. The term PDA refers to a wide variety of handheld devices, palmtops, smartphones and pocket PCs. For input, you type at a small keyboard or use a stylus - a special pen used with a touch screen to select items, draw pictures, etc.

A wearable computer runs on batteries and is worn on the user's body, e.g. on a belt, backpack or vest; it is designed for mobile or hands-free operation, often incorporating a microphone and a head-mounted display.

4. Types of keyboards (standard keyboards, multimedia keyboards, gaming keyboards, keyboards of laptops/notebooks, GKOS keyboard, virtual keyboards, on-Screen Keyboard, foldable (flexible) keyboards), groups of keys of a standard keyboard (alphanumeric keys, numeric keypad, function keys, cursor keys, Page Up, and Page Down, dedicated keys, Windows key, application key, etc).

A keyboard is a typewriter-like device that allows the user to type in text and commands to the computer. Keyboard is the earliest and one of the most commonly used input devices. There are different types of computer keyboards. Standard keyboards, such as the 101-key US traditional keyboard 104-key Windows keyboards, include alphabetic characters, punctuation symbols, numbers and a variety of function keys. Keyboards on laptops and notebook computers usually have a shorter travel distance for the keystroke and a reduced set of keys. Keyboards with extra keys, such as multimedia keyboards, have special keys for accessing music, web and other frequently used programs and features, such as a mute button, volume buttons or knob and standby (sleep) button. Gaming keyboards have extra function keys, which can be programmed with keystroke macros. For example, 'ctrl+shift+y' could be a keystroke that is frequently used in a certain game. For example, the GKOS keyboard has been designed for small wireless devices.

Virtual keyboards project an image of a full-size keyboard onto a surface. Sensors in the projection unit identify which key is being "pressed" and relay the signals to a computer or personal digital assistant. There is also a virtual keyboard, the On-Screen Keyboard, for use on Windows.

The On-Screen Keyboard is an image of a standard keyboard which the user controls by using a mouse to hover over the desired letter or symbol, and then clicks to enter the letter.. Touchscreens can be used as a keyboard - the user interacts with the computer by lightly pressing their finger on a touch-sensitive-area of the monitor screen. Foldable (flexible) keyboards are made of soft plastic which can be rolled or folded on itself for travel. A standard PC keyboard has various groups of keys. Alphanumeric keys - these represent letters and numbers, arranged as on a typewriter. A numeric keypad appears to the right of the main keyboard and contains numeric and editing keys;

Function keys appear at the top of the keyboard and can be programmed to do special jobs.

Cursor keys include 'arrow keys' which move the insertion point, and keys such as Home, End, Page Up, and Page Down, which let you move around documents. Dedicated keys are used to issue commands or produce alternative characters. For example: Ctrl changes the functions of other keys (e.g. Ctrl + X cuts the selected text).

Windows key opens the Windows Start menu. This key can also be used in combination with other keys to open utilities. For example, pressing the Windows key in combination with the F key opens the Search utility. Application key is used for quick access to shortcut menus and help assistants in Windows.