The interruption code of a new psw is not loaded
since a new interruption code is always storcd. The
instruction-length code in a new psw is similarly ig­
nored since it is unpredictable for all interruptions
other than program or supcrvisor call. The protcction
key of a new psw is stored unchanged when the pro­
tection feature is installed. When the feature is not
installed, the protection key is made zero upon storing.
Programming Note
When interruption sources are not masked off, thc
order of priority in handling the intcrruption sub­
routines is machine check, I/O, external, and program
or supervisor call. This order can be changed to some
extent by masking. The priority rule applies to inter­
ruption requests made simultaneously. An interruption
request made after some interruptions have already
been taken is honored according to the priority pre­
vailing at the moment of request.
Interruption Exceptions
The only exception that can cause a program inter­
ruption during an interruption is a specification ex­
Specification: The protection feature is not installed,
and a new psw with nonzero protection kcy is loaded.
A program interruption is taken immediately upon
loading thc new PSW, regardless of the type of inter­
ruption introducing thc erroneous protection key and
prior to any other pending interruptions. The protec­
tion key is made zero when the psw is stored.
If the new psw for the program interruption has a
nonzero protection key, another program interruption
occurs. Since this second program interruption intro­
duces the same unacceptable protection key in the
new PSW, the process is repeated with the CPU caught
in a string of program interruptions. This string can be
broken only by initial program loading or system reset.
The instruction address in a new psw is not tested
for availability or resolution as the psw is fetched
during an interruption. However, an unavailable or
odd instruction address is detected as soon as the
instruction address is used to fetch an instruction.
These exceptions are described in the section on
normal sequential operation.
If the new psw for the program interruption has an
unacceptable instruction address, another program
interruption occurs. Since this second program inter­
ruption introduces the same unaccept'able instruction
address, a string of program interruptions is estab­
lished. This string may be broken by an external or I/O interruption. If these interruptions also have an
unacceptable new psw, new supervisor information
must be introduced by initial program loading or by
manual intervention.
Transfer of information to and from main storage,
other than to or from the central processing unit or via
the direct control path, is referred to as input and out­
put operation. An inputloutput (1/0) operation in­
volves the use of an inputloutput device. Input! out­
put devices perform 110 operations under control of
control units, which are attached to the central proc­
essing unit (cpu) by means of channels.
This portion of the manual describes, from the pro­
gramming point of view, the control of 110 devices by
the channels and the CPU. The programmed control
procedures apply to all 1/0 operations and are inde­
pendent of the type of 110 device, its speed, or its
mode of op·eration. Attachment of Input/Output Devices Input / Output Devices
Input! output devices provide external storage and a
means of communication between data processing
systems or between a system and the external world.
Input! output devices include such equipment as card
read-punches, magnetic tape units, direct-access-stor­
age devices (disk or drum), typewriter-keyboard de­
vices, printers, TELE-PROCESSING devices, and process
control equipment.
Most types of 110 devices, such as printers, card
equipment, or tape devices, deal directly with external
documents, 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 reach a physical
recording medium outside main storage; the IBM 2702
Transmission Control handles transmission of informa­
tion between the data processing system and a remote
station, and its input and output are signals on a
transmission line. Furthermore, the equipment in this
case may be time-shared for a number of concurrent
operations, and it is denoted as a particular 110 device
only during the time period associated with the opera­
tion on the corresponding remote station.
Input! output devices may be accessible from one or
more channels. Devices accessible from one channel
normally are attached to one control unit only. A
device can be made accessible to two or more chan­
nels by switching it between two or more control units, Input / Output Operations
each attached to a different channel, or by switching
the control unit between two or more channels. Control Units
The control unit provides the logical capability neces­
sary to operate and control an 110 device and adapts
the characteristics of cach device to the standard form
of control provided by the channel.
All communications betwccn the control unit and
the channel take place over the 110 interface. The
control unit accepts control signals from the channel,
controls the timing of data transfer over the 110 inter­
face, and provides indications concerning thc status
of the device.
The 110 interface provides an information format
and a signal sequence common to all 1/0 devices. The
interface consists of a set of lines that can connect a
number of control units to the channel. Except for the
signal used to establish priority among control units,
all communications to and from the channel occur
over a common bus, and any signal provided by the
channel is available to all control units. At anyone
instant, however, only one control unit is logically
connected to the channel. Thc selection of a control
unit for communication with the channel is controlled
by a signal that passes serially through all control units
and permits, sequentially, each control unit to respond
to the signals provided by the channel. A control unit
remains logically connected on the interface until it
has transferred the information it needs or has, or until
the channel signals it to disconnect, whichever occurs
The 110 device attached to the control unit may be
designed to perform only certain limited operations.
A typical operation is moving the recording medium
and recording data. To accomplish these functions, the
device needs detailed signal sequences peculiar to the
type of device. The control unit decodes the com­
mands received from the channel, interprets them for
the particular type of device, and provides the signal
sequence required for execution of the operation.
A control unit may be housed separately or it may
be physically and logically integral with the 110 de­
vice. In the case of most electromechanical devices, a
well-defined interface exists betwecn the device and
the control unit because of the difference in the type of
equipment the control unit and the device contain.
These electromechanical devices often are of a type
where only one device of a group is rcquired to op- Input/Output Operations 83
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