| | | |  Kernel | | | | The inner, central part common to most operating systems, responsible for managing the machine's resources. Sometimes previously referred to as the "core" or "nucleus" and distinct from other, higher layers of the operating system's structure. Used in particular when referring to Unix/Linux-based systems. It performs the most basic, low-level functions including control of the computer's hardware - input/output devices including mouse, keyboard, displays etc (either directly or via software) - memory allocation and process control. Execution of the kernel is usually initiated by the boot process, after which it initialises and starts the first process. The kernel is critical and in constant use so is typically loaded into memory at all times. Applications or other parts of the operating system access the kernel and it's resources via System Calls and Inter-Process Communication (IPC). The efficiency and speed of the kernel determines how efficient and quickly the rest of the operating system will run and so kernel design is a critical process. Kernels may be broadly divided into two parts - Monolithic kernels and Microkernels. In the monolithic kernel all of the operating system's "Services" run in the same area of memory as the kernel. This has many advantages but can also mean that a system crash can be caused by something such as a device driver problem, due to the inter-dependancy of the system components. With a Microkernel, a much "leaner" approch is used and many services, other than minimal ones such as memory management, are performed by user-space programs on request by System Calls, leading to an easier-to-maintain kernel. There may be occassions where the system is slowed down due to the number of system calls required, however. Some kernels have been developed that are able to be used by any operating system - an example is the Mach kernel. | | | | | | FAT | | | | File Allocation Table. A file system that Microsoft developed, in the first place, for MS-DOS in 1977 (and that has since come to be known as FAT12 because cluster addresses are 12 bits long) that was used also in revised forms in the first release of Windows versions 95 as FAT16 (cluster addresses of 16 bits). Windows 95 OSR2, Windows 98, Me 2000 and XP use FAT32 with a possible 32-bit cluster address, of which 28 are currently used (although the latter two also use NTFS). The file allocation table maintains a record of where about on a hard, floppy or flash disk the clusters of data that make up a file are stored (clusters are not always contiguous, because of fragmentation). When the computer needs to use a file, it reads the FAT to determine whereabouts the different clusters are located then reassembles them into an entire file. The directory table file stores information about the address of the first cluster for each file (if there is more than one cluster). The second cluster's entry has data about the location of the third cluster etc. The entry for the last cluster contains an end of file code. FAT16 supports 16-bit cluster addresses and maximum partition sizes of up to 2GB. With FAT32 that limit is increased to a theoretical 8TB (TeraBytes) although in practice this is limited to 2TB by the boot sector. File size is still a limiting factor under even FAT32 though, with the maximum file size being 4GB (not unusual these days especially in the case of video files for example). Current "Windows PCs" generally ship with a version of NTFS (Windows NT File System) such as XP or Vista in most cases, removing this restriction and servers running a Microsoft OS with Windows Server 2003 or 2008. | | | | | | Modem | | | | Abbreviated from Modulator/Demodulator. A modem is a device (normally the term refers to hardware rather than software) that converts or "modulates" an outgoing digital signal from a computer to an analogue carrier signal prior to sending it over telephone or power cables (for example) and converts an incoming analogue signal to digital, so that it can be read by the computer. Modems are able to communicate with eachother via communcations lines that carry other information such as telephone voice calls by using i) timing signals so that they can recognise "relevant" data and ignore the rest - in the case of synchronous transmissions - and ii) using a number of protocols (e.g. TCP/IP) for error-correction in the case of asynchronous ones. Wireless modems use radio signals. The set of commands a computer uses to control a modem is called the Hayes Command Set and remain largely unchaged since their introduction by Hayes Communications in the early 1980s when, for the first time, their SmartModem was able to connect and disconnect from the phone line and dial out/hang up without manual human intervention. Modems may be external, in the form of a box that connects to the computer using the ubiquitous RS-232 port, internal, using an expansion slot such as PCI or increasingly, built in to the onboard circuitry of the motherboard as standard. Voiceband modems operate at a maximum speed of 56kbit/s and have gone through a series of "standards" as technology and the speeds possible have increased over the years, including: V.32, V.34, V.70, V.90 and the latest, V.92. With the huge uptake of broadband, however, many ISPs have never bothered upgrading their services to the V.92 standard due to the costs involved, the falling requirement for dialup connections and the fact that the maximum speed did not increase from the 56kbit/s of V.90. | | | |
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