Game Programmers Hall of Fame , The Amstrad elite.

[arnoldemu]

@AMSDOS: That is close enough

The "rupture" method has three results:
1. static panel and scrolling main area
2. You can change the screen address more than once.
3. It allows you to make a stable screen to make sure it is very smooth.

The vertical scroll is done by adding 1 or more scanlines. The whole screen moves down. To stop the panel from moving down you then add more lines but compensate for the ones you added at the start so the result is always the same number of lines. (always 312)

Scrolling: add 1 at the top. add 7 at the bottom to compensate. Result is 8.

More scrolling: add 2 at the top and 6 at the bottom. Result is 8.

When you have added 7 lines, you change the screen address to scroll vertically and reset it back to 0. scrolling vertically is normally in 8 scanlines at a time.

There would be flicker at the top of the screen, but they remove this by drawing black lines. There is no flicker at the bottom because when you add scanlines the graphics just repeat and that works fine at the bottom.

The result is perfect smooth scroll vertically.

Legend of Kage and LED Storm don't use rupture. They do add lines to do the smooth vertical scroll. The result is flicker at the top and bottom of the screen. Also, because they are adding lines and do not compensate, the length of the frame is not always the same so the screen is not stable. (frames are 312-320 lines long).

To do the compensation you must use rupture.