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

68 APPLE II COMPUTER GRAPHICSimpression of a dot travelling steadily to the right while leaving everyeighth dot on.Run the program and if the screen behaves in an unexpectedborderingon bizarre-manner, then the program is probably working perfectly.Seeing Is BelievingWhat you should really see is a dot that appears close to the upper lefthand corner of the screen, and then travels rapidly to the LEFT. Just as thefirst dot bumps into the edge of the screen, a second dot appears; it alsomoves to the left and stops where the fi rst dot began. A third dot appearsand moves left, and then a fourth and fifth and so on until the entire screenfills with a grid of little dots, all about a quarter-inch apart. (It requiresapproximately five minutes to reach this point.) The routine continues withthe next set of dots appearing immediately beneath the first set. If you letthe process continue long enough, the screen will fi nally display fortyvertical lines of dots.The display is mildly amusing, but after the novelty wears off you mightrealize that the "on" dots moved from right to left while the "on" bits inthe byte moved from left to right. Apple Hi-Res displays the bits of a bytein the reverse order, so the right hand bit turns on the left hand dot of abyte, and the left hand bit turns on the right hand dot-er, sort of. You see,there is a little more to it.Slow down the display process by changing the delay value in line 12