Retrocomputing Agon Light

Initial explorations of the Agon Light

Published · 3min

The Agon Light doesn’t have an equivalent of the “programmer’s reference guides” of old, so I’ve been doing some poking around in different places to see what information I can gather together.

There’s an agon-light tag on Github that lists a few interesting repositories. In particular, Agon Forth looks like a fun alternative to using BBC BASIC, giving it more of a Jupiter Ace. The agon-flash repo may be useful for firmware upgrades in future. The agon-utilities repo has a few things I was thinking I’d have to write myself, such as a tool for loading fonts, a text editor, and a pager for reading files on mass storage.

The Quark MOS repo contains an API manual and the README in the root of the repo clears up some issues I was having such as the star commands not always corresponding to what I expected from RISC OS (which borrowed heavily from the BBC Model B’s MOS), and pointed at a solution to a minor issue I had, which is that my Agon Light 2 was set up to use a US keyboard by default, but we use a very similar layout to the UK here in Ireland (the input method for extended characters is the main difference), and it turns out all I need to do it edit autoexec.txt and change SET KEYBOARD 1 to SET KEYBOARD 0. The VDP firmware includes a list of VDU driver command. It’s a little rough and ready, as not all the limits are explained nor is there a description of the palette (though it supports 64 colours, IIRC), but I’m expecting those can be gleaned from reading the firmware, which is written in the Arduino dialect of C++. This repo has a bit more information on the sprite VDU commands and some more information on the memory map, and some C libraries.

There’s a version of CP/M for the Agon Light, though it requires a custom VDP firmware at present to run. I haven’t looked at that as yet.

I found a repo with some useful-looking sample programs in Z80 assembler, including some include files and some information on the the spasm-ng assembler as an alterative to using Zilog Developer Studio. It’s apparently modelled off of TASM, which I used ages ago, so it shouldn’t be difficult to dig up documentation on the assembler’s directives.

The eZ80 processor seems to have to modes: classic Z80 mode, and “ADL mode”, which allows a 24-bit address space (16MB) to be accessed. I found an article describing the eZ80 and how it differs from the Z80, which may be useful for bigger programs, though the Agon Light has only 512KB of memory, not 16MB. This issue in the MOS repo implies that this may not work exactly as expected, but it’s worth experimenting with.

As far as games go, there’s a version of Sokoban.

I’m on the lookout for a VGA to HDMI converter that might, which any luck, work well with one of my monitors. The HP LA2405x on my desk really doesn’t like the Agon Light’s mode 2 VGA output, but I’ve an older Benq monitor that works with it just fine. If I can, I’d prefer to use the HP monitor, as the Benq is hooked up to the dock for my Steam Deck.