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virtual machine traces external interrupts, the virtual interval timer
assist is automatically disabled. When external interrupt tracing is
completed, virtual interval timer assist is reenabled.
THE VIRTUAL BLOCK MULTIPLEXER CHANNEL OPTION virtual machine SIO operations are simulated by CP in three ways:
byte-multiplexer, selector, and block multiplexer channel mode.
virtual byte-multiplexer mode is reserved for I/O operations that
apply to devices allocated to channel zero.
Selector channel mode (the default mode) is the mode of operation for
any channel that has an attached Channel-to-Channel Adapter (CTCA), regardless of the selected channel mode setting (the CTCA is treated as
a shared control unit and, therefore, it must be connected to a selector
channel). The user need not concern himself as to the location of the CTCA since CP interrogates the related channel linkage and marks the
channel as being in selector mode. As in real selector channel
operations, CP reflects a busy condition (condition code 2) to the
virtual machine's operating system if the system attempts a second SIO to the same device, or another device on the same channel, before the
first SIO is completed.
Block multiplexer channel mode is a CP simulation of real block
multiplexer operation; it allows the virtual machine's operating system
to overlap SIO requests to multiple devices connected to the same
channel. The selection of block multiplexer mode of operation may
increase the virtual machine's throughput, particularly for those
systems or programs that are designed to use the block multiplexer
channels.
Note: CP simulation of tlock multiplexer processing does not reflect
channel available interruptions (CAls) to the user's virtual machine.
Selecting the channel mode of operation for the virtual machine can
be accomplished by either a system generation DIRECTORY OPTION operand
or by use of the CP DEFINE command.
ALTERNATE PATH SUPPORT Through the use of the Two-Channel Switch and Two-Channel Switch
Additional Features, alternate path support for DASD or tape provides
for up to four channels on one control unit to be attached to VM/370. In addition, one device may te attached to two logical control units,
providing support for the String Switch feature. This allows the
control program up to eight paths to a given device when the maximum
number of alternate channels and alternate control units are specified. When an I/O request is received for a device which has alternate
paths defined, the VM/370 lOS supervisor searches for an available path
beginning with the primary path to the device. If the primary path is
unavailable, the search continues with the first alternate path.
Successive alternate paths are examined if required until an available
path is found. 102 TOM .J..J •. H . .l System programmer's Guide

Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829
In case where no available path to the device exists, alternate path I/O sCheduling is implemented in such a way request is queued off multiple busy/scheduled paths and the first path to become available
is the path that the I/O request is started on.
The IDS supervisor determines that a path is "available" by analyzing
the busy and scheduled software indicators in the RDEVBLOK, RCUBLOK and RCHBLOK as well as the chains of pending I/O requests which are queued
from the RCUBLOK and RCHBLOK. This processing is performed prior to
issuing the SIO. !Qte: There is no alternate path scheduling after the SID if a control
unit busy, channel busy, or not operational condition is encountered.
The I/O request will be queued in the busy conditions on the busy
control unit or channel block to wait for an interrupt which will cause
the request to be restarted. The not operational condition is presented
to the second level interrupt handlers as a fatal condition. - Part 2. control Program (CP) 102.1