1. Field of the Invention
The present invention relates to a graphics controller for controlling the display monitor of a computer. The invention also relates to a power management method for use in the graphics controller.
2. Description of the Related Art
In recent years, various portable personal computers, such as notebook-type computers, that can be driven by batteries have been developed. Power management technique is employed in portable personal computers to save power. Known as a representative power management technique is Advanced Configuration and Power Interface (ACPI) specification. ACPI is a software/hardware specification formulated to enable the operating system (OS) to control power management directly. This software/hardware specification defines techniques such as system-power management, processor-power management and device-power management.
The system-power management is concerned with the entire computer system and defines the system-power states S0 to S5. State S0 is working state in which the system is on and the software is running. State S5 is off state in which the system is off and no software is running. The other states S1 to S4 are intermediate between the working state S0 and the off state S5. In these states S1 to S4, the context of any software is saved before the system goes into the sleep state, and any software is suspended. The system-power states S0 to S5 have a power-consumption relation of: S0>S1>S2>S3>S4>S5.
The processor-power management is concerned with the CPU (processor) incorporated in the computer system. It defines six states C0 to C5. The processor-power states C0 to C5 has a power-consumption relation of: C0>C1>C2>C3>C4>C5.
The device-power management is concerned with the devices provided on the bus. It defines four device-power states D0 to D3. The device-power states D0 to D3 has a power-consumption relation of: D0>D1>D2>D3.
In the device-power management, it is required that the devices, such as LCD panel, video adapter (i.e., graphics controller), IDE device, modem and the like, should support at least two device-power states D0 and D3 each. The device-power state D0 is a working state, in which the devices are completely active. The device-power state D3 is a power-saving state (low power-consumption state), in which the devices are off.
At present, the computer system may have troubles when the operating system (OS) performs a specific device-power management, due to the problems with the specification of the OS and other software or due to the defects in the software. For example, a trouble may occur in the computer system when the graphics controller is switched to state D3 to turn off the display screen.
The graphics controller is set to the state D3 in accordance with an instruction the OS gives when, for example, the keyboard of the computer system remains not operated for a prescribed time. Upon receiving the instruction designating the state D3, the graphics controller automatically transits from the present state to state D3 and stops operating. Usually, no software accesses the graphics controller to draw images or display images, as long as the graphics controller stays in the state D3. Due to the problems with the software specification or the software defects, however, the screen saver or any other similar software may try to access the graphics controller set in the state D3, in order to display images. In this case, the graphics controller makes no responses at all. Then, the software may freeze, or the computer system may hang up.