Adaptec AHA-2740W User Manual page 112

AHA-2740W/2742W/2744W User's Guide
devices it requests data from the
first device, which temporarily
disconnects from the SCSI bus
while it is seeking the data. Dur-
ing this delay the system
requests data from the second
device, and while that device
disconnects from the bus to seek
the data, the first device starts
sending the requested data over
the bus, etc. The AHA-2740W/
2742W/2744W and all other
Adaptec host adapters fully sup-
port multithreaded I/O.
P
PC-AT
A family of small computers
sold by IBM, also called the Per-
sonal Computer/AT family of
computers. The name is trade-
marked by IBM.
PIO
See Programmed Input/Output.
Programmed Input/Output
A method of data transfer in
which the host microprocessor
transfers data to and from mem-
ory via the computer's I/O
ports. PIO enables very fast data
transfer rates, especially in
single-tasking operating systems
like DOS.
R
RAM
Random Access Memory. Mem-
ory of which any byte can be
accessed directly in a single
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memory cycle. Information can
be read from and written to the
memory.
ROM
Read-Only Memory. Memory in
which any byte can be read but
not written.
S
Scatter/Gather
A device driver feature that
allows the host adapter to mod-
ify the transfer data pointer so
that a single host adapter trans-
fer can transfer to many seg-
ments of memory in a single
transfer, minimizing interrupts
and overhead.
SCB
SCSI Control Block. The mecha-
nism used in Emulation mode to
transfer control information to
and from the board.
SCSI
Small Computer Systems Inter-
face. A bus interface standard
that defines physical and electri-
cal characteristics for hardware
devices. SCSI provides a stan-
dard interface that enables many
different kinds of devices such
as disk drives, magneto-optical
disks, CD-ROM drives, and tape
drives to interface with the host
computer.
SCSI Bus
One or more SCSI peripheral
devices and a host adapter, con-
nected by cables in a daisy-chain
Page: Glossary-6
ECN Date: 8/9/94
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