I found it difficult to find asMSX version 0.12g download online.
This was the most popular version used by many Spanish MSX fans to create games for msxdev contest.
Original file at www.robsy.net/asmsx012g.zip is no longer available. Fortunately both Windows and Linux executables are included with G-Monkey distribution at 2KBOS Game Development Contest page.
I’ll also host a copy of g-monkey.zip with asMSX 0.12g.
Please try the latest release of asMSX here: https://github.com/Fubukimaru/asMSX/releases.