When /EOSC bit is set to 1, this simply halts the internal oscillator drive circuit to the external crystal. With no oscillations (ie, no 32kHz signal) we don’t have a signal to drive the count down chain and thus no internally generated 1Hz signal and subsequently no increment of the Seconds register. All other functions are still active, such as energy to the SRAM cells to maintain their current states. So with /EOSC set to 1, there will be no timekeeping but the data that is currently stored in the DS1390 will be maintained as long as at least 1.3V remains on the backup cell. That is is called “data retention” mode.