GB2212367A - Window control using spare signal data bit - Google Patents

Abstract

A digital display system includes a CRT monitor 14 and two display controllers 10, 11. One controller, 10, displays data relating to an applications program in its respective window, whilst the other controller, 11, displays noticeboard data in its respective window. The outputs of the controllers are selectively coupled to the monitor by a multiplexer 12 which is controlled by a spare bit, VID7, of the noticeboard data. This allows the noticeboard to be at any position on the monitor's screen. <IMAGE>

Description

DIGITAL DISPLAY APPARATUS This invention relates to digital display apparatus. The invention is particularly, although not exclusively, concerned with apparatus for displaying the data output of a computer, either text or graphics, on a raster-scanned display such as a cathode-ray tube (CRT).

It is often desirable to divide a display into two or more separate areas, which are used for different purposes. For example, a part of the display may be used as a noticeboard for displaying control messages from the computer while the rest of the screen, referred to as the application area, is used to display data relating to the current processing task.

Our co-pending European patent application No.

87 30 27 05.6 describes one way of achieving this, by using two separate controllers for generating addresses for the video data, and switching between these controllers at a pre-selectable raster line of the display. One of the controllers generates addresses for the noticeboard, while the other generates addresses for the application area.

The object of the present invention is to provide an alternative way of producing separate display areas.

Summary of the invention According to the invention there is provided digital display apparatus comprising: (a) a display unit, (b) at least two display control units each of which produces a multi-bit video data output, and (c) a selection circuit for selecting between the outputs of the control units, the selected output being used to provide a video input signal for the display unit, and the selection circuit being controlled by a selection signal derived from the video data output of one of said display control units.

Brief description of the drawings One digital display apparatus in accordance with the invention will now be described by way of example with reference to the accompanying drawings.

Figure 1 is an overall block diagram of the display apparatus.

Figure 2 is a more detailed diagram showing one of two display control units forming part of the display apparatus.

Description of an embodiment of the invention.

Referring to Figure 1, the apparatus comprises two display control units 10, 11. The first of these is referred to as the application display control unit 10 and is used for generating video data relating to a current processing task. The second of these units is referred to as the noticeboard display control unit 11 and is used for generating video data relating to a noticeboard area.

Each of the control units 10, 11 produces an 8-bit video data output signal VID 0-7 which indicates the colour of the picture element (pixel) currently being scanned. In the present system, only the first six of these bits VID 0-5 are in fact used, the remaining two bits VID 6, 7 being spare.

One of the spare bits VID 7 from the noticeboard display control unit 11 is used as a control bit SEL, and is fed to the control input of a multiplexer 12. When this control signal is high, the multiplexer 12 selects the output VID 0-5 from the application display control unit 10 and when the control signal is low, the multiplexer 12 selects the output VID 0-5 from the noticeboard display control unit 11.

The six bits selected by the multiplexer 12 are fed, in pairs, to three digital-to-analog converter circuits 13, to produce three analog video signals R, G, B representing the required intensities for the three colours red, green and blue for the pixel currently being scanned. These three signals are fed to the red, green and blue video inputs of a CRT monitor 14, to produce the required display. It can be seen that each of the three colour signals can have any one of four different intensities, giving 64 different possible colours for each pixel in the display.

Referring now to Figure 2, this shows one of the display control units in more detail.

The display control unit includes a video random-access memory (RAM) 20 which holds the data to be displayed. In a graphics mode, this data represents colour values for the individual pixels of the display.

In a text mode, this data consists of character codes, identifying the characters to be displayed. The contents of the video RAM 20 can be read and updated in a conventional manner by a host computer (not shown).

The video RAM is addressed by a memory address signal MA from an address generation circuit 21 which produces a sequence of addresses for reading out data from the video RAM in the correct sequence for display.

In the graphics mode, the data output of the video RAM is applied directly to a video output circuit 22, which produces the video data output VID 0-7.

In the text mode, the data output of the video RAM is applied to a character generation circuit 23, along with a raster address signal RA from the address generation circuit 21, indicating which raster line of the row of characters is currently being scanned. The output of the character generation circuit 23 is fed to the video data output VID 0-7 for displaying the characters.

All the elements of the display control unit shown in Figure 2 may be of conventional form, and so it is not necessary to describe this unit in any further detail.

The address generation circuit 21 in each unit 10, 11 also produces two synchronisation signals HSYNC and VSYNC. The synchronisation signals from the noticeboard display control unit 11 are fed to the horizontal and vertical synchronisation inputs of the CRT monitor 14, so as to synchronise the display with the video data. The VSYNC signal from the noticeboard display control unit 11 is also connected to a synchronisation input in the application display control unit 10, so as to synchronise these two units with each other.

In summary, it can be seen that the present system has two separate display control units 10, 11.

Switching between these units is performed by a spare bit VID 7 of the video output of one of the units 11.

Thus, by writing suitable data into the video RAM in the unit 11, the multiplexer 12 can be switched between the two display control units 10, 11 at any specified positions of the display. This enables a noticeboard area to be displayed in any desired position on the screen. Furthermore, the multiplexer 12 may be switched several times during each frame, so as to allow several noticeboard areas and application areas to be displayed.

Claims (5)

1. Digital display apparatus comprising: (a) a display unit, (b) at least two display control units each of which produces a multi-bit video data output, and (c) a selection circuit for selecting between the outputs of the control units, the selected output being used to provide a video input signal for the display unit, and the selection circuit being controlled by a selection signal derived from the video data output of one of said display control units.

2. Digital display apparatus according to Claim 1 wherein said selection signal consists of one of the bits of the video data output of one of the display control units.

3. Digital display apparatus as claimed in any preceding claim, wherein said selection circuit includes a multiplexer and the selected output comprises six data bits which are fed in pairs to three digital-to-analog converter circuits to produce three outputs comprising said video input signal for the display unit.

4. Digital display apparatus as claimed in Claim 3, wherein the three outputs of the digital-to-analog converter circuits represent the required intensities of colours red, green and blue for a picture element currently being scanned.

5. Digital display apparatus substantially as hereinbefore described with reference to the accompanying drawings.

5. Digital display apparatus substantially as hereinbefore described with reference to the accompanying drawings.

CLAIMS Amendments to the claims have been filed as follows 1. Digital display apparatus for displaying data generated by a computer, the apparatus comprising: (a) a display unit, (b) at least two display control units each of which produces a multi-bit video data output, one of the display units being arranged to produce video data generated by an application program running on the computer, and the other display unit being arranged to produce noticeboard data comprising control messages from the computer, and (c) a selection circuit for selecting between the outputs of the control units, the selected output being used to provide a video input signal for the display unit, and the selection circuit being controlled by a selection signal derived from the video data output of one of said display control units.

2. Digital display apparatus according to Claim 1 wherein said selection signal consists of one of the bits of the video data output of one of the display control units.

3. Digital display apparatus as claimed in any preceding claim, wherein said selection circuit includes a multiplexer and the selected output comprises six data bits wilicil are fed in pairs to three digital-to-analog converter circuits to produce three outputs comprising said video input signal for the display unit.

4. Digital display apparatus as claimed in Claim 3, wherein the three outputs of the digital-to-analog converter circuits represent the required intensities of colours red, green and blue for a picture element currently being scanned.