You get a bit more speed out of the C64 if you disable parts of the video chip. It also avoids cycle inconsistencies every 8 scanlines (where normally, the chip

You get a bit more speed out of the C64 if you disable parts of the video chip. It also avoids cycle inconsistencies every 8 scanlines (where normally, the chip would block the bus for an additional cycle). To avoid making it a dull monochrome screen, devs tend to push the data that they process into the border color register (acting as background color in this mode). You also see this with decompressors at the beginning of (cracked) games. So the background color gets updated every few multiple of 8 pixels (1 character), and there's where the patterns come from. The slow shift of the pattern to the left indicates that the code isn't entirely balanced, but the article states as much (is it moving every 1.28 secs? I can't tell).