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A desktop computer is a personal computer (PC) in a form intended for regular use at a single location, as opposed to a mobile laptop or portable computer. Early desktop computers are designed to lie flat on the desk, while modern towers stand upright. Most modern desktop computers have separate screens and keyboards.
Prior to the widespread use of microprocessors, a computer that could fit on a desk was considered remarkably small. Early personal computers, like the IBM PC, were "desktop" machines, with a horizontally oriented computer case, usually intended to have the display screen placed on top to save space on the desktop. In modern usage the word "desktop" usually refers to tower cases that are in fact more often located on the floor under the desk than on a desk.
Technically speaking desktop and tower computers are two different styles of computer case that use desk space in varying ways.[original research?] Cases intended for home theater PC systems are usually considered to be desktop cases in both senses, regardless of orientation and placement.[original research?]
Early computers took up the space of a room. Minicomputers generally fit into one or a few refrigerator sized racks. It was not until the 1970s when computers such as the HP 9800 series desktop computers were fully programmable computers that fit entirely on top of a desk. The very first large "programmable calculators/computers" (machines lacking keyboards for text input) were marketed in the second half of the 1960s, starting with Programma 101 (1965) and HP 9100 (1968). More desktop models were introduced in 1971, leading to a model programmable in BASIC in 1972. They used a smaller version of a minicomputer design based on read-only memory (ROM) and had small one-line LED alphanumeric displays. They could draw computer graphics with a plotter. The Wang 2200 of 1973 led operating systems such as Mac (Macintosh) and Windows.
All-in-one computers (short: AIO-PC) are desktop computers that combine the computer's internal components into the same case as the monitor. Apple has manufactured several popular examples of all-in-one computers, such as the original Macintosh of the mid-1980s and the iMac of the late 1990s and 2000s. This form factor was popular during the mid-1980s for computers intended for professional use such as the Kaypro II, Commodore PET, Osborne 1, TRS-80 Model II and Compaq Portable. Other manufacturers like Commodore and Atari included the computer parts into the keyboard instead of the monitor in some of their models. All-in-one PCs are typically more portable than other desktop PCs and some have been built with carrying handles integrated into the case. They can simply be unplugged and transported to a new location.
Like laptops, all-in-one desktop computers are characterized by a comparative lack of upgradeability or hardware customization, as internal hardware is often placed in the back of the visual display unit. Furthermore, in the case of the iMac line since 2002, the CPU and other internal hardware units are, more or less, permanently attached to the motherboard due to space constraints.
However, newer models of all-in-one computers have changed their approach to this issue. Many of the current manufacturers are using standard off-the-shelf components and are designing upgrade convenience into their products.
Driven by the release of Windows 8 which supports touchscreen functionality, many of the newer all-in-one computers are now produced with touchscreen monitors.
Desktops have the advantage over laptops that the spare parts and extensions tend to be standardized, resulting in lower prices and greater availability. For example, the form factor of the motherboard is standardized, like the ATX form factor. Desktops have several standardized expansion slots, like Conventional PCI or PCI express, while laptops only tend to have one mini PCI slot and one PC card slot (or ExpressCard slot). This means that a desktop can be customized and upgraded to a greater extent than laptops. Procedures for (dis-)assembly of desktops tend to be simple and standardized to a great extent too. This tends not to be the case for laptops, though adding or replacing some parts, like the optical drive, hard disk, and adding an extra memory module is often quite simple.
Another advantage of the desktop is that (apart from environmental concerns) power consumption is not as critical as in laptop computers because the desktop is powered from the wall socket. Desktop computers also provide more space for heat to escape. The two large microprocessor manufacturers Intel and AMD develop special CPUs for mobile computers (i.e. laptops) that consume less power and lower heat, but with lower performance levels.
On the other hand, laptop computers offer portability that desktop systems can not due to their small form factor. Laptops also more commonly integrate wireless technologies like WiFi, Bluetooth and 3G, giving them a broader range of options for connecting to the internet, though this trend is changing as more desktop computers come integrated with one or more of these technologies. Wireless internet runs slower on desktop computers with outdated networking cards, and will likely affect laptops being used on the same network. 
A desktop computer needs a UPS to handle electrical disturbances like short interruptions, blackouts and spikes; achieving an on-battery time of more than 20–30 minutes for a desktop PC requires a large and expensive UPS. A laptop with sufficiently charged battery can continue to be used in case of a power outage and is not affected by short power interruptions and blackouts.
An operating system (OS) is the program that after being initially loaded into the computer by a boot program, manages all the other programs in a computer.
Most of today's desktop computers have one of the three major operating systems available. In order of usage share, they are Microsoft Windows, OS X, and Linux. Microsoft Windows and Linux can be used for almost any desktop computer, although OS X can be used on some computers that are not Apple branded, the legality of this is currently disputed. OS X has been designed by Apple to only work on Apple computers unless you have an EFI emulator which creates an environment that can boot OS X on a regular PC. New versions of each of these operating systems are released on a semi-regular basis. The newest version of Microsoft Windows is called Windows 8 which is radically different to the previous OS of Windows. The newest version of OS X is OS X Mountain Lion. Linux is available in multiple distributions, the more popular being Ubuntu, Fedora and openSUSE. Each distribution is typically free, has its own version number and bundled software, but all distributions of Linux contain a Linux kernel.
For Microsoft Windows systems, the average selling price (ASP) showed a decline in 2008/2009, possibly due to low-cost Netbooks, drawing $569 at U.S. retail in August 2008. In 2009, ASP had further fallen to US$533 by January. All in one Mac desktops such as the 21.5 inch iMac start at US$1,199 and all in one PCs utilizing GNU/Linux start as low as US$239. All in one PCs running Windows start at US$399.
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