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or when describing how sharing of the control unit
among a number of devices affects the execution of
110 operations.
Channels
A channel directs the flow of information between
110 devices and main storage. It relieves the CPU
of the task of communicating directly with the
devices and permits data processing to proceed
concurrently with 110 operations.
A channel provides a means for connecting
different types of 110 devices to the CPU and to
storage. The channel accepts control information
from the CPU in the format supplied by the
program and changes it into a sequence of signals
acceptable to a control unit and device. Similarly,
when an 1/0 device provides signals that should be
brought to the attention of the program, the
channel transforms the signals to information that
can be used in the CPU.
A channel contains facilities for the control of
1/0 operations. During execution of an 1/0
operation involving data transfer, the channel
assembles or disassembles data and synchronizes
the transfer of data bytes with storage cycles. To
accomplish this, the channel maintains and updates
an address and a count that describe the
destination or source of data in storage. When the
channel facilities are provided in the form of
separate autonomous equipment designed
specifically
tq
control 1/0 devices, 1/0 operations
are completely overlapped with the activity in the
CPU. The only storage cycles required during 1/0
operations in such channels are those needed to
transfer data and control information to or from
the final locations in storage. These cycles do not
interfere with the CPU program, except when both
the CPU and the channel concurrently attempt to
refer to the same storage area.
If separate equipment is not provided, facilities
of the CPU are used for controlling 1/0 devices.
When the CPU and channels, or the CPU,
channels, and control units, share common
facilities, 110 operations cause interference to the
CPU, varying in intensity from occasional delay of
a CPU cycle to a complete lockout of CPU activity.
The intensity depends on the extent of sharing and
on the 1/0 data rate. The sharing of the facilities,
however, is accomplished automatically, and the
program is not affected by CPU delays, except for
an increase in execution time.
Modes of Operation
An 1/0 operation occurs in one of two modes:
burst or byte-multiplex.
In burst mode, the 1/0 device monopolizes the
channel and stays logically connected to the
channel for the transfer of a burst of information.
No other device can communicate with the channel
during the time a burst is transferred. The burst
can consist of a few bytes, a whole block of data, a
sequence of blocks with associated control and
status information (the block lengths may be zero),
or status information which monopolizes the
channel. The facilities in a channel capable of
operating in burst mode may be shared by a
number of concurrently operating 110 devices.
Some channels can tolerate an absence of data
transfer during a burst-mode operation, such as
occurs when reading a long gap on magnetic tape,
for not more than approximately 112 minute.
Equipment malfunction may be indicated when an
absence of data transfer exceeds this time.
In byte-multiplex mode, the 110 device stays
logically connected to the channel only for a short
interval of time. The facilities in a channel capable
of operating in byte-multiplex mode may be shared
by a number of concurrently operating 1/0 devices.
In this mode, all 110 operations are split into short
intervals of time during which only a segment of
information is transferred. During such an interval,
only one device is logically connected to the
channel. The intervals associated with the
concurrent operation of multiple 110 devices are
sequenced in response to demands from the
devices. The channel controls are occupied with
anyone operation 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, a status report from the device, or a control
sequence used for initiation of a new operation.
Operation in burst and byte-multiplex modes is
differentiated because of the way the channels
respond to 110 instructions. A channel operating a
device in the burst mode appears busy to new 1/0
instructions, whereas a channel operating one or
more devices in the byte-multiplex mode is capable
of initiating an operation on another device. If a
channel that can operate in either mode is
communicating with an 110 device at the instant a
new 110 instruction is issued, action on the
instruction is delayed by the channel until the
current mode of operation is established.
Furthermore, the new 110 operation is initiated
only after the channel has serviced all outstanding
requests from devices previously placed in
operation.
The distinction between a short burst of data
occurring in the byte-multiplex mode and an
operation in the burst mode is in the length of the
Chapter 12. Input/Output Operations
12-3
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