UPD4990



The uPD4990 is a RTC (Real Time Clock) chip made by NEC, used on MVS boards to keep the calendar's time and date.

It shares the battery-backed RAM's battery and has a separate 32.768kHz crystal clock to run even when power is off.

Interface
The READ_CALENDAR BIOS call can be used to transparently load the uPD4990 time and date to 68k BIOS RAM.

Raw access is done through for input and  for output.

Datasheet
Official uPD4990A datasheet: []