Port Multipliers and Selectors 409
The Port Multiplier’s operation is the same, regardless of the switching type used by the host.
9.11.1 Command-Based Switching
Host controllers that do not support hardware context switching utilize a switching type called
command-based switching.
To use command-based switching, the host controller has commands outstanding to only one
device at any point in time.
By only issuing commands to one device at a time, the result is that the Port Multiplier only
delivers FISes from that device.
A host controller may support command-based switching by implementing the Port Multiplier Port
(PMP) field in the SControl register.
In order to use this mechanism, host software will set the PMP field appropriately ...