Attachment of Input/Output Devices Input/Output Devices Control Units Channels. Modes of Operation.
Types of Channels . System Operation Compatibility of Operation.
Contents Control of I nput/Output Devices Input/Output Device Addressing States of the I nput/Output System Resetting of the I nput/Output System I/O System Reset I/O Selective Reset .
Effect of Reset on a Working Device
Reset Upon Malfunction Condition Code . I nstruction Formats
List of Instructions CLEAR I/O .
HALT DEVICE HALT I/O START I/O . START I/O FAST RELEASE STORE CHANNEL ID TEST CHANNEL TEST I/O I nput/Output I nstruction Exception Handling Execution of I nput/Output Operations Blocking of Data Channel Address Word . Channel Command Word
Command Code .
Designation of Storage Area
Chaining .
Data Chaining
Command Chaining. Skipping . Program-Controlled Interruption Channel I ndirect Data Addressing
Commands
Write
Read .
Read Backward Control Sense .
Transfer in Channel. Command Retry
Conclusion of Input/Output Operations
Types of Conclusion
Conclusion at Operation Initiation I mmediate Operations .
Conclusion of Data Transfer
Termination by HALT I/O or HALT DEVICE Termination by CLEAR I/O Termination Due to Equipment Malfunction I nput/Output Interruptions I nterruption Conditions
Input/ Output Operations
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186
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199 202 204 204 206 207 208 210 210 210 210 211
212
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Input/Output Operations 185
Conclusion of Input/Output Operations (continued) Priority of Interruptions .227
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.239 Interruption Action.
Channel Status Word
Unit Status Conditions .
Attention. Status Modifier .
Control Unit End
Busy .
Channel End.
Device End .
Unit Check .
Unit Exception
Channel Status Conditions . Program-Controlled I ntHrruption Incorrect Length. Program Check . Protection Check
Channel Data Check.
Channel Control Check. Interface Control Check
Chaining Check .
Contents of Channel Status Word. Information Provided by Channel Status Word Protection Key .
Command Address
Count. Status. Channel Logout . I/O Communications Area.
The transfer of information to or from main storage,
other than to or from the central processing unit or
by means of the direct control path, is referred to as
an input or output operation. An input I output (II 0) operation involves the use of an 110 device.
Inputloutput devices perform 110 operations under
control of control units, which are attached to the
central processing unit (CPU) by means of channels.
This portion of the manual describes the program­
med control of 110 devices by the channels and by
the CPU. Formats are defined for the various types
of I/O control information. The formats apply to all 1/ 0 operations and are independent of the type of 1/0 device, its speed, and its mode of operation.
The formats described include provisions for
functions unique to some 110 device types, such as
an erase gap on a magnetic tape unit. The way in
which a device makes use of the format is defined in
the System Library (SL) or Systems Reference Li­
brary (SRL) publication for the particular device.
Attachlnent of Input/Output Devices
Input/()utput Devices
Inputloutput devices provide external storage and a
means of communication between data processing
systems or between a system and its environment.
186 System/370 Principles of Operation
Inputloutput devices include such equipment as card
readers, card punches, magnetic tape units, direct­
access-storage devices (disks and drums),
typewriter-keyboard devices, printers, teleprocessing
devices, and sensor-based .equipment.
Most types of 110 devices, such as printers, card
equipment, or tape devices, deal directly with exter­
nal media, and these devices are physically distin­
guishable and identifiable. Other types consist only
of electronic equipment and do not directly handle
physical recording media. The channel-to-channel
adapter, for example, provides a channel-to-channel
data transfer path, and the data never reaches a phys­
ical recording medium outside main storage. Similar­
ly, the IBM 2702 TransmIssion Control handles
transmission of information between the data
processing system and a remote station, and its input
and output are signals on a transmission line. Fur­
thermore, in this latter case, the 2702 may be time­
shared for concurrent operation of a number of re­
mote stations, and the 2702 is distinguished as a
particular 110 device only during the time period
associated with the operation on the corresponding
remote station.
An inputloutput device ordinarily is attached to
one control unit and is accessible from one channel.
Switching equipment is available to make some de­
vices accessible to two or more channels by switch-
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