Free operating system supporting SMP "HelenOS"

Exactly the source code is BSD license. Supported CPUs are wide, 64bit also supported.

HelenOS project
http://www.helenos.eu/

SMP is symmetric multiprocessing.

Symmetric multiprocessing - Wikipedia

The downloads and screenshots are as follows.
Download
http://www.helenos.eu/?reason=download

Screenshots
http://www.helenos.eu/?reason=screenshots