Bare Metal Raspberry Pi BASIC
highcaffeinecontent.com
highcaffeinecontent.com
(You don't have to buy it, just download the zip.)
A bare-bones version of RISC OS with the GUI desktop stripped out, leaving only the command line ("supervisor") prompt, and ARM BASIC V, which is arguably still the most featureful version of BASIC: https://en.wikipedia.org/wiki/BBC_BASIC#Acorn_Archimedes_.28...
A unique feature of ARM BASIC is the presence of an inline assembler allowing users to write ARM assembly language programs. The assembler is fully integrated into the BASIC interpreter and shares variables with it, which can be included between the [ and ] characters, saved via ★SAVE and ★LOAD, and called via the CALL or USR commands. This allows developers to write not just assembly language code, but also BASIC code to emit assembly language, making it possible to use code-generation techniques and even write simple compilers in BASIC.
It does smell of the 1980s, sporting line numbers and RESTORE/DATA and implicit variable declaration. But it's a reassuring, nostalgic smell, even if not a healthy one, a bit like four star petrol, perhaps, or stale tobacco smoke. A far cry from its Microsoft BASIC equivalent, with its two-char variable names and single-dimension arrays: nothing there but the stench of ordure and corpses.
I believe that this kind of single-purpose OS can be even easily created using an unikernel such as mirage, but that removes part of the fun of course.
edit: Looking at the source, yeah you have to use Broadcom's firmware.