IBM System/370 145 Manual page 44

interleaved, concurrent execution of multipl,e 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 bYte multiplexer,
the block multiplexer channel has multiple subchannels, each of which
has an associated UCW in control storage and can support one I/O
operation.
Block Multiplexer Channel Operation
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 or a block multiplexer channel operating in selector
mode 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 I/O device and becomes available
for an I/O operation on another device--even though the channel program
of the disconnected device 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 i t 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 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 i t 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 on block multiplexer channels, a new
interruption condition, called channel available, has been defined. At
disconnect time for a channel program, the block multiplexer channel is
available for the resumption of an uncompleted channel program
previously started, or another channel program can be initiated. A
channel available interruption 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
channel only when the channel is not busy. Thus, only cyclic
devices (such as direct access devices with rotational position
sensing) or buffered devices (such as the 2540 Card Read Punch
and the 1403 Printer) can disconnect during the execution of a
command-chained channel program on a block multiplexer channel
and resume operation later.
34 A Guide to the IBM System/310 Model 145