Affinity

Using Dual-Core CPU's, when you set a process to use just one of the two
cores, which part of the process is confined to that specific core?
Just the main thread called by CreateProcess when the program was launched,
or does the core also handle the API functions called from within the
program?
I'm asking this so I can figure out the best way to dedicate CPU time to a
running program while still allowing other processes (services) to use the
other core.

-- Andy