RCA Spectra 70 User Manual

Standard Device Byte
(Cont'd)
Sense Bytes
CHANNEL
SERVICING
Input/Output Operation
2. An input/output operation is attempted and the condition code
indicates
that status bits are stored (channel status byte, standard
device byte).
The standard device byte is defined as follows:
Bit Position 24 is the external device request interrupt pending bit.
When set, this bit indicates that a random access device, a data exchange
control or a communications device requires servicing.
Bit Position 25 is the termination interrupt pending bit. When set,
this bit indicates that a termination interrupt condition exists in an input/
output device.
Bit Position 26 is the device busy bit. When set, this bit indicates that
the specified device is busy and cannot accept another operation.
Bit Position 27 is the control busy bit. Not applicable.
Bit Position 28 is the device end bit. When set, this bit indicates that
the specified device has terminated. Another operation can be accepted by
the device if the device busy bit (26) is not set.
Bit Position 29 is the secondary indicator bit. When set, this bit indi-
cates that the specified device has additional indicators to be tested. These
indicators can be brought into main memory by using the Sense command.
Bit Position 30 is the device inoperable bit. When set, this bit indi-
cates that the specified device is inoperable.
Bit Position 81 is the status modifier bit. This bit is used with Command
chaining. When set, this bit indicates that the next Channel Command
Word is skipped. This bit is set as a result of device termination.
• The sense byte, or bytes, are brought into main memory from an
input/output device by using the Sense com'mand. These bytes contain
status information for the device referred to. The exact status information
sent is defined in the Spectra 70 input/output reference manuals for the
individual devices.
• The following sections explain in detail the three types of channel
servicing which may be performed during input/output operations. They
are: servicing a data transfer, end and chain servicing, and interrupt
servicing.
Because channel
servicing requires that the channel utilize main
memory, scratch-pad memory and non-addressable main lnemory (multi-
plexor devices), normal mode processing is held off until the servicing has
been completed. Consequently, channel servicing is time-shared with
normal mode processing. Between service requests, normal mode process-
ing is resumed, or another channel is permitted to service its device (s).
Channel servicing for a device on the multiplexor channel (multiplex
mode) requires more time than channel servicing for a device on a selector
channel. To balance the system's throughput rate, multiplexor channel
servicing is segmented to permit selector channel servicing to break-in if
any selector channels require servicing. After all selector (s) demanding
service have been satisfied, multiplexor servicing is resumed. This tech-
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RCA Spectra 70 Specifications

General

Brand	RCA
Model	Spectra 70
Category	Computer Hardware
Language	English