The present invention relates to a method of operating memory cells, comprising reading a previous user data from the memory cells; writing a new user data and merging the new user data with the previous user data into write registers; generating mask register information, and wherein the mask register information indicates bits of the previous user data stored in the memory cells to be switched or not to be switched in their logic values; counting numbers of a first logic value and a second logic value to be written using the mask register information, respectively; storing the numbers of the first logic value and the second logic value into a first counter and a second counter, respectively; and applying a programming pulse to the memory cells according to the mask register information.