Apparently a few different things have been called "memristors", but at least some of them have a resistance that depends upon the integral of current through them into the infinite past. DC current one way to write a 1, the other way to write a 0, and AC current to nondestructively read, I guess. Typically this works through ion migration in the material, and I suspect that resulted in abysmal endurance, putting an end to the promise of memristors.