IBM 3590 - Manual (Page 154)

premounted tape cartridges or the manual
loading of single tape cartridges.
CCR. Channel command retry.
channel. A device that connects the processing
unit and main storage with the I/O control unit.
channel command. An instruction that directs a
data channel, control unit, or device to perform
an operation or set of operations.
command. A control signal that initiates an
action or the beginning of a sequence of actions.
contingent allegiance. (1) A condition in which
a drive owes a response to a specific channel
path because of a unit check. (2) A condition
generated by a check condition status during
which a target preserves sense data.
control unit. A device that controls input and
output operations of one or more devices.
control unit function. A device that contains
the functional logic for controlling the input and
output operations at one or more devices.
D
DASD. Direct-access storage device.
data. Any representations such as characters or
analog quantities to which meaning is, or might
be, assigned.
data base. A set of data, consisting of at least
one file, that is sufficient for a given purpose or
for a given data-processing system.
data compression. An algorithmic
data-reduction technique that encodes data from
the host and stores it in less space than
unencoded data. The original data is recovered
by an inverse process called decompression.
data-compression ratio. The number of host
data bytes divided by the number of encoded
bytes. It is variable depending on the
characteristics of the data being processed. The
more random the data stream, the lower the
opportunity to achieve compression.
dataset. The major unit of data storage and
retrieval, consisting of a collection of data in one
of several prescribed arrangements and described
by control information to which the system has
access.
deferred unit check. A condition in which a
drive returns a unit check indication for an event
that occurred asynchronously with the channel
commands. The deferred unit check may not
refer to the command that receives the
indication.
drive loaded. A condition of a tape drive in
which a tape cartridge has been inserted in the
drive, and the tape has been threaded to the
beginning-of-tape position. Also known as a
mount.
dump. To write the contents of storage, or of a
part of storage, usually from an internal storage
to a external medium, for a specific purpose such
as to allow other use of storage, as a safeguard
against faults or errors, or in connection with
debugging.
E
effective data rate. The average number of bits,
bytes, characters, or blocks per unit time
transferred from a data source to a data sink and
accepted as valid. The rate is expressed in bits,
bytes, characters, or blocks per second, minute,
or hour.
effective recording density. The number of user
bytes per unit of length of the recording
medium.
enable. To provide the means or opportunity.
The modification of system, control unit, or
device action through the change of a software
module or a hardware switch (circuit jumper)
position.
ERA. Error-recovery action performed by the
host.
ERP. See error-recovery procedures.
134 3590 Operator Guide