1. An operating method of a memory device which controls a nonvolatile memory, the operating method comprising: managing a program depth bit map indicating an upper page program state of each of a plurality of word lines of the nonvolatile memory in response to an external write request, by a memory controller, where the program depth bit map is stored in a buffer memory in the memory controller which is separate from the nonvolatile memory; and outputting one of a plurality of different read commands to the nonvolatile memory based on information of the program depth bit map corresponding to a word line to be accessed in response to an external read request, wherein the plurality of different read commands comprises a first read command directing a read operation on a word line only having a lower page program state, and a second read command directing a read operation on a word line having an upper page program state, wherein a read time corresponding to a lower page read operation of the nonvolatile memory executed according to the first read command is the same as a read time corresponding to a lower page read operation of the nonvolatile memory executed according to the second read command, and wherein a read operation of the nonvolatile memory executed according to each of the first and second read commands does not include an operation of determining an upper page program state of the word line to be accessed.