GB1360487A - Visual display device - Google Patents

Abstract

1360487 CRT displays INTERNATIONAL COMPUTERS Ltd 12 May 1972 [15 May 1971] 15152/71 Heading H4T In an arrangement for drawing lines on a CRT display a series of pairs of co-ordinate values are produced by 2<SP>n</SP>. Successive additions of values #x/2<SP>n</SP>, #y/2<SP>n</SP> to start values X, Y, these pairs cotrolling the beam deflection. X and Y are loaded from 30 via 28, 10 and 31 into registers 5 and 15, and the line increments #X #Y into registers 7, 17. The increments are simultaneously both shifted right until neither left-hand-side 7a, 17a contains significant digits (as detected by 8, 18) and these shifted increments are then repeatedly added to the contents of 5, 15 in the lower order positions, the upper order digits used to derive analogue deflection values thus receiving periodic overflows from 5b, 15b. The number of bit positions in 5a, 15a is chosen so that a change in their lowest order corresponds to the tube spot definition; however the range of deflection values which these registers command exceed the picture area, so that the actual display is part of a larger virtual picture. The co-ordinates of the centre of the displayed picture with respect to the larger virtual picture are supplied from 9 to subtractor 10 as the start values X, Y pass through 10 as referred to above. Each addition of the shifted increment values adds a 1 from 11 (shifted downwards correspondingly to the right shift of #X, #Y) into counter 13 until its contents again reach zero indicating that the end point of the line has been reached. Field indicators 19 and 29 and AND gate 20 detect whether the current X and Y values being supplied to the D-A converters 3 and 4 are both so small as to represent a point on the tube face; when this condition does not obtain skip logic 21 is brought into operation. This examines successively the bits registered in counter 13 and controls shift and addition of the contents of 7 and 17 to complete the incrementing with a minimum number of additions. To deal with lines which start off the tube face a series of "trial sums" are attempted starting with the full increment and then successively (if necessary) ¢, “ &c. of the increment, and while the result lies outside the tube face the sum is loaded into 5, 15; if inside the field indicators 19, 29 inhibit the over writing of the old values in 5, 15. Zoom register 31, by controlling a shift of the orders in registers 5, 7, 15, 17 into which the data is originally loaded controls the scale of the display.