I Virtual 3088 Support
RCTLUNIT ADDRESS=AOO,CUTYPE=3088,FEATURE=32-DEVICE SPECIAL DIRECTORY CONTROL STATEMENT: The 3088 is a valid device for
the SPECIAL directory control statement. For example, to specify a 3088 at virtu­
al address AOO, code the SPECIAL directory control statement as follows: SPEcial AOO 3088 Use the class G DEFINE command to define a virtual 3088 device. The class G
user can define a virtual 3088 device with or without a real equivalent. The system
simulates all functions of the real 3088, except for the online testing functions, for
each virtual 3088 that you define. You must define each virtual 3088 unit address
with a single DEFINE command. Defining each virtual unit address is different
from the dedicated 3088 support where you can define mUltiple unit addresses
using a single RDEVICE macro. Refer to VM / SP Operating Systems in a Virtual Machine for examples of virtual machine usage of channel-to-channel devices.
Command Usage and 3088 Support I Channel Command Words
Support for the 3088 recognizes the 3088 as a valid device. Figure 5 outlines
commands affected by 3088 support. See the VM/SP Operator's Guide and the VM / SP CP Command Reference for General Users for the format and complete
discussion of these commands.
Command Class 3088 Support
DEFINE G The 3088 is a valid device type on
this command. The control unit
address for a CTCA and a 3088 need
not end in zero. Once you define the
control unit, you may define other vir-
tual devices for the same CTCA or 3088. ATTACH B The response to these commands is COUPLE G the same for channel-to-channel
DETACH G,B adapters (CTCAs) and 3088s. QUERY B
Figure 5. CP commands and 3088 Support
In addition to the channel commands supported in System/360 and System/370 modes, the 3088 supports the following two channel commands:
PREP ARE --the PREP ARE channel command is used to receive a channel
program without causing an attention interrupt to the side issuing the
command. • SENSE ID --the SENSE ID channel command transfers model and control
unit identification to the system issuing the command.
Performance Guidelines 43

I Diagnostic Aids
Alternate Path Support 3088 support offers online testing facilities, and messages and MNOTES as diag­
nostic aids when using the support. See VM/SP System Message and Codes for the
complete text of the messages.
ONLINE TESTING: The last address in the group of 32 or 64 addresses for each
interface attached to the 3088 is available as a dedicated diagnostic unit address.
The diagnostic unit address provides a communication path between diagnostic
programs and the 3088 microprocessor for online testing. For example, a system
attached to the 3088 may use the diagnostic unit address to read the 3088 logout
and error information.
MESSAGES AND MNOTES TO SUPPORT 3088 DEVICES: The system issues a
message or MNOTE in the following situations:
If you attempt to define a 3088 for a unit address that has previously been
defined • If the virtual channel-to-channel device specified in the COUPLE command is
busy on the receiving userid's virtual machine • If you attempt to couple a 3088 to a channel-to-channel adapter
If you specify a model on the RDEVICE macro.
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/SP (up to 2 channels per control unit in
multiprocessing configurations). In addition, one device may be 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 that has alternate paths defined, and
the primary path is unavailable, VM/370 searchs for the first available path begin­
ning with the first alternate path. Successive alternate paths are examined if
required until an available path is found. In the case where no available path to the
device exists, alternate path I/O scheduling is implemented to queue the request on
multiple busy/scheduled paths, and the first path to become available is the path
the I/O request is started on.
The VM/370 I/O Scheduler determines that a path is available by analyzing the
busy and 8cheduled software indicators in the RDEVBLOK, RCUBLOK, and RCHBLOK as well as the chains of pending I/O requests that are queued from the RCUBLOK and RCHBLOK. This processing is performed prior to issuing the SIO. The search for an available path begins with the RDEVBLOK. If the RDEVBLOK is marked busy or scheduled, the I/O request is queued on the RDEVBLOK. No alternate path scheduling is performed at the device level. If the RDEVBLOK is not busy or scheduled, the IOBLOK for this I/O request is pro­
moted upward to the RCUBLOK. If the RCUBLOK is marked busy, the 10BLOK is queued on the RCUBLOK and a search is made for an alternate control unit
44 VM/SP System Programmer's Guide