02 April 2010
01 April 2010
SeConDaRy StoRaGe
Secondary storage or external memory differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its Input/output channels to access secondary storage and transfers the desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down—it is non-volatile. Per unit, it is typically also an order of magnitude less expensive than primary storage. Consequently, modern computer systems typically have an order of magnitude more secondary storage than primary storage and data is kept for a longer time there.
• FLOPPY DISK
Floppy disk is a data storage medium that is composed of a disk of thin, flexible ("floppy") magnetic storage medium encased in a square or rectangular plastic shell.
Floppy disks are read and written by a floppy disk drive or FDD, the initials of which should not be confused with "fixed disk drive", which is another term for a (nonremovable) type of hard disk drive.
• FLOPPY DRIVES
A floppy drive grabs a disk at its centre and spin it insides its plastics jacket. The floppy drive obtains stored data and instruction from a floppy disk and stores them onto the disk. The drive is made up of a box with a slot into which a user inserts a disk. The slot has a drive gate. This drive rotates the disk with a motor inside the drive. Electronic read or write heads read data from the disk and write data to it while the disk rotates. A microcomputer usually has internal floppy drives inside the computer cabinet, but it sometimes has external floppy drive, a separate component outside the cabinet.
• HARD DISK
A hard disk drive (often shortened as hard disk, hard drive, or HDD) is a non-volatile storage device that stores digitally encoded data on rapidly rotating rigid (i.e. hard) platters with magnetic surfaces. Strictly speaking, "drive" refers to the motorized mechanical aspect that is distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.
• OPTICAL DISKS
Optical disk is a any of a variety of information storage disks that are played or read using a laser. Optical disks include compact discs (CDs and CD-ROMs), laser discs and digital versatile discs (or digital video discs; DVDs and DVD-ROMs). WORM [Write Once/Read Many] disks can be used to record data, but once data is recorded it cannot be altered except by obliterating the old version and storing the new version on a previously unused portion of the disk. Magneto-optical disks, such as the rewritable optical disk and the recordable disk used with the Mini Disc player, have a special layer, as of barium ferrite, that can be magnetically polarized by a recording head when heated with a laser. Data or sound may be recorded to and erased from any portion of a magneto-optical disk multiple times.
• MAGNETIC TAPE
Magnetic tape is sequential storage medium used for data collection, backup and archiving. Like videotape, computer tape is made of flexible plastic with one side coated with a ferromagnetic material. Tapes were originally open reels, but were superseded by cartridges and cassettes of many sizes and shapes.
Tape has been more economical than disks for archival data, but that is changing as disk capacities have increased enormously. If tapes are stored for the duration, they must be periodically recopied or the tightly coiled magnetic surfaces may contaminate each other.
• CACHE (CACHE MEMORY)
To store data locally in order to speed up subsequent retrievals. Pronounced "cash." See Web cache and cache. Reserved areas of memory in every computer that are used to speed up instruction execution, data retrieval and data updating. Pronounced "cash," they serve as staging areas, and their contents are constantly changing. There are two kinds: memory caches and disk caches.
• FLOPPY DISK
Floppy disk is a data storage medium that is composed of a disk of thin, flexible ("floppy") magnetic storage medium encased in a square or rectangular plastic shell.
Floppy disks are read and written by a floppy disk drive or FDD, the initials of which should not be confused with "fixed disk drive", which is another term for a (nonremovable) type of hard disk drive.
• FLOPPY DRIVES
A floppy drive grabs a disk at its centre and spin it insides its plastics jacket. The floppy drive obtains stored data and instruction from a floppy disk and stores them onto the disk. The drive is made up of a box with a slot into which a user inserts a disk. The slot has a drive gate. This drive rotates the disk with a motor inside the drive. Electronic read or write heads read data from the disk and write data to it while the disk rotates. A microcomputer usually has internal floppy drives inside the computer cabinet, but it sometimes has external floppy drive, a separate component outside the cabinet.
• HARD DISK
A hard disk drive (often shortened as hard disk, hard drive, or HDD) is a non-volatile storage device that stores digitally encoded data on rapidly rotating rigid (i.e. hard) platters with magnetic surfaces. Strictly speaking, "drive" refers to the motorized mechanical aspect that is distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.
• OPTICAL DISKS
Optical disk is a any of a variety of information storage disks that are played or read using a laser. Optical disks include compact discs (CDs and CD-ROMs), laser discs and digital versatile discs (or digital video discs; DVDs and DVD-ROMs). WORM [Write Once/Read Many] disks can be used to record data, but once data is recorded it cannot be altered except by obliterating the old version and storing the new version on a previously unused portion of the disk. Magneto-optical disks, such as the rewritable optical disk and the recordable disk used with the Mini Disc player, have a special layer, as of barium ferrite, that can be magnetically polarized by a recording head when heated with a laser. Data or sound may be recorded to and erased from any portion of a magneto-optical disk multiple times.
• MAGNETIC TAPE
Magnetic tape is sequential storage medium used for data collection, backup and archiving. Like videotape, computer tape is made of flexible plastic with one side coated with a ferromagnetic material. Tapes were originally open reels, but were superseded by cartridges and cassettes of many sizes and shapes.
Tape has been more economical than disks for archival data, but that is changing as disk capacities have increased enormously. If tapes are stored for the duration, they must be periodically recopied or the tightly coiled magnetic surfaces may contaminate each other.
• CACHE (CACHE MEMORY)
To store data locally in order to speed up subsequent retrievals. Pronounced "cash." See Web cache and cache. Reserved areas of memory in every computer that are used to speed up instruction execution, data retrieval and data updating. Pronounced "cash," they serve as staging areas, and their contents are constantly changing. There are two kinds: memory caches and disk caches.
PrimaRy StoRaGe
Primary storage or main memory or internal memory, often referred to simply as memory, is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner.
This led to a modern random-access memory (RAM). It is small-sized, light, but quite expensive at the same time. The particular types of RAM used for primary storage are also volatile, i.e. they lose the information when not powered.
Main memory is directly or indirectly connected to the CPU via a memory bus. It is actually two buses an address bus and a data bus. The CPU firstly sends a number through an address bus, a number called memory address that indicates the desired location of data. Then it reads or writes the data itself using the data bus. Additionally, a memory management unit (MMU) is a small device between CPU and RAM recalculating the actual memory address, for example to provide an abstraction of virtual memory or other tasks.
As the RAM types used for primary storage are volatile cleared at start up, a computer containing only such storage would not have a source to read instructions from, in order to start the computer. Hence, non-volatile primary storage containing a small start up program (BIOS) is used to bootstrap the computer, that is, to read a larger program from non-volatile secondary storage to RAM and start to execute it. A non-volatile technology used for this purpose is called ROM, for read-only memory the terminology may be somewhat confusing as most ROM types are also capable of random access.
Many types of "ROM" are not literally read only, as updates are possible; however it is slow and memory must be erased in large portions before it can be re-written. Some embedded systems run programs directly from ROM or similar, because such programs are rarely changed. Standard computers do not store non-rudimentary programs in ROM, rather use large capacities of secondary storage, which is non-volatile as well, and not as costly.
Random-access memory usually known by its acronym, RAM is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data.
By contrast, storage devices such as magnetic discs and optical discs rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than data transfer, and the retrieval time varies based on the physical location of the next item.
The word RAM is often associated with volatile types of memory such as DRAM memory modules, where the information is lost after the power is switched off. Many other types of memory are RAM, too, including most types of ROM and a type of flash memory called NOR-Flash.
Read-only memory usually known by its acronym, ROM is a class of storage media used in computers and other electronic devices. Because data stored in ROM cannot be modified (at least not very quickly or easily), it is mainly used to distribute firmware (software that is very closely tied to specific hardware, and unlikely to require frequent updates).In its strictest sense, ROM refers only to mask ROM (the oldest type of solid state ROM), which is fabricated with the desired data permanently stored in it, and thus can never be modified. However, more modern types such as EPROM and flash EEPROM can be erased and re-programmed multiple times; they are still described as "read-only memory"(ROM) because the reprogramming process is generally infrequent, comparatively slow, and often does not permit random access writes to individual memory locations.
An EPROM, or erasable programmable read only memory, is a type of memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in digital circuits. Once programmed, an EPROM can be erased by exposing it to strong ultraviolet light from a mercury-vapor light source. EPROMs are easily recognizable by the transparent fused quartz window in the top of the package, through which the silicon chip is visible, and which permits exposure to UV light during erasing.
This led to a modern random-access memory (RAM). It is small-sized, light, but quite expensive at the same time. The particular types of RAM used for primary storage are also volatile, i.e. they lose the information when not powered.
Main memory is directly or indirectly connected to the CPU via a memory bus. It is actually two buses an address bus and a data bus. The CPU firstly sends a number through an address bus, a number called memory address that indicates the desired location of data. Then it reads or writes the data itself using the data bus. Additionally, a memory management unit (MMU) is a small device between CPU and RAM recalculating the actual memory address, for example to provide an abstraction of virtual memory or other tasks.
As the RAM types used for primary storage are volatile cleared at start up, a computer containing only such storage would not have a source to read instructions from, in order to start the computer. Hence, non-volatile primary storage containing a small start up program (BIOS) is used to bootstrap the computer, that is, to read a larger program from non-volatile secondary storage to RAM and start to execute it. A non-volatile technology used for this purpose is called ROM, for read-only memory the terminology may be somewhat confusing as most ROM types are also capable of random access.
Many types of "ROM" are not literally read only, as updates are possible; however it is slow and memory must be erased in large portions before it can be re-written. Some embedded systems run programs directly from ROM or similar, because such programs are rarely changed. Standard computers do not store non-rudimentary programs in ROM, rather use large capacities of secondary storage, which is non-volatile as well, and not as costly.
Random-access memory usually known by its acronym, RAM is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data.
By contrast, storage devices such as magnetic discs and optical discs rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than data transfer, and the retrieval time varies based on the physical location of the next item.
The word RAM is often associated with volatile types of memory such as DRAM memory modules, where the information is lost after the power is switched off. Many other types of memory are RAM, too, including most types of ROM and a type of flash memory called NOR-Flash.
Read-only memory usually known by its acronym, ROM is a class of storage media used in computers and other electronic devices. Because data stored in ROM cannot be modified (at least not very quickly or easily), it is mainly used to distribute firmware (software that is very closely tied to specific hardware, and unlikely to require frequent updates).In its strictest sense, ROM refers only to mask ROM (the oldest type of solid state ROM), which is fabricated with the desired data permanently stored in it, and thus can never be modified. However, more modern types such as EPROM and flash EEPROM can be erased and re-programmed multiple times; they are still described as "read-only memory"(ROM) because the reprogramming process is generally infrequent, comparatively slow, and often does not permit random access writes to individual memory locations.
An EPROM, or erasable programmable read only memory, is a type of memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in digital circuits. Once programmed, an EPROM can be erased by exposing it to strong ultraviolet light from a mercury-vapor light source. EPROMs are easily recognizable by the transparent fused quartz window in the top of the package, through which the silicon chip is visible, and which permits exposure to UV light during erasing.
BuSeS
A bus is a common digital pathway between resources and devices. In a PC, there are two major types: the system bus and peripheral bus. The system bus, also known as the "front side bus" or "local bus," is the internal path from the CPU to memory and is split into address bus and data bus subsets. Addresses are sent over the address lines to signal a memory location, and data are transferred over the data lines to that location.
System buses transfer data in parallel. In a 32-bit bus, data are sent over 32 wires simultaneously. A 64-bit us uses 64 wires.
• Three Main Bus Architectures
ISA (Industry Standard Architecture): Pronounced "i-suh" ISA stems from the original computer. It was an 8-bit bus originally known as the PC bus and then the XT bus. It was later extended to 16 bits and became the AT bus and eventually the ISA bus.
MCA (Micro Channel Architecture): A 32-bit bus used in IBM P/S series and other IBM models. This architecture allows multi processing that allows several processors to work simultaneously. Micro channel architecture is not compatible with PC bus architecture.
EISA ( Extended Industry Standard Architecture): Pronounced "eesa" this bus was a 32-bit extension of ISA created by major vendors to counter IBM's Micro Channel. EISA slots accepted both EISA and ISA cards, but clock speed was still at the slow ISA rate. EISA was used in servers but later abandoned for PCI.
• Local Buses
A local bus is a computer bus that connects directly, or almost directly, from the CPU to one or more slots on the expansion bus. The significance of direct connection to the CPU is avoiding the bottleneck created by the expansion bus, thus providing fast throughput. There are several local buses built into various types of computers to increase the speed of data transfer. Local buses for expanded memory and video boards are the most common. There are two local-bus systems available today. That is:
VL-Bus Local Bus (VESA Local Bus): VL-Bus specification was introduced by the VESA (Video Electronics Standards Association). the VESA Local Bus was very commonplace on 486 motherboards. Probably a majority of 486-based systems had a VESA Local Bus video card, although early 486 systems never had VLB slots, as VLB debuted years after the introduction of the 486 processor. By 1996, the Pentium (driven by Intel's Triton chipset and PCI architecture) had eliminated the 80486 market and the VESA Local Bus with it. Many of the last 80486 motherboards made have PCI slots in addition to (or completely replacing) the VLB slots.
PCI (Peripheral Component Interconnect): The PCI bus, available in 32- and 64-bit versions, is the most popular bus architecture. It is used in PCs as well as many other platforms. In 2002, PCI Express was introduced, providing greatly enhanced speeds. Typical PCI cards used in PCs include: network cards, sound cards, modems, extra ports such as USB or serial, TV tuner cards and disk controllers. Historically video cards were typically PCI devices, but growing bandwidth requirements soon outgrew the capabilities of PCI. PCI video cards remain available for supporting extra monitors and upgrading PCs that do not have any AGP or PCI Express slots.
Many devices traditionally provided on expansion cards are now commonly integrated onto the motherboard itself, meaning that modern PCs often have no cards fitted. However, PCI is still used for certain specialized cards; although many tasks traditionally performed by expansion cards may now be performed equally well by USB devices.
System buses transfer data in parallel. In a 32-bit bus, data are sent over 32 wires simultaneously. A 64-bit us uses 64 wires.
• Three Main Bus Architectures
ISA (Industry Standard Architecture): Pronounced "i-suh" ISA stems from the original computer. It was an 8-bit bus originally known as the PC bus and then the XT bus. It was later extended to 16 bits and became the AT bus and eventually the ISA bus.
MCA (Micro Channel Architecture): A 32-bit bus used in IBM P/S series and other IBM models. This architecture allows multi processing that allows several processors to work simultaneously. Micro channel architecture is not compatible with PC bus architecture.
EISA ( Extended Industry Standard Architecture): Pronounced "eesa" this bus was a 32-bit extension of ISA created by major vendors to counter IBM's Micro Channel. EISA slots accepted both EISA and ISA cards, but clock speed was still at the slow ISA rate. EISA was used in servers but later abandoned for PCI.
• Local Buses
A local bus is a computer bus that connects directly, or almost directly, from the CPU to one or more slots on the expansion bus. The significance of direct connection to the CPU is avoiding the bottleneck created by the expansion bus, thus providing fast throughput. There are several local buses built into various types of computers to increase the speed of data transfer. Local buses for expanded memory and video boards are the most common. There are two local-bus systems available today. That is:
VL-Bus Local Bus (VESA Local Bus): VL-Bus specification was introduced by the VESA (Video Electronics Standards Association). the VESA Local Bus was very commonplace on 486 motherboards. Probably a majority of 486-based systems had a VESA Local Bus video card, although early 486 systems never had VLB slots, as VLB debuted years after the introduction of the 486 processor. By 1996, the Pentium (driven by Intel's Triton chipset and PCI architecture) had eliminated the 80486 market and the VESA Local Bus with it. Many of the last 80486 motherboards made have PCI slots in addition to (or completely replacing) the VLB slots.
PCI (Peripheral Component Interconnect): The PCI bus, available in 32- and 64-bit versions, is the most popular bus architecture. It is used in PCs as well as many other platforms. In 2002, PCI Express was introduced, providing greatly enhanced speeds. Typical PCI cards used in PCs include: network cards, sound cards, modems, extra ports such as USB or serial, TV tuner cards and disk controllers. Historically video cards were typically PCI devices, but growing bandwidth requirements soon outgrew the capabilities of PCI. PCI video cards remain available for supporting extra monitors and upgrading PCs that do not have any AGP or PCI Express slots.
Many devices traditionally provided on expansion cards are now commonly integrated onto the motherboard itself, meaning that modern PCs often have no cards fitted. However, PCI is still used for certain specialized cards; although many tasks traditionally performed by expansion cards may now be performed equally well by USB devices.
PorTs
Port is an external connecting socket on the outside the computer. This is a pathway into and out of the computer. A port lets users plug in outside peripherals, such as monitors, scanners and printers.
• Serial Ports
Serial port was created as an interface between data terminal equipment and data-communications equipment. It processes data sequentially, as a series of bits, and is used to connect equipment such as a modem or mouse to the computer.
• Parallel Ports
The parallel port processes several data bits in parallel and is used to connect peripherals such as computer printers and optical scanners to the computer. The parallel port is faster, but the serial port is cheaper and requires less power.
• Serial Ports
Serial port was created as an interface between data terminal equipment and data-communications equipment. It processes data sequentially, as a series of bits, and is used to connect equipment such as a modem or mouse to the computer.
• Parallel Ports
The parallel port processes several data bits in parallel and is used to connect peripherals such as computer printers and optical scanners to the computer. The parallel port is faster, but the serial port is cheaper and requires less power.
ExPanSioN SLoTS/BoarDs
• Open/Closed Architectures
Open Architecture is a system whose specifications are made public to encourage third-party vendors to develop add-on products for it. Most microcomputers adopt open architecture. They allow users to expand their systems using optional expansion boards.
Closed Architecture is a system whose technical specifications are not made public. With a machine that has closed architecture, users cannot easily add new peripherals.
• Expansion Slots
Expansion slots are receptacles inside a system unit that printed circuit boards (expansion boards) are plugged into. Computer buyers need to look at the number of expansion slots when they buy a computer, because the number of expansion slots decides future expansion. In microcomputers, the expansion slots are directly connected to the bus.
• Expansion Boards
A printed circuit board that plugs into an expansion slot on the motherboard and extends the computer's capability to control a peripheral device. Also called a "card," "interface card," "adapter" or "controller," all the printed circuit boards that plug into a computer's bus are technically expansion boards, because they expand the computer's capability. Typical examples are the display adapter, network adapter (NIC) and sound card; however, all of these circuits may be contained in chips on the motherboard.
Open Architecture is a system whose specifications are made public to encourage third-party vendors to develop add-on products for it. Most microcomputers adopt open architecture. They allow users to expand their systems using optional expansion boards.
Closed Architecture is a system whose technical specifications are not made public. With a machine that has closed architecture, users cannot easily add new peripherals.
• Expansion Slots
Expansion slots are receptacles inside a system unit that printed circuit boards (expansion boards) are plugged into. Computer buyers need to look at the number of expansion slots when they buy a computer, because the number of expansion slots decides future expansion. In microcomputers, the expansion slots are directly connected to the bus.
• Expansion Boards
A printed circuit board that plugs into an expansion slot on the motherboard and extends the computer's capability to control a peripheral device. Also called a "card," "interface card," "adapter" or "controller," all the printed circuit boards that plug into a computer's bus are technically expansion boards, because they expand the computer's capability. Typical examples are the display adapter, network adapter (NIC) and sound card; however, all of these circuits may be contained in chips on the motherboard.
SysTem CloCK
The clock is a device that generates periodic, accurately spaced signals used for several purposes such as regulation of the operations of a processor or generation of interrupts. The clock circuit uses the fixed vibrations generated from quartz crystal to deliver a steady stream of pulses to the processor. The system clock controls the speed of all the operations within a computer.
The clock speed is the internal speed of a computer. The clock speed is expressed in megahertzes (MHz). 33 MHz means 33 millions= cycles per second. A computer processor’s speed is faster if it has higher clock speed. For example, a 100-MHz processor is four times as fast internally as the same processor running at 25MHz.
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