williams-et-al-1983-apple-ii-computer-graphics

82 APPLE II COMPUTER GRAPHICSA Bit of ColorIn Chapter 8, we pointed out that the left-most bit in a Hi-Res byte was notdisplayed, and alluded to its fu nction in color control. That bit is called thecolor bit of the byte, and its setting determi nes which group of colors theremaining seven bits will display. If the color bit of a byte is off (zero), theremaining seven bits will display colors from group 1: BLACK1, GREEN,VIOLET, or WHITE1 . If the color bit is on (one), the byte will display group2 colors: BLACK2, ORANGE, BLUE, or WHITE2. Those colors will varydepending on the TV or monitor used, but you can usually adjust the tinton your screen so the proper colors are displayed. We will return to thecolor bits and color groups; meanwhile, run the following program as anexperiment.10 HGR20 HCOLOR = 330 HPLOT 1,0 TO 1,10040 GET P$: REM PAUSE50 HCOLOR = 460 FOR I = 0 TO 10070 HPLOT 0 , I80 NEXT IListing 9-1 . Vertical line.According to the Apple manuals, HCOLOR = 3 is WHITE1 (the whitebelonging to color group 1); hence, lines 10 through 30 should plot avertical white line in column 1. But when you run the program you areinstead greeted by a vertical green line! That is because you are plotting asingle dot in each row, and any isolated dot always results in a color. Butthere is more. Line 40 halts the program until you press a key, line 50 setsthe color to BLACK2, which is the black belongi ng to color group 2. Lines60, 70, and 80 plot a vertical line of black dots in column 0. Since column0 was already black, you would not expect anything to change, and yetthe green line slowly changes to orange after you press a key.The reason fo r the color change has to do with the color bit. The dots incolumn 0 and column 1 are controlled by the same byte of Hi-Res memory,and plotting WHITE1 turned on the column 1 bit of each byte and leftthe rest of the bits off, including the color bit. Plotti ng column 0 in BLACK2turned off the column 0 bits, which were off anyway, but also turned onthe color bit of each byte since BLACK2 is a group 2 color. When the colorbit of each byte was turned on, it caused the entire byte to display group 2colors; thus, the green in column 1, which is a group 1 color, changed tothe corresponding group 2 color, orange. If column 0 had been plotted inB LACK1 instead of BLACK2, the color bit of each byte would have remainedoff and no color change would have occu rred.