Encyclopedia of Computer Science and Technology

MS-DOS 321duced the mouse to millions of users of its Macintosh. Bythe early 1990s, millions more users were switching theirIBM-compatible PCs from text commands (see ms-dos)to the mouse-driven Windows interface. (See MicrosoftWindows.) Today a desktop PC without a mouse would beas unthinkable as one without a keyboard.Meanwhile, the mouse became smaller and sleeker.Instead of wheels, the contemporary mouse uses a rollingball that turns two adjacent rollers inside the mouse. Amouse pad with a special surface is generally used to provideuniform traction. A newer type of mouse uses opticalsensors instead of rollers to sense its changing position, anddoes not require a mouse pad. Some mice are also cordless,using infrared or wireless data connections.Since mice are generally impracticable for laptop use(see portable computers), designers have offered a varietyof alternatives. These include a trackball (a rolling ball builtinto the keyboard), a touch-sensitive finger pad, or a smallstub that can be moved like a joystick by the fingertip.Most mice now have at least two buttons. Generally, theleft button is used for selecting objects, opening menus, orlaunching programs. The right button is used to bring up amenu of actions that can be done with the selected object.Activating a button is called clicking. It is the operating systemthat assigns significance to clicking or double-clicking (clickingtwice in rapid succession) or dragging (holding a buttondown while moving the pointer). Some mice have a third buttonand/or a small wheel that can be used to scroll the display,but only certain software recognizes these functions.Further ReadingBrain, Marshall, and Carmen Carmack. “How Computer MiceWork.” Available online. URL: http://computer.howstuffworks.com/mouse.htm. Accessed August 15, 2007.Pang, Alex Soojung-Kim. “The Making of the Mouse.” AmericanHeritage. Available online. URL: http://www.americanheritage.com/articles/magazine/it/2002/3/2002_3_48.shtml. AccessedAugust 15, 2007.MS-DOSThe MS-DOS operating system became standard for personalcomputers built by IBM and its imitators (see IBMPC) during the 1980s. Today it has been virtually displacedby various versions of Microsoft Windows (see MicrosoftWindows). However, MS-DOS is important as an expressionof both the limitations of the first generation of personalcomputers and the remarkable patience and ingenuityof its developers and users.DevelopmentBy the end of the 1970s, there were a number of rudimentaryoperating systems for personal computers that useda variety of microprocessors. Generally, their capabilitieswere limited to loading and running programs and providingbasic file organization and access.The most sophisticated early PC operating systemwas CP/M, developed by Gary Kildall’s Digital Researchfor machines based on the Intel 8008 microprocessor. CP/M offered more advanced capabilities such as the abilityto use not only floppy but also hard disks, and includedimproved commands for listing file directories. CP/M evenoffered rudimentary programming tools, such as an editorand assembler, as well as an expandable architecture thatallowed programmers to write utilities that could be ineffect added to the operating system (see assembler).In one of computer history’s greatest missed opportunities,Kildall and IBM failed to come to an agreement in1980 for creating a version of CP/M for the IBM PC, whichwas being developed using the new 16-bit 8086 processor.IBM turned instead to Bill Gates and Microsoft, whohad achieved something of a reputation for their widelyused BASIC language package for personal computers (seeGates, William). Gates agreed to provide IBM with anoperating system, and did so by buying a program calledQDOS (“quick and dirty operating system”), which hadbeen developed by Tim Paterson of Seattle Computer Products.This program was released for the IBM PC as PC-DOSin 1981. However, Microsoft did not sell IBM an exclusivelicense, so when “clone” makers proved able to legally buildIBM-compatible machines, Microsoft could sell them ageneric version called MS-DOS. As the PC market boomed,this provided Microsoft with a large revenue stream, andthe company never looked back.FeaturesMS-DOS offered a rather “clean” design that separates theoperating system into three parts. There is a hardwareindependentI/O system (stored as the file MSDOS.SYS),which processes requests from programs for access to diskfiles or to other devices such as the screen. The routinesneeded to actually communicate with the devices are storedin a separate file, IO.SYS, which is written by each computermanufacturer. (As users from the early 1980s remember,“PC-compatible” machines often had proprietary variationsin areas such as video.) Finally, the command processor(COmmAND.COM) displays the once familiar C:\> promptand waits for the user to type commands. For example,the DIR command followed by a path specification such asC:\TEMP lists the contents of that directory. Programs, too,can be run by typing their names at the prompt.The MS-DOS file system, which remained largelyunchanged until the most recent versions of Windows, usesa FAT (file allocation table) to indicate the disk allocationunits or “clusters” assigned to each file. Starting with MS-DOS 2.0 in 1983, a hierarchical scheme of directories andsubdirectories was introduced, allowing for better organizationof the larger amount of space on hard disks.One interesting feature of MS-DOS is the ability to loada program into memory and keep it available even whileother programs are in use. This “terminate and stay resident”(TSR) function was soon used by enterprising developersto provide utilities such as notepads, calendars orshortcuts (see macro) that users could activate throughspecial key combinations.Users, however, had to struggle to keep enough memoryfree for their applications, resident programs, and devicedrivers. A combination of CPU addressing limitations andthe high price of memory meant that early IBM PCs had