I just typed in loads of detailed info about this scroll and got an error with my file attachment

bah!
Basically I was describing the use of an intemediary(sp) scroll buffer - which I have been discussing the merits of with Shockwave recently

In a nutshell, the scroll is rendered as a normal right to left operational scroller in a pre-buffer.
The faces are copied to the screen in passes - front face lower, back face lower, etc. etc. (allows the drawing of the horizontal bars) where each pass draws only the part it's interested in and works out which way up to draw the graphics etc. and as SW says, the rest is sin tables (fixed for the x offset curve (masked), and a variable array for the Y pos slots). The fixed offset controls the colour from a lookup as the fx is 256 positions wide and loops. The final effect is just controlling the Y offset in the 160 slots on the x axis (loop copy, sin data etc.)
The colours are looked up from an array made from the graphic below (256 colour values)