GB2186766A

GB2186766A - Character display apparatus

Info

Publication number: GB2186766A
Application number: GB08603850A
Authority: GB
Inventors: David Edward Penna
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1986-02-17
Filing date: 1986-02-17
Publication date: 1987-08-19
Also published as: EP0238113A3; JPS62249188A; JPH083704B2; GB8603850D0; EP0238113B1; EP0238113A2; DE3750766T2; DE3750766D1

Abstract

A logic processor-controlled data display apparatus has a display memory in which pixel data representing text for display is stored in bit-map form. The pixel data is produced for each display from character data stored in a background memory. The character data is stored in the form of machine code sub-routines. The sub-routine for a character contains instructions for identifying the shape-defining pixels (dots) of the character relative to a base dot position and the sub-routine is run to write these pixels into the display memory following location of the base dot position in the display memory. The invention requires fewer programme steps than when character data is stored in the background memory in bit-map form. <IMAGE>

Description

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SPECIFICATION

Data Display Apparatus

5 This invention relates to digitally operable data display apparatus of a type for displaying as an entity on the screen of a CRT (cathode ray tube) or other display device a quantity of data which is represented by digital codes stored in a display memory, the 10 displayed data being in the form of discrete pixels or dots, each of which has its colour and/or luminance defined by a respective digital code in the display memory at a location corresponding to the position of the pixel in the display, the apparatus including a 15 processorforcontrolling digitallythe storage, selection and display of data.

The display produced by appratus of the above type is termed a bit-map display, and is, for example, a 320 x 250 resolution dot matrix colour display. In the case 20 of a raster scan display device the digital codes stored in the display memory are accessed repeatedly by the processor to update the display in a recu rrent cycle of scanning lines which may be produced with or without interlaced field scanning.

25 Where a data display apparatus of the above type is to provide comprehensive and visually aesthetictext displays, it is usually a requirement that data fora large number of different characterfonts is provided in a background memory and the data for individual 30 characters selected for display is read from the background memory and written into the display memory.

It is known to store the data for each character of each characterfont in a bit-map form which corres-35 ponds to the image and size of the character shape. This character data, when selected, can then be transferred directly to the display memory with a minimum of processor logic. However, where the total number of character shapes is large the amount of 40 background memory needed to store all the character data can be excessive. With a view to reducing the amount of character data that has to be stored at the expense of increasing the complexity of the processor logic, it is also known to provide software algorithms 45 which can be used to scale a characterto produce displayed characters of different height and/or different width from the same character data. However,

this technique, in addition to increasing the complexity of the processor logic, has the shortcoming that the 50 resolution of some displayed characters suffers and other characters become crude in shape and no longer visually aesthetic.

It is an object of the present invention to provide for a data display apparatus of the above type the 55 character data for a large number of characterfonts, but without the above-mentioned disadvantages of excessive background memory or complex processor logic accompanied by possible degrading of displayed character shapes.

60 According to the present invention, there is provided a method of storing character data in the background memory of a data display apparatus of the type set forth above, which consists in providing a machine code sub-routine as the date for each of a 55 plurality of characters in the background memory, the selection of a characterfor display causing the appertaining sub-routine to be run to write the pixel data for the character in bit-map form in the appropriate memory location in the display memory. 70 The main advantage thatthe present invention achieves isthe significant saving in the number of processor operating steps thatare required to write thedataforacharacterinto the display memory. Therefore, for a given speed of operation of the 75 processor, the contents of the display memory can be updated for the display of a new page of text far more quickly than in apparatus where character data is stored in the background memory in bit-mapform.

The invention also extends to a memory device 80 containing character data which are stored as machine code sub-routines. Such a memory device may be a floppy diskorthe like, or a read-only memory, in which a large number of different character data can be provided in a simple and 85 relatively inexpensive way.

In further considering the nature of the invention, reference will now be made byway of exampletothe accompanying drawings of which:—

Figure 1 shows a block diagram of a data display 90 apparatus in which the present invention can be embodied;

Figure 2 shows diagramatically a bit-map display memory;

Figure 3 shows an example of character data as 95 stored in bit-mapform in a background memory; and

Figures4 and 5 showtwo machine code subroutines which form character data in a background memory in accordance with the invention.

Referring to the drawings, the data display appar-100 atus shown in Figure 1 comprises a display device 1, a display generator 2,a processor3,a background memory 4, a display memory 5 and user interface apparatus 6 and 7. The display device is suitably a colourtelevision monitorwhich is connected to 105 receive R, G, B, video signals from the display generator 2. These R, G, B, video signals are produced in the display generator 2 by three digital-to-analogue converters 8,9 and 10, respectively. The display generator 2 also included a colour look-up table 11 110 which is suitably a read/write memory and is responsive to dot information received from the display memory 5 over a bus 12 to produce digital signals for driving the converters 8,9 and 10. A display timer 13 in the display generator 2 provides line and field 115 synchronisation signals LS and FSforthe television monitorl over a connection 14.Thetimer13also provides over a connection 15 timing signalsTfor controlling the transfer of dot information from the display memory 5 to the colour look-up table 11. 120 The display memory 5 is a random-access memory which has a capacity for storing dot information for at least one display frame. The dot information comprises digital codes composed of one or more bits per dot to be display, depending ontherangeof colours

The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

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afforded by the colour look-up table 11. A combined address/data bus 16 interconnects the display generator 2 and the display memory 5 with the processor 3. The background memory 4, which is also at least 5 partially a random-access memory, is also connected to the address/data bus 16. The background memory 4 may also have a read-only memory part of which contains permanentprogram data forcontrollingthe "house-keeping" operations »fthe processor3. The 10 user interface apparatus comprises a keyboard data entry device 6 and a writing tablet 7. Such interface apparatus is well-known in the art and specific details thereof are unnecessary for an understanding of the present invention. The processor 3 can be a commeri-15 cially available microprocessor, for instance the signetics S68000 |ip.

Consider now in detail the nature of the invention forstoring character data in a background memory. By way of illustration it will be assumed that a character 20 which is available for display, and the data for which is stored in the background memory, is a simple cross (+) composed of five vertical pixels and five horizontal pixels. It will also be assumed that this cross (+) is to be written into a bit-map display memory BM/DM in 25 the position CP shown in Figure 2. The bit-map display memory represented in Figure 2 is only of small size (15x15 pixels) which is adequate for the present description. In practice, a bit-map display memory would normally be much larger,forexample,a384x 30 256pixel display. Whenthecharacterdataforthis cross (+) is stored in the background memory in bit-map form in a cell pattern, it can take the form shown in Figure 3. This character data comprises a 5 x 5 bit matrix CHin which logic '1's representthe 35 charactershapeand logic'O's represent background data. Ifthe background memory contains memory locations of 8-bit bytes, as is common, then five such bytes b 1 to bb are required to store the character data and the byte area signified at B is redundant. 40 In orderto write the character data into the position CP the following programme steps have to be performed by the processor.

1. Determine the xsize and the y size of the character.

45 2. Determinethecharacterpositioninthebit-map memory.

3. Foryfrom start /to stop y (5 times) andforxto stop x (5 times), that isfor each of the 25 pixels in the character cell, read the bit data in the background 50 memory and ifthe bit is a '1'write the bit information forthe pixel concerned in the display memory.

The programme step will take of the order of 6-8 instructions, for each bit, so that a total of up to 200 instructions will be necessary to write the data for the 55 cross (+) into the display memory.

The number of instructionsforwriting character data into the display memory can be reduced considerably by means of the invention. Figure4shows the machine code instructions that are required to 60 write the cross (+) into the position CP. Before writing into the display memory, data which identifies the colourthatthe cross (+) is to be displayed in is entered into a first register (DO). Data which identifies a base dot position forthe character position isthen entered 65 into another register (AO). The background memory includes an address table for each possible character shape that can be displayed. When a character is selected by keyboard operation, the selected character is identified and the address forthe character data is read from the address table. This character data, which is in the form of machine coded instructions is then accessed by the processorto write the character data into the display memory.

The instruction (1) in Figure 4 provides the character colour. The instruction (2) provides the base dot address which is pixel bit positions 82 in the bit-map display memory BM/DM. The remaining instructions (3) to (11) cause the processorto step in turn to each of the pixel dot positions which are to contain a pixel for the charactershape. Thus, writing into the display memory is achieved using farfewer programme instructions than are required when character data is stored in the background memory in bit-mapform as in cell pattern.

The machine code instructions can also be arranged to write into the display memory the character data starting from a base dot position which is the first actual pixel position for a character.

Figure 5 shows an example of these alternative machine code instructions. In this instance, the register AO is initially set to pixel position 148, and instruction (2) writes in a pixel at that position. Instructions (3) to (6) then successively write into pixel positions 132,116,100 and 84. Different instructions (7)to (10)then writeintothe remaining pixel positions 114,115,117and118.

Claims

1. A digitally operable data displayapparatusfor displaying as an entity on the screen of a display device a quantity of data which is represented by digital codes stored in a display memory, the displayed data being intheform of discrete pixels or dots, each of which has its colour and/or luminance defined by a respective digital code in the display memory at a location corresponding to the position of the pixel in the display, the apparatus including a processorfor controlling digitally the storage, selection and display of data, characterised in thata method of storing character data in the background memory consists providing a machine code sub-routine as the data for each of a plurality of characters, the selection of a character for display causing the appertaining subroutine to be run to write the pixel data for the character in bit-mapform in the appropriate memory location in the display memory.

2. A data display appratus as claimed in Claim 1, characterised in that the machine code sub-routine which forms the stored data for a character comprises a series of instructions which respectively identify the pixel position of pixels, which define the shape of the character, with respect to a base dot position which locates the screen position of the character.

3. Adata display apparatus as claimed in Claim 2, characterised in thatfor writing the pixel data forthe character into the display memory, the machine code sub-routine includes a first further instruction which specifies in a first register data which identifies the colour in which the character is to be displayed, and a second further instruction which specified in a second register the bit position in the background memory

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that is to be base dot position forthe character.

4. A data display apparatus as claimed in Claim 2 or Claim 3, characterised in that each pixel position is identified in the relevant machine code instruction by

5 a direct count of the pixels line-by-linefrom the base dot position.

5. A data display apparatus as claimed in Claim 2 or Claim 3, characterised in that each pixel position is identified in the relevant machine code instruction by

10 a pixel position count which includesan offset number corresponding to one or more whole pixel position lines that exist between the line containing the base dot position and the pixel position that is being written into.

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6. Adatadisplayapparatusinwhichcharacter data is stored in a background memory as a machine-code sub-routine, by the method substantially as hereinbefore described.

7. A data display apparatus substantially as he-

20 reinbefore described with reference to the accompanying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office by the Tweeddale Press Group, 8991685, 8/87 18996. Published at the Patent Office, 25 Southampton Buildings, London WC2A 1AY, from which copies may be obtained.