Types Of Channels - IBM 4300 Manual

bursts of data. A channel that can operate in
either mode determines its mode of operation by
timeout. Whenever the burst causes the device to
be connected to the channel for more than
approximately 100 microseconds, the channel is
considered to be operating in the burst mode.
Ordinarily, devices with a high data-transfer rate
operate with the channel in burst mode, and slower
devices run in byte-multiplex mode. Some control
units have a manual switch for setting the mode of
operation.
Types of Channels
A system can be equipped with three types of
channels: selector, byte multiplexer, and block
multiplexer.
The channel facilities required for sustaining a
single I/O operation are termed a subchannel. The
subchannel consists of internal storage used for
recording the addresses, count, and any status and
control information associated with the I/O
operation. The capability of a channel to permit
multiplexing depends upon whether it has more
than one subchannel.
A selector channel, which contains a minimum of
facilities, has one subchannel and always forces the
I/O device to transfer data in the burst mode. The
burst extends over the whole block of data, or,
when command chaining is specified, over the
whole sequence of blocks. A selector channel
cannot perform any multiplexing and therefore can
be involved in only one I/O operation or chain of
operations at a time. In the meantime, other I/O
devices attached to the channel can be executing
previously initiated operations that do not involve
communication with the channel, such as
backspacing tape. When the selector channel is not
executing an operation or a chain of operations and
is not processing an interruption, it monitors the
attached devices for status information.
A byte-multiplexer channel contains multiple
subchannels and can operate at anyone time in
either byte-multiplex or burst mode. The channel
operates most efficiently when running I/O devices
that are designed to operate in byte-multiplex
mode. The mode of operation is determined by the
I/O device, and, during data transfer, the mode can
change at any time. Unless data transfer is
occurring, the mode of operation has no meaning.
The data transfer associated with an operation can
occur partially in the byte-multiplex mode and
partially in the burst mode.
A block-multiplexer channel contains multiple
subchannels and can only operate in burst mode.
The channel operates most efficiently when running
12-4 IBM 4300 Processors Principles of Operation
devices that are designed to operate in burst mode.
When mUltiplexing is not inhibited, the channel
permits multiplexing between blocks, between
bursts, or when command retry is performed. On
most models, the burst is forced to extend over the
block of data, and multiplexing is permitted either
between blocks of data or when command chaining
is specified. Whether or not multiplexing occurs
depends on the design of the channel and I/O
device and on the state of the block-multiplexing-
control bit.
When the block-multiplexing-control bit, bit 0 of
control register 0, is zero, multiplexing is inhibited;
when it is one, multiplexing is allowed.
Whether a block-multiplexer channel executes an
I/O operation with multiplexing inhibited or
allowed is determined by the state of the block-
multiplexing-control bit at the time the operation is
initiated by START I/O or START I/O FAST
RELEASE and applies to that operation until the
involved subchannel becomes available.
For brevity, the term "multiplexer channel" is
used hereafter when describing a function or
facility that is common for both byte-multiplexer
and block-multiplexer channels. Multiplexer
channels vary in the number of sub channels they
contain. When multiplexing, they can sustain
concurrently one I/O operation per subchannel,
provided that the total load on the channel does
not exceed its capacity. Each subchannel appears
to the program as an independent selector channel,
except in those aspects of communication that
pertain to the physical channel (for example,
individual subchannels on a multiplexer channel are
not distinguished as such by the TEST CHANNEL
instruction or by the masks controlling I/O
interruptions from the channel). When a
multiplexer channel is not servicing an I/O device,
it monitors its devices for data and for status
information.
Subchannels on a multiplexer channel may be
either nonshared or shared.
A sub channel is referred to as nonshared if it is
associated with and can be used only by a single
I/O device. A nonshared subchannel is used with
devices that do not have any restrictions on the
concurrency of channel-program operations, such
as the IBM 3211 Printer Modell or one drive of
an IBM 3330 Disk Storage.
A subchannel is referred to as shared if data
transfer to or from a set of devices implies the use
of the same sub channel. Only one device
associated with a shared sub channel may be
involved in data transmission at a time. Shared
subchannels are used with devices, such as