Usillg tile Extended Control-Program Support: VM/3 70
Restricted Use of ECPS I Channel Usage Extended Control-Program Support: VM/370 (ECPS) is controlled at two levels:
the VM/SP system and the virtual machine.
At the VM/SP system level, ECPS is automatically enabled when the system is
loaded. The class A command:
set cpassist off
disables both CP assist and expanded virtual machine assist. The class A
command:
set sassist off
disables only the expanded virtual machine assist part of ECPS as well as the virtu­
al machine assist. CP assist is the only part of ECPS that is truly independent.
At the virtual machine level, whenever ECPS is enabled on the system, both
expanded virtual machine assist and virtual interval timer assist are automatically
enabled when you log on. If you issue the class G command:
set assist off
both assists as well as the existing virtual machine assist are disabled. If you issue:
set assist notmr
only the virtual interval timer assist is disabled. If CP assist is disabled for the sys­
tem, the class A command:
set sassist on
enables the virtual machine assist. You can then enable virtual machine assist and
virtual interval timer assist for your virtual machine by issuing the class G
command:
set assist on tmr
The restrictions on the use of ECPS are the same as those described for the virtual
machine assist feature with one addition. Whenever a 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
block mUltiplexer channel mode.
Virtual byte-multiplexer mode is reserved for I/O operations that apply to devices
allocated to channel zero.
Performance Guidelines 41

Multisystem Communications
In virtual 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 opera­
tion; 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 block multiplexer processing does not reflect channel
available interruptions (CAIs) to the user's virtual machine.
Selecting the channel mode of operation for the virtual machine can be accom­
plished by either a DIRECTORY OPTION operand or by use of the CP DEFINE
command.
The IBM 3088 Multisystem Channel Communication Unit (MCU) is an
input! output device that interconnects as many as eight systems using block
multiplexer channels. The 3088 Modell interconnects up to four systems, while
the 3088 Model 2 interconnects up to eight systems.
The 3088 supports the PREPARE channel command which can be used in certain
situations to prevent attention interrupts on the side issuing the PREP ARE com­
mand. See "Channel Command Words" later in this discussion for a description of 3088 channel command words.
The 3088 is compatible with existing channel-to-channel usage. In addition, 3088 support extends existing CTCA addressing and scheduling by:
Allowing multiple unit addresses per control unit
Implementing block multiplexer channel scheduling for both real and virtual
CTCAs and 3088. System Programmer Considerations
At system generation time, the system programmer must code parameters in the
RDEVICE macro and the RCTLUNIT macro to define the 3088 to the control
program. See the VM / SP Installation Guide for the format of these macros.
RDEVICE MACRO: When you code the RDEVICE macro, specify the address
and device type. For example, to define a maximum of 32 sequential unit
addresses at AOO, code the RDEVICE macro as follows:
RDEVICE ADDRESS=(AOO,32),DEVTYPE=3088
RCTLUNIT MACRO: When you code the RCTLUNIT macro, you must indicate
the address and the control unit type. In addition, since the 3088 supports a maxi­
mum of 32 or 64 devices, you must also specify the number of sequential unit
addresses using the FEATURE=xxx-DEVICE operand. For example, if you want
to generate 32 devices at channel address AOO, code the RCTLUNIT macro as fol­
lows:
42 VM/SP System Programmer's Guide