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 REG_STATUS_A for input and REG_RTCCTRL for output.

Official uPD4990A datasheet: [[1]]