erate at a time (magnetic tape units and disk access
mechanisms, for example), and the control unit is
shared among a number ofI/O devices. On the other
hand, in electronicI/O devices such as the channel-to
channel adapter, the control unit does not have an
identity of its own.
From the user's point of view, most functions per
formed by the control unit can be merged with those
performed by theI/O device. In view of this, the con
trol unit normally is not identified, and execution ofI/O operations is described in this manual as if the I/O devices communicated directly with the channel. Ref
erence is made to the control unit only when a func
tion performed by it is emphasized or when sharing
of the control unit among a number of devices affects
the execution ofI/O operations. Channels The channel directs the flow of information between I/O devices and main storage. It relieves the CPU of
the task of communicating directly with the devices
and permits data processing to proceed concurrently
withI/O operations.
The channel provides a standard interface for con
necting different types ofI/O devices to the CPU and
to main storage. It accepts control information from
theCPU in the format supplied by the program and
changes it into a sequence of signals acceptable to a
control unit. After the operation with the device has
been initiated, the channel assembles or disassembles
data and synchronizes the transfer of data bytes over
the interface with main-storage cycles. To accomplish
this, the channel maintains and updates an address
and a count that describe the destination or source of
data in main storage. When anI/O device provides
signals that should be brought to the attention of the
program, the channel again converts the signals to a
format compatible to that used in theCPU. The channel contains all the common facilities for
the control ofI/O operations. When these facilities are
provided in the form of separate autonomous equip
ment designed specifically to controlI/O devices, I/O operations are completely overlappcd with the activity
in theCPU. The only main-storage cycles rcquired
duringI/O operations in such channels are those need
ed to transfer data and control information to or from
thefinal locations in main storage. These cycles do
not interfere with theCPU program, except when both
theCPU and the channel concurrently attempt to refer
to the same main storage.
Alternatively, the system may use to a greater or
lesser extent the facilities of theCIJU for controlling I/O devices. When the CPU and the channel share common
84
equipment, interference varies from delaying theCPU by occasional cycles to a complete lockout of CPU activity, depending on the extent of sharing and on the I/O data rate. The sharing of the equipment, however,
is accomplished automatically, and the program is not
aware ofCPU delays, except for an increase in execu
tion time.
Modesof Operation
Data can be transferred between main storage and anI/O device in two modes: burst and multiplex.
In burst mode, theI/O device monopolizes all chan
nel controls and stays logically connected on theI/O interface for the transfer of a burst of information. Only one dcvice can be communicating with the
channel during the time a burst is transferred. The
burst can consist of a few bytes, a whole block of data,
or a sequence of blocks with associated control and
status information.
In multiplex mode, the facilities in the channel may
bc shared by a number of concurrentI/O operations.
The multiplex mode causes allI/O operations to be
split into short intervals of time during which only a
segment of information is transferred over the inter
face. The intervals associated with different operations
are intermixed in response to demands from theI/O devices. The channel controls are occupied with any
one opcration only for the time required to transfer a
segment of information. The segment can consist of a
single byte of data, a few bytes of data, or a control
sequence such as initiation of a new operation or a
status report from the device.
Short bursts of data can appear in both the burst
and multiplex modes of operation. The distinction be
tween a short burst occurring in the multiplex mode
and an operation in the burst mode is in the length of
the bursts. Whenever the burst causes the device to
be connccted to the channel for more than approxi
mately100 microseconds, the channel is considered to
be operating in the burst mode.Operation in burst and multiplex modcs is differ
entiated because of the way the channels respond to
r/o instructions. A channel operating in the burst mode
appears busy to newI/O instructions, whereas a chan
nel operating in the multiplex mode is available for
initiation of new operations. A channel that can op
erate in both modes determines its mode of operation
by time-out. If such a channel happens to be com
municating with anI/O device at the instant a new I/O instruction is issued, action on the instruction is
delayed until the current mode of operation is estab
lished. NewI/O operations are initiated only after the
channel has serviced all outstanding requests for data
transfer for previously initiated operations.
mechanisms, for example), and the control unit is
shared among a number of
hand, in electronic
channel adapter, the control unit does not have an
identity of its own.
From the user's point of view, most functions per
formed by the control unit can be merged with those
performed by the
trol unit normally is not identified, and execution of
erence is made to the control unit only when a func
tion performed by it is emphasized or when sharing
of the control unit among a number of devices affects
the execution of
the task of communicating directly with the devices
and permits data processing to proceed concurrently
with
The channel provides a standard interface for con
necting different types of
to main storage. It accepts control information from
the
changes it into a sequence of signals acceptable to a
control unit. After the operation with the device has
been initiated, the channel assembles or disassembles
data and synchronizes the transfer of data bytes over
the interface with main-storage cycles. To accomplish
this, the channel maintains and updates an address
and a count that describe the destination or source of
data in main storage. When an
signals that should be brought to the attention of the
program, the channel again converts the signals to a
format compatible to that used in the
the control of
provided in the form of separate autonomous equip
ment designed specifically to control
in the
during
ed to transfer data and control information to or from
the
not interfere with the
the
to the same main storage.
Alternatively, the system may use to a greater or
lesser extent the facilities of the
84
equipment, interference varies from delaying the
is accomplished automatically, and the program is not
aware of
tion time.
Modes
Data can be transferred between main storage and an
In burst mode, the
nel controls and stays logically connected on the
channel during the time a burst is transferred. The
burst can consist of a few bytes, a whole block of data,
or a sequence of blocks with associated control and
status information.
In multiplex mode, the facilities in the channel may
bc shared by a number of concurrent
The multiplex mode causes all
split into short intervals of time during which only a
segment of information is transferred over the inter
face. The intervals associated with different operations
are intermixed in response to demands from the
one opcration only for the time required to transfer a
segment of information. The segment can consist of a
single byte of data, a few bytes of data, or a control
sequence such as initiation of a new operation or a
status report from the device.
Short bursts of data can appear in both the burst
and multiplex modes of operation. The distinction be
tween a short burst occurring in the multiplex mode
and an operation in the burst mode is in the length of
the bursts. Whenever the burst causes the device to
be connccted to the channel for more than approxi
mately
be operating in the burst mode.
entiated because of the way the channels respond to
r/o instructions. A channel operating in the burst mode
appears busy to new
nel operating in the multiplex mode is available for
initiation of new operations. A channel that can op
erate in both modes determines its mode of operation
by time-out. If such a channel happens to be com
municating with an
delayed until the current mode of operation is estab
lished. New
channel has serviced all outstanding requests for data
transfer for previously initiated operations.