Abstract:
A display apparatus includes a display memory for storing an image of a first area forming a portion of an entire image; a display section including a display screen having a display area usable for displaying at least a partial image of the first area; a case for exposing the display screen; a scroll unit operable to move a display screen area appearing on the display screen over the image in accordance with a movement of the case; and a controller operable to provide a virtual memory area for storing an image of a second area having a size greater than the size of the first area in the image when the scroll unit moves the display screen area beyond the first area.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority from Japanese Patent Application No. JP 2005-050948 filed on Feb. 25, 2005, the disclosure of which is hereby incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a display apparatus capable of outputting a scroll display. More particularly, the present invention relates to a display apparatus for detecting a movement of a case for accommodating the display apparatus and outputting a scroll display in accordance with the movement. 
   In general, the amount of information that can be shown on the display screen of a display apparatus mounted on a device such as a portable terminal or a hand phone is small. Thus, the display apparatus is provided with a function referred to as the so-called scroll display function by which, if an entire image cannot be shown on the display screen, only a partial range of the image is shown on the display screen and the display range of the image can then be shifted by operating a button. In general, this scroll display function is implemented in an apparatus such as a personal computer by adoption of a display operation method. Since the size of an operation section of a portable terminal is also determined under strict constraints, however, an operation carried out on a button to implement the scroll display function in such a portable terminal cannot be said to be an operation that can be performed with a high degree of convenience. 
   For the reason described above, Japanese Patent Laid-open No. 2002-7027 discloses a technology for detecting a movement of a display apparatus in the forward, backward, left or right and scrolling the display of an image as well at the same time to the direction of the movement. In accordance with the technology, it is possible to get rid of the cumbersomeness of the operation to be otherwise carried out to scroll the display of the image and thus implement the scroll display function matching the intuition of a human being. 
   The technology disclosed in Japanese Patent Laid-open No. 2002-7027 is a technology adopted in a display apparatus as a technology capable of outputting a scroll display of an image if the data of the image has been stored in a display memory employed in the display apparatus. Thus, if the amount of the data of the image is greater than the storage capacity of the display memory, the technology is not capable of implementing the scroll display function for the image. 
   In order to solve this problem, it is conceivable to increase the storage capacity of the display memory employed in the display apparatus. In general, however, it is difficult to increase the storage capacity of the display memory employed in the display apparatus, once the memory has been incorporated in the apparatus. In particular, in the case of a display apparatus mounted on a device such as a portable terminal or a hand phone, the case for incorporating the display apparatus is small. Thus, it is even more difficult to increase the storage capacity of the display memory employed in such a display apparatus. 
   In addition, increasing the storage capacity of such as a display memory raises new problems such as a higher cost and rising power consumption. 
   SUMMARY OF THE INVENTION 
   Addressing the above problems, the inventors of the present invention have devised a display apparatus capable of displaying an image having a size greater than the storage capacity of a display memory by a simple operation. 
   In order to solve the problems described above, the present invention provides a display apparatus having a display memory for storing an image of a first area forming a portion of an entire image; a display section including a display screen having a display area usable for displaying at least a partial image of the first area; a case for exposing the display screen; scroll means for moving a display screen area appearing on the display screen over the entire image in accordance with a movement of the case; and control means for providing a virtual memory area for storing an image of a second area having a size greater than the size of the first area in the entire image when the scroll means moves the display screen area beyond the first area. 
   In this case, the image is a concept of showing a display object that can be recognized by an eye. The substance of the image can be video data, a pattern or text. 
   The display apparatus may further include detection means for detecting the direction of movement of the case and, based on the detected direction of movement, the control means identifies a partial area of the second area and limits the virtual memory area to be set to the partial area. 
   The control means may manage the second area by dividing the second area into a plurality of partial areas and may set the virtual memory area allocated to the partial areas in a memory device selected from a plurality of memory devices including external memory devices. 
   The control means may set the virtual memory area in a memory device selected from a plurality of memory devices based on the amount of image data of the partial areas and the storage capacity and/or access speed of each of the memory devices. 
   The display apparatus may further include communication means connectable to a plurality of networks. The control means may manage the second area by dividing the second area into a plurality of partial areas and may set the virtual memory area allocated to the partial areas in a memory device selected from a plurality of memory devices connected to the networks. 
   The memory device setting the virtual memory area may be selected based on the amount of image data of the partial areas and the communication speed and/or band of each of the networks to which the plurality of memory devices are connected. 
   The display apparatus may further include a buffer for storing a portion of the image. The control means may transfer the image of a third area including a particular partial area included in the second area as a partial area approached by the display screen area or closest to the display screen area from a particular virtual memory area allocated to the particular partial area to the buffer while the display screen area still exists in the first area; and when the scroll means has moved the display screen area to a location outside the first area, an area to be accessed for displaying the display screen area is switched from the display memory to the buffer. 
   Operations of the present invention are described as follows. 
   The scroll means moves the display screen area appearing on the display screen over the entire image in the case. During the operation to move the display screen area, the image of the first area is read out from the display memory as a portion of the entire image to be displayed on the display screen as long as the display screen area is moving within the first area. Then, when the scroll means has moved the display screen area to a location outside the first area, the control means sets a virtual memory area used for storing the image of a second area with a size exceeding the size of the first area in the entire image. 
   The virtual memory area can be set in a memory employed in the display apparatus as a memory other than the display memory or in an external memory device. The virtual memory area is allocated to image data included in the entire image as image data that cannot be stored in the display memory. 
   That is to say, when the scroll means moves the display screen area to a location outside the first area, image data is read out from the virtual memory area to be displayed on the display screen. 
   In accordance with the present invention, it is possible to display an image having a size greater than the storage capacity of a display memory by a simple operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing a perspective view of the external appearance of a display apparatus according to an embodiment of the present invention schematically; 
       FIG. 2  is a block diagram showing the hardware configuration of the display apparatus according to the embodiment; 
       FIG. 3  is a block diagram showing the configuration of a circuit for controlling a movement of an image and displaying an image; 
       FIG. 4  is a diagram showing a model of a relation between the area of an entire image and a display screen area; 
       FIG. 5  is a diagram showing a relation between the display area of a display screen and memory areas; 
       FIG. 6  is a diagram showing a state in which an entire image is divided into a plurality of partial images in accordance with a second embodiment of the present invention; 
       FIG. 7  is a diagram showing a model of a case in which a virtual memory area including a plurality of partial areas is set in a plurality of memory devices located at distributed locations; 
       FIG. 8  is a diagram showing a table included in a control section as a table of management states of the virtual memory area set in a plurality of memory devices shown in  FIG. 7 ; and 
       FIG. 9  is a diagram showing a state in which an entire image is divided into a plurality of partial images in accordance with a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   First Embodiment 
   An embodiment of the present invention is explained by referring to diagrams as follows. 
     FIG. 1  is a diagram showing a perspective view of the external appearance of a display apparatus  1  according to the embodiment of the present invention schematically. 
   As shown in  FIG. 1 , the display apparatus  1  has a case  2  with a size allowing the case to be held for example by a hand. A display section  3  and an operation section  4  are provided on the front face  2   a  of the case  2 . On a side face  2   b  of the case  2 , a mounting section  6  is provided. The mounting section  6  allows a disk-shaped storage medium  5  to be mounted on the mounting section  6  and dismounted from the mounting section  6  with a high degree of freedom. The display section  3  is typically a color LCD (Liquid Crystal Device). The operation section  4  is provided with operations buttons  41  to  43  to be operated by the user to carry out operations on the display apparatus  1 . 
   [Configuration of the Display Apparatus 1] 
     FIG. 2  is a block diagram showing the hardware configuration of the display apparatus  1 . 
   As shown in  FIG. 2 , the display apparatus  1  includes the display section  3  mentioned above, the operation section  4  cited above, a CPU serving as a control section  7 , a ROM  8 , a main memory  9 , a display memory  10 , a storage medium interface (storage medium I/F)  14 , a sensor interface (sensor I/F)  12  and a communication interface (communication I/F)  13 , which are connected to each other by a system bus  15 . 
   The control section  7  includes a CPU as the principal component. The control section  7  is a section for controlling the whole display apparatus  1 . The ROM  8  is a memory used for storing programs necessary for controlling the display apparatus  1 . In addition, the control section  7  also implements a scroll display function to be described later. 
   The storage medium interface  14  is connected to the mounting section  6  so that the storage medium interface  14  is capable of communicating data with the storage medium  5  mounted on the mounting section  6 . If necessary, a virtual memory area to be described later is set on the storage medium  5 . 
   The sensor interface  12  is connected to a position detection sensor  11 . The position detection sensor  11  is a sensor for detecting a vector quantity representing the distance of a movement of the case  2 . 
   An acceleration sensor can be employed as a typical position detection sensor  11 . The acceleration sensor is a device for detecting accelerations in two axial or three axial directions as accelerations of the display apparatus  1  typically held by a hand of the user and moved in a space. 
   Let us assume for example that the acceleration sensor is a sensor for detecting accelerations in two axial directions. In this case, the acceleration sensor detects the vertical-direction and horizontal-direction components of an acceleration applied to the display apparatus  1  and integrates each of the components with respect to the time axis in order to compute respectively vertical-direction and horizontal-direction components of the velocity and displacement for the acceleration. In addition, an acceleration sensor, an angular-velocity sensor and an earth magnetic sensor can be combined to form the position detection sensor  11 . 
   The movement distance obtained as a detection result produced by the position detection sensor  11  is used in the scroll display function described later as a function to display an image. 
   The ROM  8  is used for storing a basic program required for operating the display apparatus  1  and a program for executing the function to display an image. 
   The main memory  9  is a RAM having a large storage capacity. A program stored in the ROM  8  is loaded into the main memory  9  to be executed by the CPU employed in the control section  7 . 
   The main memory  9  is also used for storing image data, which is also properly referred to hereafter as an image. In addition, a virtual memory area to be described later is set in the main memory  9  when necessary. 
   The display memory  10  is typically a RAM used for storing data of an image to be displayed. The image data stored in the display memory  10  as data of an image to be displayed on the display screen of the display section  3  is data transferred from the main memory  9 . In the case of an image having a very large size, however, the data of the image cannot be all stored in the display memory  10 . In this case, only a portion of the image data is transferred from the main memory  9  to the display memory  10 . 
   It is to be noted that, in some cases, the display memory  10  is a partial area of the main memory  9  or a memory integrated in with the display section  3 . 
   As described above, the operation section  4  is provided with the operations buttons  41  to  43  to be operated by the user to carry out operations on the display apparatus  1 . For example, the user operates one of the operations buttons  41  to  43  in order to select an image to be displayed from those stored in the main memory  9 . In addition, the user is also capable of operating one of the operations buttons  41  to  43  in order to activate or deactivate the scroll display function. 
   The communication interface  13  is a communication processing section for connecting the display apparatus  1  to an external network. For example, the communication interface  13  connects the display apparatus  1  to the Ethernet in order to exchange data with an external memory device through the Ethernet. 
   Scroll Display Function 
   The display apparatus  1  has the scroll display function mentioned earlier. The scroll display function can be implemented by hardware, software or both. The scroll display function of the display apparatus  1  is explained by referring to  FIG. 3  as follows. 
     FIG. 3  is a block diagram showing the configuration of a circuit for controlling a movement of an image and displaying an image. 
   The configuration of the circuit includes the display section  3 , the position detection sensor  11 , a base address register  71 , a display address counter section  72  and the display memory  10 . The base address register  71  and the display address counter section  72  are each a component included in the control section  7 . 
   The display address counter section  72  includes an X counter  72 A and a Y counter  72 B. The X counter  72 A is a counter for generating X-coordinate data (X) denoted by reference numeral  722  on the basis of address data  721  showing a scanning point (x, y) scanning the whole area of the display screen of the display section  3 . By the same token, the Y counter  72 B is a counter for generating Y-coordinate data (Y) denoted by reference numeral  723  on the basis of the address data  721  showing a scanning point (x, y) scanning the whole area of the display screen of the display section  3 . 
   On the other hand, the base address register  71  is a register for holding X base data X b  and Y base data Y b  as a base address of a reference point of an image stored in the display memory  10  and displayed on the display screen of the display section  3 . 
   In the display memory  10 , an address area in the range (X b , Y b ) to (X b +X, Y b +Y) is read out from the display section  3  and displayed on the display screen. 
   At a point of time data of an image is transferred to the display memory  10 , the X base data X b  and the Y base data Y b , which are output by the base address register  71  as the coordinates of an origin of a display, coincide with the addresses of an origin of a storage area in the display memory  10 . The origin of a storage area in the display memory  10  is a point set in advance to serve as a reference. 
   The origin of a storage area in the display memory  10  is set typically at a point on the left lower corner of a display screen area of the display section  3  for a case in which the position of the display screen area is adjusted to coincide with the center of the area of the display memory  10 . 
   In this state, when the display apparatus  1  is moved by a hand holding the display apparatus  1 , the value S 11  of a movement distance detected by the position detection sensor  11  is supplied to the base address register  71  as the X base data X b  and the Y base data Y b  to be used for updating the coordinates of the origin. The updated coordinates of the origin are used in the display memory  10  as the coordinates of a reference point of an area to be cut out from an image to be displayed on the display screen of the display section  3 . 
   Thus, image data S 10  read out from the display memory  10  changes in accordance with the value S 11  of a movement distance detected by the position detection sensor  11 . 
   With regard to the scroll display function, a relation between an entire image and a display screen area is described by referring to  FIG. 4 . 
   In this case, the entire image is like an image stored in the main memory  9  and read out by an operation carried out by the user. However, the entire image is used from a standpoint of distinguishing the entire image from a displayed image, which is defined as an image displayed on the display screen of the display section  3 . Thus, by execution of the scroll display function, a portion of the entire image is displayed as a display image on the display screen of the display section  3 . 
     FIG. 4  is a diagram showing a model of a relation between the area of an entire image and the display screen area. In the diagram, reference notation WI denotes the area of an entire image and reference notation DA denotes the display screen area of the display section  3 . The area of an entire image is also referred to hereafter as an entire image area WI. 
   In the case shown in  FIG. 4 , when the display apparatus  1  is moved in the left direction, the position detection sensor  11  detects the movement and the entire image area WI is scrolled in the right direction over the display screen area DA. When the display apparatus  1  is moved in the right direction, on the other hand, the position detection sensor  11  detects the movement and the entire image area WI is scrolled in the left direction over the display screen area DA. By the same token, when the display apparatus  1  is moved in the downward direction, the position detection sensor  11  detects the movement and the entire image area WI is scrolled in the upward direction over the display screen area DA. When the display apparatus  1  is moved in the upward direction, on the other hand, the position detection sensor  11  detects the movement and the entire image area WI is scrolled in the downward direction over the display screen area DA. 
   As is obvious from the above description, by moving the display apparatus  1  in the left, right, upward and downward directions, the user is capable of viewing the entire image area WI. 
   It is to be noted that, in order to make the explanation simple, instead of explaining that the entire image area WI serving as a background image moves over the fixed display screen area DA in a scroll operation, the following description is mainly given by assuming that the display screen area DA moves over the fixed entire image area WI. It is needless to say, however, that motion of the entire image area WI over the fixed display screen area DA is essentially the same phenomenon as the motion of the display screen area DA over the fixed entire image area WI. 
   Virtual Memory Area 
   In addition, in the display apparatus  1  according to the embodiment, a virtual memory area is provided so as to allow an entire image with a size exceeding the storage capacity of the display memory  10  to be displayed by execution of the scroll display function. The virtual memory area is explained as follows. 
     FIG. 5  is a diagram showing a relation between the display area of the display screen and memory areas. In the figure, reference notation DM denotes the display memory area, which is defined as the area of the display memory  10 , and reference notation IM denotes the virtual memory area. 
   It is to be noted that the display memory area DM corresponds to a first area provided by the present invention. On the other hand, an outer-side area included in the entire image as an area outside the display memory area DM corresponds to a second memory provided by the present invention. 
   As described earlier, the origin serving as the reference of a screen display on the display memory area DM is set so as to position the display screen area DA at the center of the display memory area DM. 
   Data of an image having a very large size cannot all be transferred from the main memory  9  to the display memory  10 . Thus, only a portion of the data or a portion of the entire image is transferred from the main memory  9  to the display memory  10 . The portion transferred from the main memory  9  to the display memory  10  is a portion that can be accommodated in the display memory  10 . In this case, the entire image area WI is greater than the display memory area DM as shown in  FIG. 5 . 
   The virtual memory area IM is set for displaying a portion included in the entire image area WI as a portion outside the display memory area DM. It is desirable to set the virtual memory area IM at a location included in the main memory  9 , which can be accessed at a high speed. However, the location of the virtual memory area IM is not limited to such a location. For example, the virtual memory area IM can also be set in the storage medium  5 . As another alternative, the virtual memory area IM can also be set in an external memory device that can be accessed through the communication interface  13 . 
   Processing to set the virtual memory area IM is explained as follows. 
   By execution of the scroll display function, the display screen area DA is moved in the upward, downward, left or right direction from a position shown in  FIG. 5 . If the range of the movement of the display screen area DA is within the range of the display memory area DM, a scroll display process is carried out in accordance with the operation explained before by referring to  FIG. 3 . 
   Then, as the execution of the scroll display function in the same direction is continued, the display screen area DA moves beyond the range of the display memory area DM. That is to say, on the basis of the movement distance found from the output of the position detection sensor  11 , the control section  7  determines that the display screen area DA has moved to a location outside the range of the display memory area DM. At that time, the virtual memory area IM is allocated in the main memory  9  and used for storing the image data, that is, the data of the entire image. An operation to read out the image from the virtual memory area IM to the display section  3  is the same as the operation explained earlier by referring to  FIG. 3 . 
   As described above, for an image having a size greater than the size of the display memory area DM, an virtual memory area IM is set if necessary. Thus, by execution of the scroll display function, the user is capable of viewing the entire image. 
   It is to be noted that, while the display screen area DA still exists in the range of the display memory area DM, the range reserved as a range in which the virtual memory area IM to be set may be limited to a smaller storage area in the main memory  9  on the basis of the movement direction found from the output of the position detection sensor  11  as the direction of the movement of the display screen area DA. In this way, the area of the main memory  9  can be utilized with a higher degree of efficiency. 
   For example, if the display screen area DA is moving in range of the display memory area DM in the left direction, a virtual memory area IM may be set in an area included in the main memory  9  as an area corresponding to only on the left side adjacent to the display memory area DM. 
   Second Embodiment 
   Next, a second embodiment provided by the present invention as an embodiment for implementing a display apparatus provided by the present invention is explained. 
   In the case of the display apparatus according to the first embodiment described above, the virtual memory area IM is all set in the main memory  9 . In the case of the second embodiment, however, the virtual memory area IM can also be set in the storage medium  5  or an external memory device, which can be accessed through the communication interface  13 . 
   Thus, this embodiment is effective for a case in which the amount of data of an image is extremely large so that the storage capacity of the main memory  9  is not large enough for storing all the data of the image. 
   In addition, if the amount of data of an image is extremely large, the image is divided into a plurality of partial images, and a virtual memory area IM may be allocated in a plurality of memory devices including the storage medium  5  and the main memory  9  as a memory used for storing the partial images. 
     FIG. 6  is a diagram showing a state in which an entire image is divided into a plurality of partial images. In this diagram, a typical virtual memory area IM allocated in the entire image area WI to a portion included in the entire image as a portion outside the display screen area DA includes a plurality of partial areas V 11  to V 44 . It is to be noted that the number of partial areas is not limited to the number in the state shown in the figure. That is to say, the number of partial areas can be changed. 
   In the display apparatus according to this embodiment, each memory device is selected from a plurality of memory devices so that the memory device is optimally allocated to each of the partial areas. 
   For example, the amount of data of each partial area varies from image to image. In this case, the setting position of the virtual memory area IM consisting of the partial areas may be changed in dependence on the amount of image data. That is to say, a rank is assigned to each of the memory devices in advance from the data-amount point of view, that is, from the standpoint of the variable storage capacity of each of the memory devices. Then, in dependence on the amount of data of each partial area, the virtual memory area IM is set in a series of memory devices selected in a decreasing-rank order starting with the memory device having the high rank. By selecting memory devices in this way, the load of swapping data among the memory devices can be reduced. 
   In addition, a rank may also be assigned to each of the memory devices in advance from the access-speed point of view, that is, from the standpoint of the variable access speed of each of the memory devices. Also in this case, the virtual memory area IM is set in a series of memory devices selected in a decreasing-rank order starting with the memory device having the high rank in dependence on the amount of data of each partial area. By selecting memory devices in this way, it is possible to improve a display response characteristic showing a response to a transition of the display screen area DA from the display memory area DM to the virtual memory area IM. 
   A rank may also be assigned to each of the memory devices in advance on the basis of a combination of the variable storage capacity and variable access speed of each of the memory devices. 
   On the other hand, the virtual memory area IM consisting of the partial areas may be set in a plurality of memory devices connected to the display apparatus by a network. Also in this case, in the same way, the setting location of the virtual memory area IM consisting of the partial areas is changed on the basis of the amount of data in each of the partial areas. For example, a rank is assigned to each of the memory devices in advance from the standpoint of the communication speed and/or band of a network, through which accesses are made to the memory device, and a virtual memory area IM is set in memory devices selected sequentially in a decreasing-rank order starting with a memory device with a highest rank in accordance with the amount of data in each of the partial areas. It is thus possible to improve the display responsiveness, which is exhibited when the display screen area DA transits from the display memory area DM to the virtual memory area IM. 
     FIG. 7  is a diagram showing a model of a case in which the virtual memory area IM consisting of a plurality of partial areas is set in memory devices installed at distributed locations. The memory devices include external memory devices  80  and  81 , memory devices  90  to  93  connected to a network A as well as memory devices  94  to  97  connected to a network B. In this way, it is possible to set the virtual memory area IM for executing the scroll display function in memory devices accessible to the display apparatus  1  without regard to whether accesses to any of the memory devices are made by the display apparatus  1  through radio or wire transmission. 
     FIG. 8  is a diagram showing a table of management states of the virtual memory area IM set in a plurality of memory devices as shown in  FIG. 7 . A device name on a column of the table shown in  FIG. 8  represents reference numerals each denoting a memory device as shown in  FIG. 7 . As shown in the table, the control section  7  determines a memory device, in which a virtual memory area IM consisting of any particular one of the partial areas V 11  to V 44  is set, on the basis of the size of an image file displayed on the particular partial area. 
   As described above, in accordance with the display apparatus  1  according to this embodiment, in the case of an image with a size exceeding the size of the display memory area DM, the virtual memory area IM is set in external memory devices if necessary in order to allow the user to view the whole of the image without modifying and/or increasing the display memory  10  and/or the main memory  9 , which have already been built in the display apparatus  1 . 
   Third Embodiment 
   Next, a third embodiment of the present invention is explained. 
   In the display apparatus  1  according to the first and second embodiments described above, partial areas included in the entire image area WI as portions existing outside the display memory area DM are set in the virtual memory area IM. However, the display responsiveness, which is exhibited by an external apparatus including a virtual memory area IM when the display screen area DA transits from the display memory area DM to the virtual memory area IM, may be slow in some cases. 
   For example, in the case of the example shown in  FIGS. 7 and 8 , the image data of the partial area V 12  exists in a virtual memory area set in the memory device  91  connected to a network. By the same token, the image data of the partial area V 13  exists in a virtual memory area set in the memory device  92  connected to the network. With the display screen area DA existing in the display memory area DM, image data is read out from the display memory area DM and displayed at a high speed. As soon as the display screen area DA transits from the display memory area DM to the partial areas V 12  and V 13 , however, image data must be read out from the virtual memory areas through the network A. At that time, the operation to read out the image data is slow due to a communication delay and other causes. 
   In order to solve the problem seen from this point of view, the present invention provides a third embodiment devised by inventors of the present invention as an embodiment for improving the display response characteristic even for an event in which the display screen area DA transits from the display memory area DM to the virtual memory area IM. 
   To put it concretely, in order to improve the display response characteristic even for such an event, the display apparatus according to this embodiment is provided with a buffer memory serving as an interface between the display memory and the virtual memory. The buffer memory is used for storing image data of a portion of the entire image area WI. Image data of the portion corresponding to a particular virtual memory area selected dynamically in dependence on a location included in the display memory area DM as the location of the display screen area DA is transferred in advance from the particular virtual memory area to the buffer memory. Thus, it is possible to improve the display response characteristic even for an event in which the display screen area DA transits from the display memory area DM to the particular virtual memory area. Since it is not necessary to transfer all the image data of the entire image area WI from all virtual memory areas to the buffer memory, the efficiency to utilize the buffer memory provided in the display apparatus can be improved. 
   It is to be noted that, in the configuration of the display apparatus  1  shown in  FIG. 2 , the buffer memory can be provided as an area inside the main memory  9  or as an independent buffer connected to the system bus  15 . 
   The operation of the display apparatus  1  according to this embodiment is explained by referring to  FIG. 9  as follows. 
   Much like  FIG. 6 ,  FIG. 9  is a diagram showing a state in which an entire image is divided into a plurality of partial images. The diagram of  FIG. 9  is different from the diagram of  FIG. 6  in that, in the case of the diagram of  FIG. 9 , the display memory area DM is managed by dividing the display memory area DM into a plurality of display partial areas R 1  to R 4 . 
   At a point of time an image is initially displayed on the display screen area DA, that is, in an initial state, the origin serving as the reference of the screen display on the display memory area DM is set at a position making the center of the display screen area DA coincide with the center of the display memory area DM. As the scroll display function is executed, the display screen area DA moves from this initial state over the display memory area DM in an upward, downward, left or right direction. 
   As shown in  FIG. 9 , a point at the center of the display screen area DA is referred to as the reference point Ref of the display screen area DA. The reference point Ref may move to any one of the display partial areas R 1  to R 4 . The virtual memory area IM from which image data is to be transferred to the buffer memory is determined in accordance with a display partial area to which the reference point Ref has moved. It is to be noted that, the image data is transferred from a virtual memory area IM to the buffer area in advance while the entire display screen area DA still exists in the display memory area DM. 
   For example, the scroll display function is executed to move the reference point Ref from the initial state to the display partial area R 1  in the display memory area DM as described above. In this case, image data of the display partial area R 1  of the display memory area DM and image data in the partial areas V 11 , V 12  and V 21  of the virtual memory area IM are transferred in advance to the buffer memory. Typically, the partial areas V 11 , V 12  and V 21  of the virtual memory area IM have been set in memory devices. That is to say, pieces of image data are transferred from the partial areas R 1 , V 11 , V 12  and V 21  to the buffer memory as parts of the entire image. 
   Then, as the display screen area DA goes beyond the display memory area DM as a result of execution of the scroll display function by the user, the access object for a screen display is switched from the display memory  10  to the buffer memory. At a point of time the access object for a screen display is switched from the display memory  10  to the buffer memory, necessary image data has been stored in the buffer memory. Thus, it is not necessary to make an access to a virtual memory area set in an external memory device. As a result, the response characteristic of the screen display is improved. 
   By the same token, if the scroll display function is executed to move the reference point Ref from the initial state to the display partial area R 2 , R 3  or R 4  in the display memory area DM, the following pieces of image data are transferred in advance from a virtual memory area to the buffer memory for cases (1) to (3) as follows: 
   Case (1): The reference point Ref is moved from the initial state to the display partial area R 2 . 
   In this case, pieces of image data are transferred from the partial areas R 2 , V 13 , V 14  and V 22 . 
   Case (2): The reference point Ref is moved from the initial state to the display partial area R 3 . 
   In this case, pieces of image data are transferred from the partial areas R 3 , V 31 , V 41  and V 42 . 
   Case (3): The reference point Ref is moved from the initial state to the display partial area R 4 . 
   In this case, pieces of image data are transferred from the partial areas R 4 , V 32 , V 43  and V 44 . 
   It is to be noted that a group of partial areas, from which pieces of image data are transferred to the buffer memory, corresponds to a third area of the present invention. In addition, the number of partial areas shown in  FIG. 9  is typical. In other words, it is needless to say that the number of partial areas can be properly changed. 
   Also in the display apparatus  1  according to this embodiment, any one of the partial memories V 11  to V 44  included in the virtual memory area IM as a portion that cannot be accommodated in the display memory area DM is optimally set in one selected among memory devices existing at distributed locations. 
   For example, as explained in the description of the second embodiment, the setting position of the virtual memory area IM consisting of the partial areas may be changed in accordance with the amount of data included in each of the partial areas. That is to say, a rank is assigned to each of a plurality of memory devices in advance in accordance with an assignment method seen from the standpoints of the storage capacity (or the amount of data) and/or access speed of each of the memory devices. Then, the virtual memory area IM is set in memory devices selected sequentially in a decreasing-rank order starting with a memory device with a highest rank in accordance with the amount of data in each of the partial areas. It is thus possible to improve the efficiency of the operation to transfer data from a virtual memory area to the buffer memory. 
   If the virtual memory area IM consisting of partial areas is set in a plurality of memory devices connected to a network, on the other hand, a rank is assigned to each of the memory devices in advance from the standpoints of the communication speed and/or band of a network, through which accesses are made to the memory device, and a virtual memory area IM is set in memory devices selected sequentially in a decreasing-rank order starting with a memory device with a highest rank in accordance with the amount of data in each of the partial areas. It is thus possible to improve the efficiency of operations to transfer data from the memory devices to the buffer memory. 
   Elements of the first to third embodiments described above can be combined properly to create a new embodiment. In addition, embodiments of the present invention are not limited to the first to third embodiments described above. A person skilled in the art is capable of creating a variety of modified versions of the embodiments within a range not deviating from essentials of the present invention. 
   In addition, it should be understood by those skilled in the art that a variety of modifications, combinations, sub-combinations and alterations may occur in dependence on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.