With code embedded into the application or external to the application?
For any 6 cores of 8 core processor or a specific set of 6 cores of 8 core processor?
Both Windows and Linux have a mens to control the applicaiton (also known as a process) to a specific hardware thread or set of hardware threads. Where the documentation usually refers to these as "processor". The key words to google are:
affinityprocess restrict or affinity programming site:wikipedia.org
The above searches should yield some useful links.