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[back to SCSI Terminology index page]
D_________________________
Daisy Chain
A method of connecting devices on a bus. On a daisy chained bus, devices not requesting a signal respond to it by passing it on. The daisy chain scheme permits assignment of device priorities based on the electrical position of the device on the bus.
DARPA
The US Defence Advanced Research Projects Agency, which developed the Internet Protocols.
DAT (Digital Audio Tape)
DAT cartridges can hold 1.3 to 2.2 GB of data; these cartridges are small and compact. In DAT drives, the tape moves past the recording head at 0.31 inches per second. The tape heads move in a spiral rotation from the bottom to the top of the tape to write each track diagonally on the tape. The combined movements of the tape and the rotating recording heads deliver a recording speed equivalent to 123 inches per second. Since the recording heads are wider than the tape, each new track overlaps the previous track to achieve a high data density. Alternate tracks are written at alternate angles to distinguish one track from another.
Data Area
In ISO 9660, the space on a CD-ROM where the user data is written. It begins at the address 00:02:16.In ISO 9660, the space on a CD-ROM where the user data is written. It begins at the address 00:02:16.
Data Transfer Rate
A measure of how quickly information can be passed between the computer and another device or between devices. The higher the data transfer rate, the less you'll have to wait for data to get where it needs to go.
DDR SDRAM
Double Data Rate-Synchronous DRAM (DDR SDRAM) is a type of SDRAM that supports data transfers on both edges of each clock cycle, effectively doubling the memory chip's data throughput. DDR-SDRAM is also called SDRAM II.
DDR SDRAM will provide up to 3 times the speed of current solutions, including those of EDO DRAM and conventional SDRAM. Similar to traditional SDRAM, DDR SDRAM moves data on each clock edge, doubling the peak bus bandwidth. A DDR SDRAM rated at 100MHz, will have a burst rate of 200MHz. 100MHz SDRAM will take precedence over the 50MHz regular DRAM.
You may notice "Double Clock", which is, method by which pseudo-synchronization with both rising and falling edges of clock signals provides two times faster data transfer. This is achieved using very fast synchronous DRAM, called DDR SDRAM. It starts from 64-Mbit DDR SDRAM capacity.
High speed is the goal of DDR SDRAM, it represents one of the latest high-speed chip technologies. DDR SDRAM will begin to be used for high-end PCs in early 1999, and emerge as the memory technology of choice for PCs from second half of 1999 through 2000.
DEC
Data Communications Equipment, for examplea modem.
Device
Generally refers to equipment that can be connected to the computer, such as printers hard disks, scanners, and modems. Devices can also be interface cards, such as video cards, SCSI cards, and sound cards. The computer itself may also be referred to as a device.
Device Driver
A software program designed to drive or operate a peripheral device through a specific Operating system. It is operating system and device dependent.
Most SCSI devices require that you add device drivers to your system. Device drivers make the operating system aware that SCSI devices exist and define the configuration of SCSI devices on bootup. Most device drivers also assign a drive letter to the device.
Device ID
See SCSI ID.
Digital Audio Extraction
The process of copying CD-DA audio tracks digitally from your CD-Writer or another CD-ROM drive, to hard disk or to recordable CD. Not all CD-ROM drives support this!
Digital Audio Tape
See DAT
Differential
A SCSI bus configuration in which ezch signal is sent on two wires. The signal is derived by taking the difference in voltage between the two wires, effiectively eliminating unwanted noise in the wire. Maximum cable length of 25 meters. Electrically incompatible with Single Ended devices.
Differential SCSI
A hardware configuration for connecting SCSI devices. It uses a pair of lines for each signal transfer (as opposed to single-ended SCSI which references each SCSI signal to a common ground.)
DIMM
Dual Inline Memory Module. A type of pre-assembled RAM unit, used in some new computers, including the Macintosh PCI models. DIMMs support a wider data path than SIMMs, allowing faster memory access.
DIP Switch
Dual Inline Package. Small on-off switches mounted in computer hardware for purposes of user configuration. When adding expansion cards to computers, DIP switches must often be set to make the card compatible with the computer's configuration.
Direction Keys
Also called Arrow Keys. Keys used at times to navigate around the screen- especially in text areas. Direction keys have arrows pointing up, down, right, and left.
Direct RDRAM
Direct RDRAM technology was developed by the American chip design house, Rambus. Intel's next generation Core Logic chip code name as "Camino" will support only Direct RDRAM. It promises to drastically improve PC performance by speeding up data transfer between the main memory and microprocessor. Concurrent RDRAM will be followed by Direct RDRAM, the technology supported by Intel for PC main memory in the future. DDR SDRAM may find the way into PC main memory as a stepping stone to Direct RDRAM.
Direct DRAM technology is intended to open the data path wider to speed up data rates. It's known to be capable of increasing data rates up to 1.6GB per second, over three times higher than the current RDRAM's data rates of 500MB per second. It will start from 64Mb, and will reach 256Mb by the end of the decade.
In PCs, Direct DRAM will be implemented on memory boards very similar to today's DIMMs, careful mainboard designs will enable bandwidth to reach 800Mbps per pin.
Disc-at-Once
A method of writing in which one or more tracks are written in a single operation. The laser will not stop until the whole disc recording is finished and the disc is closedr --hence no Run blocks and no clicks between audio tracks. (See also Track-at-Once.) Disc-at-Once has only been added/available with the recent generation of CD recorders and recent firmware upgrades. For more information, read "Disk-at-Once, Track-at-Once and Packet Writing".
Disconnect/Reconnect
The host adapter uses this option to allow a device to temporarily disconnect from the SCSI bus. This allows the host adapter to perform other functions on the bus while the device is temporarily disconnected. The device and the host adapter can then reconnect when the bus is needed. The ability of a device to remove itself from a bus to perform a task (such as a tape drive fast-forwarding) and then connecting itself back to the bus after completion of the task.
Disk
A medium for storing information. Information stored on a disk remains there even when you turn your computer off, unlike information stored in memory (RAM). A floppy disk can be inserted and removed from a floppy disk drive, whereas a hard disk is permanently mounted inside its drive.
Disk Cache
A portion of RAM set aside for temporarily holding information read from a disk. It often makes data retrieval faster.
Disk Drive
The mechanism that holds and spins a disk, reading and writing data to it.
Display Adapter
Hardware that converts (adapts) information in video memory to video output.
DLL
Dynamic Link Library. A Windows file, that contains code that can be added to a WIndows program while it is running.
DMA
Direct Memory Access. The ability of an I/O system to transfer data to and from a memory subsystem without CPU intervention. DMA is handled by a DMA controller chip (8237) in the computer(third-party DMA) or by the device itself (bus mastering or first-party DMA).
DMA Bus Master
A feature that allows a peripheral to control the flow of data to and from system memory by blocks, as opposed to PIO (Programmed I/O) where the processor is in control and the flow is by byte.
DOS
Disk Operating System. A single-tasking operating system for the PC. The most common version of DOS is developed by Microsoft.
DOS Protected Mode Interface
See DPMI.
DoubleSpeed SCSI
See Fast-20.
DPMI
DOS Protected Mode Interface. An API that allows programs to use memory beyond the 640K limitation imposed by DOS.
DRAM (Dynamic RAM)
A type of physical memory used in most personal computers. Dynamic RAM uses internal capacitors to store data (a single transistor turns it on of off) which lose their charge over time, so they need constant refreshing to retain data content, otherwise 1 state will turn to 0 state. The end result is that between every memory access is sent an electrical charge that refreshes the chip's capacitors to keep data in a fit state, which cannot be reached while recharging is going on. Reading a DRAM discharges its contents, so they have to be written back to immediately to keep the same data content.
The term dynamic indicates that the memory must be constantly refreshed (reenergized) or it will lose its contents. RAM (random-access memory) is sometimes referred to as DRAM to distinguish it from static RAM (SRAM). Static RAM is faster and more stable than dynamic RAM, but it requires more power and is more expensive.
Driver
See device driver.
D-sub Connector
A widely used family of connectors probably deriving its name from its "D" shape. Specific connectors are denoted by a letter for its size and a number for pin configuration. For example, a DB-15 connector is D-sub connector of size B with pin configuration number 15.
DTE
Data Terminal Equipment, for example, a computer.
Dynamic Link Library
See DLL.
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