That is outside the scope of MATLAB and is handled by the operating system.
If you are using MS Windows you can set processor affinity after the process is running; see https://technet.microsoft.com/en-us/library/ee851672.aspx
Normally the operating system dynamically assigns cores as required, and avoids assigning extra work to busy cores if there are free cores. Typically setting processor affinity is only needed for accounting / management purposes (e.g., one group might have contributed to the funding under the priviso that they get access to a minimum number of cores.) Sometimes it can make a bit of a difference to assign to a core that is on a board that has special hardware properties -- for example interrupt latency might be lower if you assign the handler process to the board that has the device interface attached. Or sometimes difference CPUs are different models and that might make some kind of difference.
But typically you do not need to set affinity.
