It is based on an algorithm published on pouet in 2021. One its implementation (v2) is for the 6502 under Basic, another is for the Z80 under CP/M (v4), and yet another is for the Z80 under ROM Basic (v2). All variants use dithering to simulate 8 colors, 4x1 texture bricks are used per pixel. Generated pictures are 512x256 raster images. It is very likely that this algoritm implementation is the fastest known Mandelbrot set computation routine for the 6502 and Z80.
All ready-to-run binary programs are available in a zipped archive.
Sources are available on github.