Glossary 5

boot sector
The part of th e boot block tha t contains t he operatin g system load er, a progra m that
starts by itself and loads the operating system.
bps
Bits per second. The number of bits of data that can be transmitted in one second.
Because data compression schemes enable more than one bit per voltage transition,
bps is equivalent to baud only if no compression is used.
buffer
A place, especially in RAM, for the temporary storage of data for the purpose of
speeding up a n operation su c h as printing or drive acces s . Data from a b uffer is
available more quickly than data from where the buffer got it. Typically buffers get
data before it is needed so it is rea dy quickly wh en needed. Simila r to cache.
burst mode
When a device s eizes control of t he bus, sends da ta, then relin quishes contr ol of the
bus. Any time a device sends data without interruption instead of taking turns with
other devices.
bus
A parallel electrical pathway on the system board, connecti ng and shared by the
parts of a computer system (especially the CPU, its support circuitry, memory, and
expansion cards), used for transmitting data or electrical power from one device to
another. Typically the lines in a bus are dedicated to specific functions, such as
control lines, address lines, and data lines. Different bus architectures have different
numbers and a rrangement s of these lin es , and differ ent names (f or example, IS A,
PCI). The most useful way of distinguishing bus architectures is by the number of
simultaneous data bits they can carry. The ISA is a 16-bit bus, while PCI is a 32-bit
bus.
bus master
The ability of an expa nsion card to c ontrol the bus w i t hout needing interventi o n
from the CPU.
byte
Group of eight cont i guous bits. Frequently wr itten as an ei ght-digit bina ry number
or a two-digit hexadecimal number. One letter of the alphabet in ASCII code takes
one byte.
C
cache
A special bloc k of fast memor y u sed for temp or ary stora ge of frequentl y used data
for quick retr ieval. A memor y cache, somet i mes called a ca che store or RAM
cache, is a por tion of memory ma de of high-sp eed static RAM ( SRAM) instead of
the slower and cheaper dynamic RAM (DRAM) used for system memory. Memory
caching is effective because most pr ograms 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.”