Smooth scrolling screen on Commodore 64
Here is some sample code to implement horizontal smooth scrolling on a C64.
The algorithm in pseudo-code is:
when_graphics_chip_is_at_line_3() {
if (xscroll == 0) {
shift_upper_half_of_color_ram();
}
}
when_graphics_chip_is_at_vblank() {
xscroll--;
if (xscroll == 4) {
shift_upper_half_of_screen_ram_to_back_buffer();
}
else if (xscroll == 2) {
shift_lower_half_of_screen_ram_to_back_buffer();
}
else if (xscroll < 0) {
swap_screen_buffer();
shift_lower_half_of_color_ram();
draw_next_column_to_screen_and_color_ram()
}
}
See http://1amstudios.com/2014-12-07-c64-smooth-scrolling for more details
DustLayer is a fanstastic site devoted to C64 internals with great descriptions and tutorials. Highly recommended!