UBASIC 9 by Yuji KIDA [email protected]
I did not write this. I used UBASIC in high school, and in 2014 I asked him about getting the source. He emailed it to me, and I just dug it out to make it available. It is entirely written in assembly.
My plan was to fork some version of BASIC to include all the functionality that UBASIC has, so any UBASIC source could run on modern PCs. I never got around to it, unfortunately. He was planning to make a newer version using C/C++ but I haven't seen it yet.