Output Device

CRT

The cathode ray tube (CRT) is a vacuum tube containing an electron gun (a source of electrons or electron emitter) and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam(s) onto the fluorescent screen to create the images. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets and others. CRTs have also been used as memory devices, in which case the visible light emitted from the fluoresecent material (if any) is not intended to have significant meaning to a visual observer (though the visible pattern on the tube face may cryptically represent the stored data)

LCD

A liquid-crystal display (LCD) is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals. Liquid crystals do not emit light directly.

Input Device

In computing, an input device is any peripheral (piece of computer hardware equipment) used to provide data and control signals to an information processing system such as a computer or other information appliance. Examples of input devices include keyboards, mice, scanners, digital cameras and joysticks.

Input Devices Before computers, the phrase "input device" probably was not on the tip of anyone's tongue. It has a dry and impersonal sound, sorta technical - rather like computer talk, don't you think? The phrase is easy to define because a "device" is an instrument that performs a simple task. "Input" is also easy - something put into a system. So in the technology world...
an input device is any tool that feeds data into a computer. For example, a keyboard is an input device, whereas a display monitor is an output device. Mice, trackballs, and light pens are all input devices. Isn't it amusing that under the dry title of "input devices," we get such fun words as mice, joystick, trackball, touch pad, light pen, and puck?

Keyboards

The input of information into your computer is generally done through your keyboard and your mouse or mouse substitute. The keyboard has the distinction of being the standard input device.
The variety of keyboards is almost staggering. No one has to stick with the standard keyboard that comes with the computer. Ergonomically designed keyboards often have unusual sculpted or contoured shapes and a space age look. These keyboards are designed to help avoid repetitive stress injury to the wrists. Some of these keyboards come with foot pedals to spread the work to other limbs. Some keyboards are even designed to take advantage of the strength in the thumbs.
Other keyboards come with extra large keys to help users with sight or hand coordination problems. Wireless keyboards can offer the freedom from being tethered to the computer. In keyboards, if you shop around, you will find that there really is something for everyone.
Your choice of keyboard, like your choice of any input device, is a very personal matter. If only we could test each type of keyboard or mouse for an extended period of time before buying one, but that just doesn't happen in today's marketplace. Computer users should research the many different products offered. Make sure you know if and how you can customize the input device for your own use. Only you can decide which kind of device is most comfortable for you. And that applies to all input devices.

Operating System

Types of Operating System

Types of Operating System

Overlays

     One of the main limitations imposed on programmers in the early days of computing was the size of the computer's memory. If the program was larger than the available memory, it could not be loaded, which placed severe restrictions on program size. The obvious solution would be to increase the amount of memory available, but this would significantly increase the cost of the computer system. One way for a programmer to overcome these limitations was to use overlays. The programmer divided the program into a number of logical sections. A small portion of the program had to remain in memory at all times, but the remaining sections (or overlays) were loaded only when they were needed. The use of overlays allowed programmers to write programs that were much larger than physical memory, although responsibility for managing memory usage rested with the programmer rather than the operating system.

• The entire program and data of a process must be in the physical memory for the process to execute.
• The size of a process is limited to the size of physical memory.
• If a process is larger than the amount of memory, a technique called overlays can be used.
• Overlays is to keep in memory only those instructions and data that are needed at any given time.
• When other instructions are needed, they are loaded into space that was occupied previously by instructions that are no longer needed.
• Overlays are implemented by user, no special support needed from operating system, programming design of overlay structure is complex

Page Stealing

When this supply becomes low, OS uses page stealing to replenish it, that is, it takes a frame assigned to an active user and makes it available for other work. The decision to steal a particular page is based on the activity history of each page currently residing in a central storage frame. Pages that have not been active for a relatively long time are good candidates for page stealing.

Belady's Anamaly

This is a stage which occurs in FIFO page replacement algorithm, in which  a large number of page fault happens.

Preemptive scheduling

Task are usually assigned with priority at times it is necessary to run a certain task that has higher priority before another task although it is running  therefore the running task is interrupted for some time and resumes later when the priority task has finished its execution. This is called preemptive scheduling

E.g. Round-Robin

Non-Preemptive 

 In non preemptive scheduling a running task is executed till. it cannot be interrupted

E.g. FIFO 

Computer Memory

Memory

In computing, memory refers to the physical devices used to store programs (sequences of instructions) or data (e.g. program state information) on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which function at high-speed (i.e. RAM), as a distinction from secondary memory, which are physical devices for program and data storage which are slow to access but offer higher memory capacity. Primary memory stored on secondary memory is called "virtual memory".

Characteristics

Volatile memory

Volatile memory is a type of storage whose contents are erased when the system's power is turned off or interrupted. For example, RAM is volatile; meaning users will lose a document if they do not save their work to a non-volatile classification of memory, such as a hard drive, before shutting down the computer.

Non-volatile

Non-volatile is a term used to describe any memory or storage that is saved regardless if the power to the computer is on or off. The best example of non-volatile memory and storage is a computer hard drive, flash memory, and ROM. If data is stored on a hard drive, it will remain on that drive regardless if the power is interrupted, which is why it is the best place to store your data and documents. This is also how your computer keeps the time and other system settings even when the power is off.