Chapter 2. The ccw bus
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# Chapter 2. The ccw bus

The ccw bus typically contains the majority of devices available to a s390 system. Named after the channel command word (ccw), the basic command structure used to address its devices, the ccw bus contains so-called channel attached devices. They are addressed via I/O subchannels, visible on the css bus. A device driver for channel-attached devices, however, will never interact with the subchannel directly, but only via the I/O device on the ccw bus, the ccw device.

## I/O functions for channel-attached devices

Some hardware structures have been translated into C structures for use by the common I/O layer and device drivers. For more information on the hardware structures represented here, please consult the Principles of Operation.

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