In order to support automatic system and memory configuration, the VXIbus standard specifies a minimum
amount of capability on each device regardless of its function. A VXIbus device (see Section C.2, "Device
Operation"), being the lowest element of a hierarchy, has a set of "Configuration Registers", all totally
accessible from P1 on VMEbus which allow the system to identify the device, its class, model and
manufacturer, address space (A16, A24, A32, A64) and memory requirements. VXIbus devices with only this
minimum level of capability are called REGISTER BASED DEVICES. 
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VXIbus also defines MEMORY DEVICES, (See Section C.2.3, "Memory Devices") that can be identified as RAM, ROM or other memory types and be configured in contiguous blocks of memory based on speed or memory type.
If a higher level of communication ability between modules is desired in the system, VXIbus defines a class of
devices called MESSAGE BASED DEVICES. In addition to the configuration registers mentioned above,
Message Based Devices are required to have communication registers that are accessible to other modules in
the system. Each device in the system can then use specific communication protocols such as VXIbus's Word
Serial Protocol (See Section C.3.3.1, "Word Serial Protocols") to communicate with other devices. 
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Communication between VXIbus devices is based on a hierarchical device relationship involving
COMMANDERS and SERVANTS. For many systems, such as the single CPU system shown in Figure C.1,
only the first level of this hierarchy exists. In that example, the CPU and host interface device is a Commander which controls the three instrument Servants. A Commander is able to initiate communication with its
Servants based on their capabilities. If the Servants are Message Based Devices, such communication can take
place using COMMANDS which are described by the Word Serial Protocols. Communication with Register
Based Servants is device dependent and may vary between systems. The Commander/Servant hierarchy can
be nested since a Message Based Device may be a Commander as well as a Servant to the next level above it.

C.2.1.1.1 Device Addressing
All VXIbus devices have registers located within 64 byte blocks in the A16 address space. The base address of
a device's registers is determined by the device's unique Logical Address. The Logical Address selector is an
eight (8) bit selector which has 256 unique settings. The Logical Address is the value indicated by this selector
and corresponds to bits 6 →13 of the device register base address. Bits 14 and 15 of the base address are both
1 and the base address is:
     V × 64 + 49152
where V is the device's Logical Address.