CompTIA A+ Certification All-in-One Exam Guide

account database and a common security policy. On the Internet, a domain is
a group of computers that share a common element in their hierarchical
name. Other types of domains exist—e.g., broadcast domain, etc.
domain controller A computer running Windows Server that stores a set of
domain accounts.
domain-based network Network that eliminates the need for logging on to
multiple servers by using domain controllers to hold the security database for
all systems.
DoS (denial of service) An attack on a computer resource that prevents it
from performing its normal operations, usually by overwhelming it with large
numbers of requests in an effort to monopolize its resources.
DOS (Disk Operating System) First popular operating system available for
PCs. A text-based, single-tasking operating system that was not completely
replaced until the introduction of Windows 95.
dot-matrix printer Printer that creates each character from an array of dots.
Pins striking a ribbon against the paper, one pin for each dot position, form
the dots. May be a serial printer (printing one character at a time) or a line
printer.
double-sided RAM RAM stick with RAM chips soldered to both sides of
the stick. May only be used with motherboards designed to accept doublesided
RAM. Very common.
dpi (dots per inch) Measure of printer resolution that counts the dots the
device can produce per linear (horizontal) inch.
DRAM (dynamic random access memory or dynamic RAM) Memory
used to store data in most personal computers. DRAM stores each bit in a
“cell” composed of a transistor and a capacitor. Because the capacitor in a
DRAM cell can only hold a charge for a few milliseconds, DRAM must be
continually refreshed, or rewritten, to retain its data.
drive cloning Taking a PC and making a duplicate of the hard drive,