Tag Archives: Retro
Cosmac 1802 Assembler in Python
So back in the day, one of my Cosmac 1802 projects was getting FIG-Forth up and running on the 1802.
I laboriously converted the printed listings to a digital assembler source file. This file was then assembled with a cross-assembler that was written in BASIC.
As part of that effort I also modified the Forth system to run from EPROM.
Since I wanted to get Forth running on the 1802 Membership Card (see earlier blog post), I needed the ability to assemble that source, and do some updates etc.
In the same way it’s been a long time since I’ve had a computer with a parallel port, it’s been a long time since I’ve had a computer with a BASIC interpreter installed.
The obvious next step was to write an assembler in a more modern language. Again Python was my tool of choice.
This is a fairly simple assembler, just sufficient to assemble my ancient FIG-Forth listings. It is not a macro-assembler and I’m sure has other limitations relative to a “real” 1802 assembler.
The source is available here: https://github.com/donmeyer/cosmac-assembler
This plays nicely with my 1802 Membership Card Loader. The loader can invoke the assembler automatically when given a source file, allowing you to give one single simple command line that assembles the source and downloads it to the Membership Card.
1802 Membership Card Loader
So I built Lee Hart’s slick little 1802 Membership Card and its Front Panel card. Next I wanted to load some programs onto it! There was that nice DB-25 connector all ready to connect to my PC’s parallel port. However… I’ve got a Mac, and haven’t seen a computer parallel port in perhaps a decade.
My solution to this problem was to create my own interface hardware based on an Arduino. I considered using a Spark Core to allow doing things wirelessly via WiFi, but decided that for this purpose the Arduino was something more people were likely to have, and it was just a somewhat simpler solution.
The final design uses an Arduino, a 16-line I2C port expander chip, and some software. The software consists of the Arduino program and a Python program that runs on my Mac. Since it’s Python, it should also run on a Windows system, a Linux box, etc.
You can also forego the Python side of things and directly control the Arduino loader from a terminal, or from your own software if you prefer.
Why go to all this trouble? Just a fun little project!
It’s been a good mental exercise and a nice trip down memory lane.
Here is the Git repository with the source code and documentation: https://github.com/donmeyer/membership-card
Coming up next – an 1802 Emulator
Muddlin' Through 