Apple-1 Mandelbrot

Mandelbrot fractal explorer for the Apple-1

• mandelbrot.bas is the Mandelbrot program written in Applesoft BASIC.
• mandelbrot.py is a test version in Python.

This simple Mandelbrot program works with Applesoft Lite, a version of Applesoft BASIC modified to work on the Apple-1 by Tom Greene. The Apple-1 display is limited to displaying 40x24 characters from the Signetics 2513 character ROM, so each pixel of the Mandelbrot set is plotted as a text character. The pixels are considered to be 4-bit grayscale, mapping to the following characters with increasing intensity: .:,;!-^+=/&*%#@ . (This is the same mapping used in the Apple-I 30th birthday program originally by Dave Schmenk. I referred to this version ported to the Apple II by J.B. Langston.)

This version runs quite slowly, taking just over 5 minutes to plot the Mandelbrot shown above! This is because Applesoft BASIC treats all numeric variables as 40-bit floating point (a representation that is emulated by the 8-bit 6502 CPU.)

A few ways to emulate Applesoft BASIC programs for the Apple-1:

• The Apple 1js emulator by Will Scullin works well in a modern web browser. Select Load > Applesoft to load Applesoft Lite from a built-in virtual tape. This takes about a minute. (Watch the red loading bar.) After loading is complete, you should see the BASIC ] prompt where you can paste the program listing.
• OpenEmulator is a nice Apple-1 and Apple II emulator for macOS. Select the Apple-1 system and enable the virtual R&D CFFA1 expansion card (which adds more RAM and allows for reading/writing from a CF card). You can then open a CF disk image such as ULTIMATE APPLE1 CFFA 3.3.po from this Apple I Software Archive, which contains a variety of programs including Applesoft Lite. Start the CFFA1 menu by entering 9000R at the WOZ Monitor prompt, select P then switch to the ASOFT directory, then select L to load APPLESOFT into to memory at address $6000. Then hit Q to quit the CFFA1 menu and return to the WOZ Monitor prompt. You can then run Applesoft BASIC by running 6000R. Once you see the BASIC ] prompt you can paste the program listing here.