It is based on an algorithm published on pouet in 2021. It uses 16 colors. Generated pictures are 256x128 raster images. It is very likely that this algoritm implementation is the fastest known Mandelbrot set computation routine for the 6809 and 6803. However, the plot is made rotated because the CoCo3 is not capable to show 256 raster lines, and the rotated plot is slower than the normal.
An archive with executables is here. The programs may be run directly from the Xroar emulator. As an extra it contains a fullscreen (320x225, 16 colors) Mandelbrot generator which can draw, for instance the next image.
Sources are available on github.