Uninterruptible power supply providing continuous power mainstore function for a computer system

A method for providing back-up power for system mainstore memory and an uninterruptible power system (UPS) for a computer system are provided. The computer or data processing system includes a system memory and a system power supply. The uninterruptible power supply (UPS) includes an AC output and a DC output, an inverter circuit, a battery and an interface to the system power supply. The inverter circuit supplies AC power to the AC output. The battery supplies DC power to the DC output and to the inverter circuit. The interface enables the inverter circuit to initially supply AC power to the system power supply via the AC output upon detection of a utility power loss and then, after a predetermined period of time, activates the DC output to supply DC power to the system memory via the system power supply, and disables the inverter circuit to stop the initial supply of AC power to the system power supply.