Method for operating non-volatile memory device and memory controller

A method of controlling a memory device, the method comprising:

• counting first memory cells of the memory device each having a threshold voltage belonging to a first voltage range between a first read voltage and a second read voltage;
• counting second memory cells of the memory device each having a threshold voltage belonging to a second voltage range between the second read voltage and a third read voltage;
• comparing a number of the first memory cells counted and a number of the second memory cells counted;
• and
• determining a fourth read voltage to be applied to memory cells of the memory device sequentially after the third read voltage based on a comparison result.

A method, comprising:

• counting first memory cells of a memory device each having a threshold voltage belonging to a first voltage range between a first read voltage and a second read voltage;
• counting second memory cells of the memory device each having a threshold voltage belonging to a second voltage range between the second read voltage and a third read voltage;
• comparing a number of the first memory cells counted and a number of the second memory cells counted to produce a comparison result;
• and
• determining, based on the comparison result, a third voltage range for counting memory cells of the memory device having a threshold voltage belonging to the third voltage range.