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bus master
The ability of an expansion card to control the bus without needing intervention from the
CPU.
byte
Group of eight contiguous bits. Frequently written as an eight-digit binary number or a two-
digit hexadecimal number. One letter of the alphabet in ASCII code takes one byte.
C
cache
A special block of fast memory used for temporary storage of frequently used data for quick
retrieval. A memory cache, sometimes called a cache store or RAM cache, is a portion of
memory made of high-speed static RAM (SRAM) instead of the slower and cheaper
dynamic RAM (DRAM) used for system memory. Memory caching is effective because
most programs access the same data or instructions over and over. By keeping as much of
this information as possible in SRAM, the computer avoids accessing the slower DRAM.
See "write-back" and "write-through."
chassis
The metal frame to which the electronic components of the computer (such as the system
board, power supply, and drive bays) are attached. The chassis goes inside the system unit
cover.
checksum
A number, calculated from a block of data, used to verify the integrity of that data. For
example, a modem could send a block of data and include the number of 1's that occur in
the block. The receiving modem could count the number of 1's it receives and compare its
own number with the transmitted number. If the numbers are the same, the transmission was
probably OK.
clock
Electronic timer (oscillator) used to synchronize computer operations. The oscillator
generates the timing pulses that coordinate and enable the flow of data within a digital
device. Also a circuit powered by battery that keeps track of the date and time for human
and various system requirements.
clock speed
The frequency at which a clock oscillates. In microcomputers, measured in MHz. The faster
the clock, the faster the computer can compute.
CMOS
Complementary Metal Oxide Semiconductor. A chip on the system board that contains
nonvolatile memory. CMOS is backed up by an internal lithium battery that preserves
clock/calendar data and system configuration parameters stored in CMOS.
codec
Coder-decoder. Used primarily with Internet phone and video conferencing products, this
software compresses digitized data and decompresses it at the other end, allowing for faster
data transfer and less delays.
Glossary 5
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