IBM System/370 Manual page 36

later that the I/O operation could not be started, an I/O interrupt
occurs to indicate this fact .•
Block multiplexer mode is designed to increase system throughput
by increasing the amount of data entering and leaving the system in
a given period of time (the effective data rate). Better use of 2880
channel time is achieved by operating the channel in block multiplexing
mode. A single 2880 block multiplexer channel can support interleaved,
concurrent execution of multiple high-speed channel programs. The
block multiplexer channel can be shared by multiple high-speed I/O
devices operating concurrently, just as the byte multiplexer can be
shared by multiple low-speed devices.
Like the 2870 Multiplexer, the 2880 Block Multiplexer Channel has
multiple subchannels., each of which has an associated UCW and can
support one I/O operation. A 2880 channel has either 64 nonshared
subchannels or one shared and 56 nonshared subchannels.
A block multiplexer channel functions differently from a selector
channel in the way in which i t handles command-chained channel programs.
A selector channel executing a command-chained channel program is busy
during the entire time the channel program is in operation, whether
data transfer is occurring or not. A block multiplexer channel
executing a command-chained channel program has the ability to
disconnect from the operational channel program during certain non-
data transfer operations. That is, a block multiplexer channel can
be freed during a nonproductive activity, for example, during disk
seeking and most record positioning, thereby allowing more data to
be transferred per unit of channel busy time.
Block mUltiplexing operates as follows. Assume a block multiplexer
channel is executing a channel program consisting of multiple command-
chained CCW's. When channel end is presented without concurrent device
end., the channel disconnects from the channel program and becomes
available for an I/O operation on another device, even though the
disconnected channel program is not complete. At channel disconnect
time the subchannel and the device'S control unit retain the information
necessary to restart the disconnected channel program.
When the device signals that it is again ready for the channel (by
presenting device end), its control unit attempts to regain use of
the channel. If the channel is free at this time, the active channel
registers are reloaded with the information previously saved (in the
device's UCW) and the disconnected channel program is resumed at the
appropriate CCW. If the channel is busy when reconnection is requested,
the device must wait until it becomes available. Once multiple channel
programs have been initiated on one channel, the interleaving of data
transfer operations is controlled by block multiplexer channel hardware
and the control units of the devices operating in block multiplexing
mode.
To facilitate channel scheduling, a new interrupt condition, called
channel available, has been defined for block multiplexer channels.
At disconnect time for a channel program, the channel is available
for the resumption of an uncompleted channel program previously started,
or another channel program can be initiated. A channel available
interrupt occurs at disconnect time to indicate channel availability
if a START I/O, TEST I/O, TEST CHANNEL, or HALT DEVICE instruction
was issued previously while the block multiplexer channel was busy.
Two additional facts should be noted about block multiplexer channel
operations:
1. When multiple channel programs are operating concurrently in
block multiplexing mode, a device can regain control of the
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