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The data rate a channel can accommodate
depends also on the way the I/O operation is
programmed. The channel can sustain its highest
data rate when no data chaining is specified. Data
chaining reduces the maximum allowable rate, and
the extent of the reduction depends on the
frequency at which new CCWs are fetched and on
the address resolution of the first byte in each new
storage area. Furthermore, since a channel shares
storage with the CPU and other channels, activity
in the rest of the system affects the accessibility of
storage and, hence, the instantaneous load the
channel can sustain.
In view of the dependence of channel capacity
on programming and on activity in the rest of the
system, an evaluation of the ability of elements in a
specific I/O configuration to function concurrently
must be based on a consideration of both the data
rate and the way the I/O operations are
programmed. Two systems differing in
performance but employing identical complements
of I/O devices may be able to execute certain
programs in common, but it is possible that other
programs requiring, for example, data chaining,
may not run on one of the systems because of the
increased load caused by the data chaining.
Control of Input/Output Devices
The CPU controls I/O operations by means of
eight I/O instructions: CLEAR I/O, HALT
DEVICE, HALT I/O, START I/O, START I/O
FAST RELEASE, STORE CHANNEL ID, TEST
CHANNEL, and TEST I/O.
The instructions TEST CHANNEL and STORE
CHANNEL ID address a channel; they do not
address an I/O device. The other six I/O
instructions address a channel and a device on that
channel.
Input / Output Device Addressing
An I/O device and the associated access path are
designated by an I/O address. The 16-bit I/O
address consists of two parts: a channel address in
the leftmost eight bit positions and a device address
in the rightmost eight bit positions.
The channel address provides for identifying up
to 256 channels. Channels are numbered 0-255. /'
Channel 0 is a byte-multiplexer channel, and each
of channels 1-255 may be a byte-multiplexer,
block-multiplexer, or selector channel.
The number and type of channels and
subchannels available, as well as their address
assignment, depend on the system model and the
particular installation.
The device address identifies the particular I/O
device and control unit on the deSignated channel.
The address identifies, for example, a particular
magnetic-tape drive, disk-access mechanism, or
transmission line. Any number in the range 0-255
can be used as a device address, providing facilities
for addressing up to 256 devices per channel. An
exception is some multiplexer channels that provide
fewer than the maximum configuration of
sub channels and hence eliminate the corresponding
unassignable device addresses.
Devices that do not share a control unit with
other devices may be assigned any device address
in the range 0-255, provided the address is not
recognized by any other control unit. Logically,
such devices are not distinguishable from their
control unit, and both are identified by the same
address.
Devices sharing a control unit (for example,
magnetic-tape drives or disk-access mechanisms)
are assigned addresses within sets of contiguous
numbers. The size of such a set is equal to the
maximum number of devices that can share the
control unit, or 16, whichever is smaller.
Furthermore, such a set starts with an address in
which the number of low-order zeros is at least
equal to the number of bit positions required for
specifying the set size. The high-order bit positions
of an address within such a set identify the control
unit, and the low-order bit positions designate the
device on the control unit.
Control units designed to accommodate more
than 16 devices may be assigned nonsequential sets
of addresses, each set consisting of 16, or the
number required to bring the total number of
.assigned addresses equal to the maximum number
of devices attachable to the control unit, whichever
is smaller. The addressing facilities' are added in
increments of a set so that the number of device
addresses assigned to a control unit does not
exceed the number of devices attached by more
than 15.
The control unit does not respond to any address
outside its assigned set or sets. For example, if a
control unit is designed to control devices having
only the values 0000 to 1001 in the low-order bit
positions of the device address, it does not
recognize addresses containing 1010 to 1111 in
these bit positions. On the other hand, a control
unit responds to all addresses in the assigned set,
regardless of whether the device associated with the
address is installed.
If
no control unit responds to
an address, the
110
device appears not operational.
If
a control unit responds to an address for which
Chapter 12. Input/Output Operations
12-7
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