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

April 1, 1981
102.2 IBM VM/370 System Programmer's Guide