I have a F type visa (double entry, 90 days stay).

Does that mean that I am allowed to be in China for 90 days, leave the country, re-enter, and stay for another 90 days on the same F type? Or am I only allowed to be in PRC for 90 days in total on this F type? Or am I only allowed to be in the country for 90 days staring on the day of my first entry no matter if I leave the country for some days and then re-enter?