| OC |
| (Optical Carrier) The
transmission speeds defined in the SONET specification. OC defines transmission by optical
devices, and STS is the electrical equivalent. See DS. Service Speed (Mbps) OC-1 STS-1 51.84 (28 DS1s or 1 DS3) OC-3 STS-3 155.52 (3 STS-1s) OC-3c STS-3c 155.52 (concatenated) OC-12 STS-12 622.08 (12 STS-1, 4 STS-3) OC-12c STS-12c 622.08 (12 STS-1, 4 STS-3c) OC-48 STS-48 2488.32 (48 STS-1, 16 STS-3) OC-192 STS-192 9953.28 (192 STS-1, 64 STS-3) OC-768 STS-768 38813,12 (768 STS-1, 256 STS-3) |
| operating system |
| The master control program that runs the
computer. The first program loaded when the computer is turned on, its main part, the
"kernel," resides in memory at all times. The operating system sets the
standards for all application programs that run in the computer. The applications
"talk to" the operating system for all user interface and file management
operations. Also called an "executive" or "supervisor," an operating
system performs the following functions. User Interface All graphics based today, the user interface includes the windows, menus and method of interaction between you and the computer. Prior to the Mac, Windows and Motif (UNIX) interfaces, all interaction was based on commands entered by the user. Operating systems may support optional interfaces and allow a new shell, or skin, to be used instead. Job Management Job management controls the order and time in which programs are run and is more sophisticated in the mainframe environment where scheduling the daily work has always been routine. IBM's job control language (JCL) was developed decades ago. In a desktop environment, batch files can be written to perform a sequence of operations which can be scheduled to start at a given time. Task Management Multitasking, which is the ability to simultaneously execute multiple programs, is available in all operating systems today. Critical in the mainframe and large server environment, applications can be prioritized to run faster or slower depending on their purpose. In the desktop world, multitasking is necessary just for keeping several applications open at the same time so you can bounce back and forth between them. See multitasking. Data Management Data management keeps track of the data on disk, tape and optical storage devices. The application program deals with data by file name and a particular location within the file. The operating system's file system knows where that data are physically stored (which sectors on disk) and interaction between the application and operating system is through the programming interface. Whenever an application needs to read or write data, it makes a call to the operating system (see API). Device Management Device management controls peripheral devices by sending them commands in their own proprietary language. The software routine that knows how to deal with each device is called a "driver." The operating system contains all the drivers for the peripherals attached to the computer. When a new peripheral is added, that device's driver is installed into the operating system. See driver. Security Multiuser operating systems provide password protection to keep unauthorized users out of the system. Large operating systems also maintain activity logs and accounting of the user's time for billing purposes. They also provide backup and recovery routines for starting over in the event of a system failure. History The earliest operating systems were developed in the late 1950s to manage tape storage, but programmers mostly wrote their own I/O routines. In the mid-1960s, operating systems became essential to manage disks, complex timesharing and multitasking systems. Today, all multi-purpose computers from micro to mainframe use an operating system. Special-purpose devices (appliances, games, toys, etc.) generally do not. They usually employ a single program that performs all the required I/O and processing tasks. Common Operating Systems The primary operating systems in use are the many versions of Windows (95, 98, NT, ME, 2000, XP), the many versions of UNIX (Solaris, Linux, etc.), the Macintosh OS, IBM mainframe OS/390 and the AS/400's OS/400. DOS is still used for some applications, and there are other special-purpose operating systems.    |
| optical disk |
| A direct access disk
written and read by light. CD, CD-ROM, DVD-ROM and DVD-Video are
read-only optical disks that are recorded at the time of manufacture
and cannot be erased. CD-R, DVD-R, WORM and magneto-optic (in WORM
mode) disks are write-once. They are recorded in the user's
environment, but cannot be erased. CR-RW, DVD-RAM, DVD-RW and MO
disks are rewritable. Rewritable disks use either magneto-optic (MO) or phase change technology. Used in libraries that hold multiple cartridges, magneto-optic (MO) disks are extremely robust. Phase change disks (CD-RW, DVD-RAM, etc.) are lower cost consumer-oriented products, and DVD-RAM is expected to become very popular. Optical disks have some advantages over magnetic disks. They have higher capacities as removable modules, and they are not subject to head crashes or corruption from stray magnetic fields. They also have a 30-year life and are less vulnerable to extremes of hot and cold. See holographic storage. |
| OS |
| See operating system. |
| OverDrive CPU |
| Intel's trade name for its CPU upgrade chips. For the 486, there are 486 and Pentium OverDrives. Depending on the motherboard, the old chip is either replaced or the new one is installed in the upgrade socket, leaving the old chip intact or removing it. Pentium Overdrive chips install in the original socket. |