Patent Publication Number: US-6983384-B2

Title: Graphics controller and power management method for use in the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-263874, filed Aug. 31, 2000, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   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 S 0  to S 5 . State S 0  is working state in which the system is on and the software is running. State S 5  is off state in which the system is off and no software is running. The other states S 1  to S 4  are intermediate between the working state S 0  and the off state S 5 . In these states S 1  to S 4 , the context of any software is saved before the system goes into the sleep state, and any software is suspended. The system-power states S 0  to S 5  have a power-consumption relation of: S 0 &gt;S 1 &gt;S 2 &gt;S 3 &gt;S 4 &gt;S 5 . 
   The processor-power management is concerned with the CPU (processor) incorporated in the computer system. It defines six states C 0  to C 5 . The processor-power states C 0  to C 5  has a power-consumption relation of: C 0 &gt;C 1 &gt;C 2 &gt;C 3 &gt;C 4 &gt;C 5 . 
   The device-power management is concerned with the devices provided on the bus. It defines four device-power states D 0  to D 3 . The device-power states D 0  to D 3  has a power-consumption relation of: D 0 &gt;D 1 &gt;D 2 &gt;D 3 . 
   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 D 0  and D 3  each. The device-power state D 0  is a working state, in which the devices are completely active. The device-power state D 3  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 D 3  to turn off the display screen. 
   The graphics controller is set to the state D 3  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 D 3 , the graphics controller automatically transits from the present state to state D 3  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 D 3 . 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 D 3 , 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. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of this invention is to provide a graphics controller that can prevent troubles from occurring from an access, if any, made by software after the instruction representing transition to a low power-consumption state has been issued. 
   Another object of the invention is to provide a power management method that can enable a graphics controller to prevent troubles from occurring from an access, if any, made by software after the instruction representing transition to a specific state has been issued. 
   A graphics controller according to an aspect of the invention, which controls a display monitor of a computer, comprises a register, a logic unit, and a state controller. The register can be accessed through a bus of the computer and can store state control data representing a state of the graphics controller. The logic unit can operate in a working state and a low power-consumption state and can transit to the working state or the low power-consumption state in accordance with the state control data stored in the register. The state controller can invalidate the state control data stored in the register and designating the low power-consumption state, thereby to maintain the logic unit in the working state in spite of the state control data designating the low power-consumption state. 
   The state controller may preferably operate in a first mode to prohibit the logic unit from transiting to the low power-consumption state and in a second mode to allow the logic unit to transit to the low power-consumption state. The graphics controller may further comprise a register configured to be accessed through the bus of the computer and to store mode-designating data that designates the first mode and the second mode. 
   In this case, the state controller can be easily controlled by software to prohibit the logic unit from transiting to the low power-consumption state when it is predicted that troubles occur in the computer, and to allow the logic unit to transit to the low power-consumption state when it is not predicted that troubles occur in the computer. 
   The state controller may include a selector configured to transmit to the logic unit either fixed data of the same value as state control data representing the working state or the state control data to be stored into the register. The selector selects the fixed data while the state control logic unit is operating in a first mode to prohibit the logic unit from transiting to the low power-consumption state. While the state control logic unit is operating in a second mode to allow the logic unit to transit to the low power-consumption state, the selector selects the register. Thus, the logic unit can be prohibited from transiting to the low power-consumption state and allowed to transit thereto, thanks to the use of the switch circuit that is simple in structure. 
   Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
       FIG. 1  is a block diagram showing the computer system relating using the first embodiment of the present invention; 
       FIGS. 2A and 2B  are diagrams explaining a state transition of the graphics controller incorporated in the computer system of  FIG. 1 ; 
       FIG. 3  is a diagram explaining the hierarchical structure of the software for controlling the graphics controller incorporated in the computer system of  FIG. 1 ; 
       FIG. 4  is a block diagram illustrating the graphics controller incorporated in the computer system of  FIG. 1 ; 
       FIG. 5  is a flowchart explaining how the computer system of  FIG. 1  operates in the normal mode; 
       FIG. 6  is a flowchart explaining how the computer system of  FIG. 1  operates in the D3-bypass mode; 
       FIG. 7  is a block diagram of a graphics controller that may be used in the computer system of  FIG. 1 , in place of the graphics controller shown in  FIG. 4 ; 
       FIG. 8  is a block diagram of a modification of the computer system illustrated in  FIG. 1 ; and 
       FIG. 9  is a flowchart explaining how the computer system of  FIG. 8  operates in D3-bypass mode. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention will be described, with reference to the accompanying drawings. 
     FIG. 1  shows a computer system incorporating one embodiment of the present invention. The computer system is a notebook-type personal computer. As  FIG. 1  shows, the notebook-type personal computer comprises a CPU  11 , host bridge  12 , main memory  13 , graphics controller  14 , sound controller  15 , PCI-ISA bridge  16 , I/O controller  17 , hard disk drive (HDD)  18 , BIOS-ROM  19  and keyboard controller (KBC)  20 . 
   The CPU  11  is a processor for controlling the other components of the computer system. The CPU  11  executes various programs loaded in the main memory  13 , such as a operating system (OS), and application programs. The CPU executes BIOS (Basic Input Output System) and the like, too. Both OS and BIOS accord with the Advanced Configuration and Power Interface (ACPI) specification. 
   The host bridge  12  is a bridge LSI designed to connect the CPU bus  1  and the PCI bus  3 . The host bridge  12  includes a memory controller and an AGP (Accelerated Graphics Port) bridge. The memory controller controls the access to the main memory  13 . The AGP bridge supplies and receives data to and from the graphics controller  14  via the AGP bus  2 . The AGP bus  2  is dedicated to data transfer between the host bridge  12  and the graphics controller  14 . It is one of the extension items of the PCI bus specification. 
   The graphics controller  14  is a display control device. It controls the LCD  143  and the external CRT display  142 . Note that the LCD  143  and CRT display  142  are the display monitors of the computer system. The graphics controller  14  is comprised of a one-chip LSI. The graphics controller  14  works as a graphics accelerator. The controller  14  can write image data into the video memory (VRAM)  141 , convert the data in the VRAM  141  to a display signal and output the display signal to the display monitors. The graphics controller  14  is a device that accords with the Advanced Configuration and Power Interface (ACPI) specification. It supports at least two of the device states D 0  to D 3 , i.e., state D 0  (working state) and state D 3  (off state). (Recall that the device states D 1  to D 3  are low power-consumption states.) The graphics controller  14  incorporates a PCI-configuration register that is used to control the device state and the like. The graphics controller  14  goes into the device state D 3  when the CPU  11  writes the state control data designating the state D 3  into the PCI configuration register of the graphics controller  14  through the AGP bus  2 . The state D 3  is an off state in which the graphics controller  14  is turned off. Hence, in the device state D 3 , the device context is usually erased in the graphics controller  14 . 
   The graphics controller  14  can operate in a special mode called “D3 bypass mode.” In the D3-bypass mode, the state control data designating the state D 3  and held in the PCI configuration register is neglected, whereby the graphics controller  14  is prohibited from transiting to the state D 3 . Thus, in the D3-bypass mode, the controller  14  remains in the state D 0 , not transiting to the state D 3 , in spite of the state control data designating the state D 3  that is stored in the PCI configuration register. The D3-bypass mode can be validated and invalidated in accordance with the BIOS or with the hardware of the computer system. 
   The sound controller  15  is the sound source of the computer system. It can reproduce audio data and output the same. The sound controller  15  is another device that accords with the Advanced Configuration and Power Interface (ACPI) specification. It supports at least two of the device states D 0  to D 3 , i.e., state D 0  (working state) and state D 3  (off state). The sound controller  15  incorporates a PCI-configuration register that is used to control the device state and the like. The sound controller  15  goes into the device state D 3  when the CPU  11  writes the state control data designating the state D 3  into the PCI configuration register of the sound controller  15  through the PCI bus  3 . The state D 3  is an off state in which the sound controller  15  is turned off. In the device state D 3 , the device context is usually erased in the sound controller  15 . 
   The PCI-ISA bridge  16  is a bridge LSI designed to connect the PCI bus  3  and the ISA bus  4 . To the ISA bus  4  there are connected the BIOS-ROM  19  and ISA devices such as the keyboard controller (KBC)  20 . The BIOS-ROM  19  stores the system BIOS (Basic Input Output System). The system BIOS is composed of a POST routine for initializing and testing various devices when the system is powered on, BIOS drivers for controlling various types of hardware, and system management routines. The BIOS drivers include VGA-BIOS that controls the graphics controller  14 . The system management routines provide various power management functions such as memory suspend and hibernation. 
   The memory suspend is a function that powered off almost all components of the computer system, except for the main memory  13 , after the contexts of the CPU 11  and the other devices, which are required to restore the present operating environment, have been stored in the main memory  13 . The hibernation is a function that powered off almost all components including the main memory  13 , after the contents of the main memory  13  and the contexts of the other devices, which are required to restore the present operating environment, have been stored in the prescribed region of the hard disk drive  18 . 
   In the present embodiment, the memory suspend is used in the system state S 3 , and the hibernation is used in the system state S 4 . The system states S 0  to S 5  are not directly related to the device states D 0  to D 3 . Rather, the OS can usually transit any device to any one of the states D 0  to D 3 , whenever necessary, while the computer system remains in the system state S 0  (i.e., the working state). 
   The state transition of the graphics controller  14  will be described with reference to  FIGS. 2A and 2B . 
     FIG. 2A  shows how the graphics controller  14  transits from one state to another in the normal mode.  FIG. 2B  shows how the graphics controller  14  transits from one state to another in the D3 bypass mode. 
   The normal mode is a mode in which the graphics controller  14  can transit to the state D 3 ; that is, the D3 bypass mode is invalidated. In the normal mode, the graphics controller  14  can be set into the state D 0  or the state D 3  by an access the OS or any other software makes to the PCI configuration register provided in the graphics controller  14 . If the graphics controller  14  supports not only the states D 0  and D 3 , but also the states D 1  and D 2 , it can of course transit from any of the states D 0  to D 3  to another thereof in accordance with the state control data supplied from the OS or any other software to the PCI configuration register. 
   In the D3 bypass mode, the state control data representing the state D 3  and supplied to the PCI configuration register is neglected. The graphics controller  14  remains in the state D 0  and will not transit to the state D 3  even if the state control data representing the state D 3  is supplied from the OS and stored into the PCI configuration resister. 
   While the graphics controller  14  stays in the state D 0 , the system BIOS can directly control the core logic of the graphics controller  14 , thereby decreasing the power consumption of the graphics controller  14 . That is, the graphics controller  14  assumes a so-called “special power-save mode.” In the special power-save mode, the following events take place:
     (1) The supply of the operation clock signal to the DAC (D/A Converter) provided in the graphics controller  14  is stopped.   (2) Selected ones of the many hardware components of the core logic is disabled.   

   The power consumption of the graphics controller  14  can thereby be lowered even if the core logic of the graphics controller  14  is inhibited from entering the state D 3 . 
     FIG. 3  illustrates the hierarchical structure of the software for controlling the graphics controller  14 . To display images, the graphics controller  14  is accessed usually through a PCI configuration driver, and a display driver and the like, under the control of the operating system (OS). To set the graphics controller  14  into the state D 3 , the graphics controller  14  is accessed directly by the PCI configuration driver, not through the display driver, in accordance with a command supplied from the operating system (OS). 
   The BIOS has a communication interface for the display driver. It is however difficult for the BIOS to hook the access made to set the PCI configuration register into the state D 3 . This is because the graphics controller  14  is accessed not through the display driver. Therefore, in the present embodiment, the operation of the above-mentioned D3 bypass mode is accomplished by the use of hardware in the graphics controller  14 . More precisely, the hardware logic (i.e., D3 bypass circuit) incorporated in the graphics controller  14  invalidates the state control data representing the state D 3 . 
   The hardware configuration of the graphics controller  14  will be described with reference to  FIG. 4 . 
   As  FIG. 4  shows, the graphics controller  14  comprises a PCI configuration register  201 , D3 bypass register  202 , I/O registers  203 , bypass circuit  204  and core unit (core logic unit)  205 . 
   The core unit  205  is the main component of the graphics controller  14 . The core unit  205  performs two functions. First, it performs graphics-drawing functions. Second, it controls the display monitors of the computer system. The core unit  205  can assume the states D 0  and D 3 . As shown in  FIG. 3 , the core unit  205  has two graphics engines  301  and  302 , a 2D-engine  303 , a 3D-engine  304 , an bus interface unit (BIU)  305 , a memory interface unit (MIU)  306 , a D/A converter (DAC)  307 , an LCD gate array (LCD GA)  308 , and a TV D/A converter (TV DAC)  309 . The graphics engines  301  and  302  are provided to accomplish dual display control. The 2D-engine  303  processes data to generate 2D (two-dimensional) image data. The 3D-engine  304  processes data to generate 3D (three-dimensional) image data. The bus interface unit  305  transfers data through the AGP bus  2 . The memory interface unit  306  is used to control the VRAM  141 . The D/A converter  307  outputs analog video signals to the CRT display  142 . The LCD gate array  308  is provided to control the LCD  143 . The TV D/A converter  309  outputs video signals to television (TV) sets. 
   The PCI configuration register  201  can be accessed via the AGP bus  2 . The register  201  is defined in a configuration space in which the various items of the operating environment of the graphics controller  14  are stored. The register  201  stores the state control data designating the state (either D 0  or D 3 ) into which the graphics controller  14 , more correctly the core unit  205 , should be set. The bypass circuit  204  is a state controller that invalidates the state control data representing the state D 3 , which is stored in the PCI configuration register  201 . The circuit  204  has a switch circuit SW that supplies to the core unit  205  either the state control data stored in the register  201  or a fixed data of the same value as state control data representing the state D 0 . 
   The core unit  205  transits to the state D 0  or D 3  in accordance with the state control data supplied via the bypass circuit  20  and representing the state D 0  or D 3  (power management # 1 ). 
   The D3 bypass register  202  can be accessed through the AGP bus  2 . It is a special register defined in a configuration space or an I/O space. The BIOS writes the mode control data designating the D3 bypass mode or the normal mode into the D3 bypass register  202 . If the mode control data designating the normal mode is written in the D3 bypass register  202 , the switch circuit SW selects the PCI configuration register  201 . If the mode control data designating the D3 bypass mode is written in the D3 bypass register  202 , the switch circuit SW selects the fixed data that represents the state D 0 . The normal mode may be defined as the default-operating mode of the graphics controller  14 . 
   I/O registers  203  include control registers, each designed to control the power consumption in the core unit  205  while the unit  205  remains in working state D 0 . The control registers can be accessed through the AGP bus  2 . The BIOS set commands and parameters. The commands and the parameters are set to the I/O register  203  and directly control the core unit  205  that stays in the working state D 0 . The graphics controller  14  can therefore operate in the special power-save mode (power management # 2 ). To reduce the power consumption of the core unit in the state D 0 , some operation controls are performed. More specifically, the graphics engines  301  and  302 , 2D-engine  303 , 3D-engine  304 , BIU  305 , MIU  306 , DAC  307 , LCD GA  308  and TV DAC  309  are selectively stopped, or the clock speed of the particular units such as DAC  307  is changed. 
   How the computer system operates in the normal mode and how it operates in the D3 bypass mode will be explained, with reference to the flowcharts of  FIGS. 5 and 6 . 
   (Normal Mode) 
   The operating system (OS) may detect, on the basis of its policy, that a prescribed condition that should be satisfied to, for example, turn off a display screen. If this is the case, the operating system issues an instruction for setting the graphics controller  14  into the state D 3  (Step S 101 ). The instruction is supplied to the PCI configuration driver. In response to the instruction, the PCI configuration driver accesses the graphics controller  14  through the AGP bus  2  and writes the state control data representing the state D 3  into the PCI configuration register  201  of the graphics controller  14  (Step S 102 ). 
   The state control data representing the state D 3  is supplied to the core unit  205 , whereby the core unit  205  transits from the state D 0  to the state D 3 . If the screen saver or any other software accesses the graphics controller  14  due to problems with the software specification or the software defects, the software will freeze, or the computer system may hang up. In order to achieve memory suspend in normal way, it is necessary to save the context of the graphics controller  14  into the main memory  13  before the graphics controller  14  transits to the state D 3 . 
   (D3 Bypass Mode) 
   As shown in  FIG. 6 , the BIOS accesses the graphics controller  14  via the AGP bus  2 , for example when the computer system is powered on, thus writing the mode control data representing the D3 bypass mode, into the D3 bypass register  202  (Step S 111 ). Therefore, the bypass circuit  204  selects the fixed data (=D 0 ), regardless of the contents of the PCI configuration register  201 . 
   The operating system (OS) may detect, on the basis of its policy, that the prescribed condition that should be satisfied to turn off a display screen. In this case, the operating system generates an instruction for setting the graphics controller  14  into the state D 3  (Step S 112 ). The instruction is supplied to the PCI configuration driver. In response to the instruction, the PCI configuration driver accesses the graphics controller  14  through the AGP bus  2  and writes the state control data representing the state D 3  into the PCI configuration register  201  (Step S 113 ). However, the state control data representing the state D 3  is not supplied to the core unit  205 . Therefore, the graphics controller  14  does not transit the state D 3 . 
   Thereafter, the BIOS performs, on the basis of its policy, the power management # 2  on the graphics controller  14  (Step S 114 ). More precisely, the BIOS controls the core unit  205  by way of the I/O register  203 . If the screen saver or any other software accesses the graphics controller  14  due to problems with the software specification or the software defects, the software will not freeze, or the computer system will not hang up. This is because the graphics controller  14  is not set in the state D 3 . In the power management # 2  the BIOS performs, the context of the graphics controller  14  would not be erased at all. Hence, memory suspend can be achieved in normal way and at any time, without the necessity of saving the context of the graphics controller  14  into the main memory  13  beforehand. 
   The graphics controller  14  may be replaced by a graphics controller  14 ′ illustrated in  FIG. 7 . This graphics controller  14 ′ will be described below. 
   The graphics controller  14 ′ shown in  FIG. 7  is designed to use a hardware strap  401  to control the bypass circuit  204 , not by using the bypass register  202  as in the graphics controller shown in  FIG. 4 . The graphics controller  14 ′ is identical to the graphics controller  14  in any other respects. The hardware strap  401  is connected to one input pin P 1  of the graphics controller  14 ′ that is a one-chip LSI. It supplies a mode control signal through the input pin P 1  to the bypass circuit  204 , to set the graphics controller  14 ′ into either the normal mode or the D 3  bypass mode. In the graphics controller  14 ′, the input pin P 1  is connected by a pull-up resistor R 1  to a power-supply terminal, as is illustrated in  FIG. 7 . 
   The hardware strap  401  is mounted of the system board. As  FIG. 7  shows, it comprises a jumper switch  500  and a pull-down resistor R 2 . When the jumper switch  500  is turned on, the input pin P 1  is connected to the pull-down resistor R 2 . The mode control signal “0” that represents the D3 bypass mode is thereby supplied to the bypass circuit  204 . On the other hand, when the jumper switch  500  is turned off, the control signal “1” that represents the normal mode is supplied to the bypass circuit  204 . The jumper switch  500  may be preset either turned on state or turned off state prior to shipment of the computer system, in accordance with the type and/or version of the OS used in the computer system. Also, the jumper switch  500  may be set either turned on state or turned off state by user, after the computer system has be shipped. 
   The jumper switch  500  may be replaced by a dipswitch. The jumper switch may be dispensed with, if the graphics controller  14 ′ is made to operate always in the D3 bypass mode. In this case, the input pin P 1  may be soldered to the ground terminal provided on the system board. 
     FIG. 8  shows a modification of the computer system illustrated in  FIG. 1 . The modified computer system is identical to the system of  FIG. 1 , except that not only the graphics controller  14 , but also the sound controller  15  can operate in the D3 bypass mode. 
   The sound controller  15  is of the same structure as the graphics controller  14  shown in  FIG. 4 . The sound controller  15  comprises a bypass circuit and a D3 bypass register. In the sound controller  15 , too, the bypass circuit invalidates the state control data representing the state D 3 , which is stored in the PCI configuration register of the sound controller  15 . The sound controller  15  can therefore be prohibited from transiting to the state D 3 . 
   How the computer system of  FIG. 8  operates in the D3 bypass mode will be explained, with reference to the flowchart of  FIG. 9 . 
   (D3 Bypass Mode) 
   As shown in  FIG. 9 , the BIOS accesses the graphics controller  14  via the AGP bus  2 , for example when the computer system is powered on, thus writing the mode control data representing the D3 bypass mode, into the D3 bypass register  202  (Step S 201 ). Therefore, the bypass circuit  204  selects the fixed data (=D 0 ), regardless of the contents of the PCI configuration register  201 . The graphics controller  14  is thereby set into the D3 bypass mode. Then, the BIOS accesses the sound controller  15  via the PCI bus  3 , writing the mode control data representing the D3 bypass mode into the D3 bypass register incorporated in the sound controller  15 . The sound controller  15  is set into the D3 bypass mode, too (Step S 202 ). 
   The operating system (OS) may detect, on the basis of its policy, that the prescribed condition that should be satisfied to turn off a display screen. In this case, the operating system generates an instruction for causing the PCI configuration driver to set the graphics controller  14  into the state D 3  (Step S 203 ). The instruction is supplied to the PCI configuration driver. In response to the instruction, the PCI configuration driver accesses the graphics controller  14  through the AGP bus  2  and writes the state control data representing the state D 3  into the PCI configuration register  201  (Step S 204 ). However, the state control data representing the state D 3  is not supplied to the core unit  205 . Therefore, the graphics controller  14  does not transit the state D 3 . The holds true of the sound controller  15 . 
   The operating system may generate an instruction for causing the PCI configuration driver to set the sound controller  15  into the state D 3 . In response to the instruction, the PCI configuration driver accesses the sound controller  15  through the PCI bus  3  and writes the state control data representing the state D 3  into the PCI configuration register of the sound controller  15  (Step S 205 ). However, The sound controller  15  does not transit to the state D 3 . 
   Thereafter, the BIOS performs, on the basis of its policy, the power management # 2  on the graphics controller  14  or the sound controller  15  (Step S 206 ). More precisely, the power management # 2  for the sound controller  15  is performed as the BIOS controls the hardware components of the sound controller  15  by way of the I/O register of the sound controller  15 . 
   Even if the screen saver or any other software accesses the graphics controller  14  or the sound controller  15  due to problems with the software specification or the software defects, the software will not freeze, or the computer system will not hang up. This is because neither the graphics controller  14  nor the sound controller  15  is set in the state D 3 . In the power management # 2  the BIOS performs, the context of the graphics controller  14  and that of the sound controller  15  would not be erased at all. Hence, memory suspend can be accomplished in normal way and at any time, without the necessity of saving the contexts of the graphics controller  14  and the sound controller  15  into the main memory  13  beforehand. 
   The configuration of the present embodiment can be applied to various devices connected to a computer (the devices provided on the bus, the peripheral devices, and the like). In the normal mode, any device that can operate in at least a working state and a low power-consumption state can transit from the working state to the low power-consumption state upon receipt of a state transition instruction. In the bypass mode, the device is prohibited from transiting from the working state to the low power-consumption state, notwithstanding the state transition instruction. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.