JPH10161638A - Image display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten the time required for loading a display data from a video memory in the display of text mode. SOLUTION: This device has a font data converting circuit 22 and a font address generating circuit 23. A video memory plane 3 is added to a video memory 36, and in conformation to the output of a scanning line count means for designating a scanning line for character display, the font data of each character is continuously accessed and loaded in the order of character text code every scanning line from the continuous space of the video memory plane 3 through a memory interface 24, and an image signal is outputted from a display circuit 25 to a CRT device 37.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image display device, and more particularly to an image display device for accessing font data stored in a video memory.

[0002]

2. Description of the Related Art Conventionally, in this type of image display device, during a display period, display data must be loaded from a memory called a video memory or a video buffer within a predetermined time to output an image signal. This fixed time is determined by the display clock frequency or the resolution. If the load of the display data cannot keep up with the output of the image signal at the display speed, correct display is not performed.

There are two methods of storing display data in a video memory: a plane type and a packed pixel type. In the plane type, 1-bit display data per pixel is stored in one plane. In the case of a 4-plane configuration, the display data per pixel is 4 bits. In the packed pixel type, there is only one plane, and the display data per pixel is 1 plane.
It is stored in bytes or 2 bytes.
Since this patent relates to a method of using a plane type video memory, a conventional plane type technology will be described below.

[0004] The plane type display mode includes a graphic mode and a text mode, and a method of storing display data in a video memory and a method of loading display data differ depending on each mode.

In the graphic mode, color data corresponding to one pixel is stored in a video memory. For example, in the case of a 16-color graphic, one pixel is made up of 4-bit display data, and each of the four planes stores one-bit data.

In the graphic mode, display data continuous in the display scan line direction is accessed from a DRAM constituting a video memory by an access method called high-speed access or page access (hereinafter referred to as high-speed access), and 64 pixels are accessed. , Or in 128-pixel transfer units. This transfer unit depends on the size of the FIFO of the image display device. In the graphic mode, the display data loaded is converted into a palette to generate RGB data.

On the other hand, in the text mode, the storage method and the loading method of the display data in the video memory are different from those in the graphic mode. In the video memory, a character code and an attribute code of a text to be displayed from the upper left on the screen are stored at consecutive addresses. The character code is usually 8 bits, and one character can be selected from 256 characters. The attribute code is also usually 8 bits, the lower 4 bits,
The upper 4 bits indicate a foreground color and a background color, respectively. These character codes and attribute codes are updated by the CPU when scrolling, clearing the screen, and the like. In addition to these character codes and attribute codes, the video memory stores character font data. This character font data is bitmap data of a character font that is not frequently updated from the CPU like a character code and an attribute code after being loaded by the CPU when the text mode is set, and exists in a fixed area of the video memory. Usually, character code,
The attribute code and the font data are stored separately in plane 0, plane 1 and plane 2.

In the text mode, when loading display data, first, a character code and an attribute code are loaded in the horizontal scan line direction in a transfer unit such as 8 characters or 16 characters. This transfer unit also depends on the size of the FIFO of the image display device, as in the graphic mode. The load at this time can use the high-speed access of the DRAM. This is because data in a continuous area in the video memory is read. When the loading of the character code and the attribute code is completed, the font data is loaded next. When loading the font data, the address of the font data is calculated from the character code and the display scan line value.

[0009] Programmers Guide to the EGA and VGA C
ards Second Edition (1990, Richard F. Ferrar
o)), the font data is stored in a continuous area of the video memory per character, for example, in a continuous space of 32 bytes, as shown in FIG. Normally, font data per character is 8 pixels wide and 32 pixels wide.
It consists of 32 bytes of pixels. For the font data of the character code 00h, the head scan line data is stored at the address 00h of the video memory plane 2;
The character code 01h is the address 20 of the plane 2
h stores the first scan line data.

As described above, in the text mode, the font data is loaded following the loading of the character code and the attribute code. The major difference from the graphic mode is that in the graphic mode, the loaded display data can be used directly as pixel data, whereas in the text mode, the font data must be loaded following the loading of character codes and attribute codes. That is. For this reason, the text mode requires about twice as much video memory access time as the graphic mode.

Since the character code and the attribute code in the text mode are stored in the continuous space of the video memory according to the display direction, it is possible to use the high-speed access of the DRAM when loading. High-speed access is, for example, about four to five times faster than random access with a transfer time of eight addresses.

On the other hand, loading font data in the text mode does not always enable high-speed access to the DRAM. The reason is that the address calculated from the character code and the display scan line value may not be the address of the same page of the DRAM.
DRAM high-speed access can be used only when accessing the same page address. In FIG.
7 shows how character codes, attribute codes, and font data are loaded in a text mode. FIG. 8 shows an example in which all font data loads are random access.

As shown in FIG. 7, the font data is stored in a plane 2 of the video memory in a continuous space of 32 bytes per character. Fonts of adjacent character codes, for example, character codes 00h, 01h, 02h, etc.
Present on the same page of DRAM. However, font data whose character code is distant, for example, character code 00h
And the font data of the character code FFh exist in different pages of the DRAM. The order of the character codes to be displayed is 0
0h, FFh, 01h, FEh, 02h, FDh, 03
In cases such as h and FCh, the loading of font data from the DRAM is all random access.

[0014]

A problem with the conventional image display device is that the high-speed access of the DRAM can be used for loading character codes and attribute codes in the text mode, but the high-speed access is not necessarily used for loading font data. That is not possible.

The reason is that, since each character font is stored in a continuous video memory space, it is not possible to access the address of the same page of the DRAM depending on the character code arrangement to be displayed.

In a conventional image display device, 640 × 400
At a low resolution such as (80 rows × 25 columns), there is no problem because there is a margin in the load time of display data with respect to the display speed. However, 1056x400 (132
When the resolution becomes high (eg, row x 25 columns), the display speed increases, and the load time of the display data must be reduced. If the font data is loaded by random access, it is difficult to reduce the loading time. If the load time cannot be reduced, normal display at high resolution is not performed.

The present invention has been made in consideration of the above circumstances, and has as its object to reduce the time required to load display data from a video memory in text mode display.

[0018]

Therefore, an image display apparatus according to the present invention provides a font for each character from a continuous space of a video memory plane corresponding to an output of a scan line counting means for specifying a scan line for character display. The data is successively accessed and loaded in the order of the character text code for each scan line unit as display data, and an image signal is output.

Also, at the time of system startup or during a non-display period, font data of each character previously stored in a continuous space of another video memory plane in scan line order for each character text code unit is read out and decomposed in scan line units. A font data conversion means for converting the character text code in scan line units and storing the converted data in a continuous space of the video memory plane; and accessing and loading the font data of each character as one of display data. Font address generation means for generating font addresses of font data stored in the memory plane in the order of character text codes for each scan line.

Further, the font data conversion means includes:
An address conversion circuit is provided which shift-converts a read address of the other video memory plane and outputs it as a write address of the video memory plane.

Further, the font address generating means includes:
A font address bus is provided, wherein the output of the FIFO means for loading the character code to be displayed is the lower bit part of the font address and the output of the scan line counting means is the upper bit part of the font address.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the image display device of the present invention. Referring to FIG. 1, an image display device 35 according to this embodiment has a memory interface 24, a display circuit 25, and other components, as in a conventional image display device. Access and load stored display data. The same blocks as those of the conventional image display device will not be described repeatedly.

The image display device 35 of this embodiment further includes a font data conversion circuit 22 and a font address generation circuit 23. The video memory plane 3 is added to the video memory, and a scan line for specifying a scan line for character display is provided. Corresponding to the output of the counting means, the font data of each character is successively accessed and loaded from the continuous space of the video memory plane 3 as one of the display data in scan line units in scan line order,
The image signal is output to the device 36.

Here, as in the conventional image display device, in the text mode, the display characters are as follows.
Identified by 1 byte character code and 1 byte attribute code,
It is assumed that the font data of each character is stored in the plane 0 and the plane 1 of the video memory, respectively, and the font data of each character is stored in advance in the continuous space of the plane 2 of the video memory in the scan line order for each character text code unit. The plane 3 of the video memory is the image display device 3 of this embodiment.
5 is used as a storage conversion area for font data of plane 2 of the video memory.

The font data conversion circuit 22 reads out font data of each character previously stored in the continuous space of the plane 2 of the other video memory in the order of the character text code when the system is started or during the non-display period, and scans the data in units of scan lines. It is decomposed, divided into scan line units, converted into character text code order for each scan line unit, and stored in the continuous space of plane 3 of the video memory.

FIG. 2 shows the font data conversion circuit 22.
FIG. 3 is a block diagram illustrating a detailed configuration example. The font data conversion circuit 22 includes a source address counter circuit 6,
A destination address conversion circuit 4 and a font data latch 5 are provided. The source address counter circuit 6 further includes a source address register 2 and an incrementer 3, and is sequentially incremented from 0h to sequentially generate a read address of the plane 2 of the video memory. The font data latch 5 latches source font data read from the plane 2 of the video memory and outputs destination font data which is write data of the plane 3 of the video memory. Further, the destination address conversion circuit 4
Shifts the read address of plane 2 of the video memory and outputs it as the write address of plane 3 of the video memory. At this time, in the read address of the plane 2 of the video memory, the scan line of the character display and the lower bit portion and the upper bit portion corresponding to the character code are shifted and converted into the upper bit portion and the lower bit portion.

In the font data conversion circuit 22, the source address counter circuit 6 is incremented sequentially from 0h, and the font data is converted by reading the source font data from the plane 2 and writing the destination font data to the plane 3. Is repeated. This operation is performed for all font address areas of plane 2. The font data conversion work is started by the CPU so as to perform the conversion work when the system is started or during display blanking.

FIG. 3 shows the font data conversion circuit 22.
FIG. 9 is an explanatory diagram for describing a storage format of font data in a plane 3 converted by the above-described method. All 256 types of character font data are stored in scan line order. It is easy to understand when compared with the storage form of FIG. 7 which is a conventional technique. In this storage form, all 256 types of character font data are stored in a form divided into scan lines. The font data of scan line 0 of each character font is stored in a continuous 256-byte area from the top of plane 3. Scan line 1 is stored in the next continuous 256-byte area. In this way, the font data up to the scan line 31 is stored in the order of the scan lines 2, 3, and 4.

As described above, by storing the data in units of scan lines, the font data is loaded when the font data is loaded.
Fast access to RAM can be used. This is because a general DRAM address has a row address and a column address of 9 bits each, and the size of one page is 512 bytes (2 9). Therefore, since the font data of the same scan line is 256 bytes, the font data of two scan lines can fit in the same page. For this reason, when displaying one horizontal scan line, the font data can be loaded using the high-speed access of the DRAM.

The font address generation circuit 23 uses the D
When accessing and loading the font data of each character as one of the display data by the high-speed access of the RAM, the font address of the font data stored in the plane 3 of the video memory is generated in the order of the character text code for each scan line unit.

FIG. 4 shows the font address generation circuit 2
FIG. 3 is a block diagram showing a detailed configuration example of No. 3; The font address generation circuit 23 has a FIFO 15 for storing character codes for eight characters and a scan line counter circuit 7, and uses the output of the FIFO 15 as the lower bit part of the font address and outputs the output of the scan line counter circuit 7 to the font address. And a font address bus as an upper bit portion of The scan line counter circuit 7 further includes a character vertical width register 9, a comparator 10, an incrementer 11, a multiplexer 13, a scan line register 14, and the like, and outputs a signal of a vertical scan line value of a display character. For example, in the case of 8 × 16 font data, the scan line counter 7 indicates 0
Increment from h, count up to 15h and return to 0h again. At this time, the character vertical width register 9 stores 15
h. Since the font address of the same scan line changes only in the lower 8 bits that are the character code portion, the font address of the same scan line changes within a range of 256 bytes. This font address is converted into a high-speed access sequence of the DRAM and used for data access of the video memory.

FIG. 5 is a waveform diagram showing an operation example of the image display device of this embodiment, and shows the timing of access to the video memory. First, eight character codes and attribute codes are loaded from the video memory. Next, font data of character codes for eight characters is loaded. Since the font data is stored in a 256-byte video memory area that is continuous for each scan line,
It can be loaded using fast access to RAM. The operation of loading the character codes, attribute codes, and font data for eight characters is repeated 10 times in one horizontal scan line in the example of the 80-digit × 25-line mode. In this way, the display data character code, attribute code,
Since all the loading of the font data is performed using the high-speed access of the DRAM, the time required for the loading is reduced to about one half compared with the related art.

As a modification of the image display device of this embodiment, font data of an even scan line is stored in the plane 2 of the video memory 36,
6 plane 3 can store font data of odd scan lines. FIG. 6 is an explanatory diagram for describing a storage format of font data in planes 2 and 3 of video memory 36 in this modified example. In this storage mode, font data of two scan lines, an even scan line and an odd scan line, can be loaded by accessing one font address. Therefore, it is not necessary to load the font data every scan line, and it is sufficient to load the font data once every two scan lines. In this modified example, the number of accesses to the video memory accompanying the loading of the display data can be reduced by half compared to the present embodiment, so that the power consumption can be reduced.

[0035]

As described above, the image display device according to the present invention has an effect that the load time of display data can be reduced. In addition, there is an effect that text display at a high resolution can be dealt with.

The reason is that by storing and converting font data in the order of the character text code for each scan line, the font data at the time of display of the horizontal scan line is always DR rather than random access of the DRAM.
This is because reading and loading can be performed by high-speed access of the AM.

[Brief description of the drawings]

FIG. 1 is a partial block diagram illustrating an embodiment of an image display device of the present invention.

FIG. 2 is a block diagram illustrating a configuration example of a font data conversion circuit in the image display device of FIG. 1;

FIG. 3 is an explanatory diagram for describing a storage format of font data in a video memory converted by the font data conversion circuit of FIG. 2;

FIG. 4 is a block diagram illustrating a configuration example of a font address generation circuit in the image display device of FIG. 1;

FIG. 5 is a waveform chart showing an operation example of the image display device of FIG. 1;

FIG. 6 is an explanatory diagram for explaining a font data storage form in a video memory according to a modification of the embodiment of FIG. 1;

FIG. 7 is an explanatory diagram for describing a font data storage form in a video memory by a conventional image display device.

FIG. 8 is a waveform chart showing an operation example of a conventional image display device.

[Explanation of symbols]

 1 Font address of video memory plane 3 2 Source address register 3 Incrementer 4 Destination address conversion circuit 5 Font data latch 6 Source address counter circuit 7 Scan line counter circuit 9 Character vertical width register 10 Comparator 11 Incrementer 12 Constant value 0h 13 Multiplexer 14 Scan line register 15 Character code FIFO 16 Row address of character code and attribute code 17 Column address of character code and attribute code 18 Read data of character code and attribute code 19 Row address of font data 20 Column address of font data 21 Font Data read data 22 Font data conversion circuit 24 Memory interface 25 Display circuit 26 Character code 27 Attribute code 28 Font address of video memory plane 2 29 Row address of character code and attribute code 30 Column address of character code and attribute code 31 Read data of character code and attribute code 32 Row address of font data 33 Font data Column address 34 Read data of font data 35 Image display device 36 Video memory 37 CRT device

Claims (4)

[Claims] In accordance with an output of a scan line counting means for designating a scan line for character display, font data of each character from a continuous space of a video memory plane is set as one of display data as character text for each scan line unit. An image display device that continuously accesses and loads in code order and outputs an image signal. 2. During system startup or during a non-display period,
The font data of each character previously stored in the continuous space of another video memory plane in the scan line order for each character text code unit is read out, decomposed in scan line units, converted into the character text code order for each scan line unit, and converted to the video data order in the scan line unit. A font data converting means for storing the font data of each character in the continuous space of the memory plane, and a font address of the font data stored in the video memory plane when accessing and loading the font data as one of the display data on a scan line basis. The image display device according to claim 1, further comprising: a font address generation unit that generates a character text code for each character. 3. The image display device according to claim 2, wherein said font data conversion means comprises an address conversion circuit for shift-converting a read address of said another video memory plane and outputting it as a write address of said video memory plane. 4. A font in which the font address generating means sets an output of a FIFO means for loading a character code to be displayed as a lower bit part of the font address and an output of the scan line counting means as an upper bit part of the font address. 3. An address bus, comprising:
The image display device as described in the above.