A display device for use in a computer system comprising the display device and a computing device external to the display device , the display device comprising: - interface circuitry configured for receiving a display signal from the external computing device , - a display screen configured for displaying the received display signal; - an energy management function configured for controlling the operating state of the display device ; - a sensor configured for providing at least one control signal indicative of the presence of a user in a predefined area in front of the display screen ; and - a control unit configured for selecting an energy saving mode from a plurality of energy saving modes based at least on the control signal provided by the sensor ; wherein the energy management function is configured to control the operating state based on the selected energy saving mode; and wherein the interface circuitry is configured for transmitting control information regarding a determination of an operating state of the external computing device back to the external computing device by means of a manufacturer-specific virtual control panel, 'VCP' in the monitor command control set, 'MCCS' language according to the Display Data Channel, 'DDC', standard, wherein the control information comprises control data, comprising data regarding the presence of the user in the predefined area in front of the display screen and the energy saving mode selected by the control unit, and wherein the control information comprises an interrupt request, in particular an unsolicited attention call, to alert the external computing device to the selection of the energy-saving mode by the control unit. - interface circuitry configured for receiving a display signal from the external computing device , - a display screen configured for displaying the received display signal; - an energy management function configured for controlling the operating state of the display device ; - a sensor configured for providing at least one control signal indicative of the presence of a user in a predefined area in front of the display screen ; and - a control unit configured for selecting an energy saving mode from a plurality of energy saving modes based at least on the control signal provided by the sensor ; wherein the energy management function is configured to control the operating state based on the selected energy saving mode; and wherein the interface circuitry is configured for transmitting control information regarding a determination of an operating state of the external computing device back to the external computing device by means of a manufacturer-specific virtual control panel, 'VCP' in the monitor command control set, 'MCCS' language according to the Display Data Channel, 'DDC', standard, wherein the control information comprises control data, comprising data regarding the presence of the user in the predefined area in front of the display screen and the energy saving mode selected by the control unit, and wherein the control information comprises an interrupt request, in particular an unsolicited attention call, to alert the external computing device to the selection of the energy-saving mode by the control unit. <EOS>