5 ms·

where do the visual effects (those blinking bars) come from? Are they deliberate, or some sort of side effect?

• vardump 9y ago
  They're deliberate, done by incrementing border color register. One of the only things possible with the available CPU time.
  • tonysavon 9y ago
    The three-byte codebook version (the mazinger demo) can't even afford the increment, and just does a sty $d020, which takes 2 cycles less, hence the colorbars look a bit different on that one. But the audio sounds better.
• csixty4 9y ago
  They're raster lines. At some point in the code, or multiple points, the program increments the register holding the border color so the video chip starts drawing a different color. This can happen in the middle of a line, which is why you get multiple "columns". Edit: vardump is right, the screen would be "turned off" in this case so just the border color is used. This gives a slight performance boost.
• pgeorgi 9y ago
  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).
  • sp332 9y ago
    Yes, it does jump every 1.28 seconds :)