TeX was and is available on a large number of OSes. I was very happy being to run a 'large' TeX instance on my Atari ST with 1 MB of ram, it could easily typeset my undergraduate thesis.
Indeed. TeX later moved to Pascal, but was originally written in SAIL for use on WAITS. GLS ported it to ITS (the first ever port of TeX), and also was responsible for lobbying Knuth to make it Turing-Complete.
Man, is there any part of modern computing that doesn't have GLS's fingerprints on it?
