Someone recommended a nice (German) talk:
https://media.ccc.de/v/ds24-394-linux-hello-world-nur-mit-einem-hex-editor
Luckily, everything™ is easier™ on DOS with .COM files. A fun little time killer to make a HELLO.COM using only a hex editor, the Intel docs and the DOS interrupt list.
That ModR/M stuff is easy in the end, but it took me quite some time to understand it. 🥴
(I’m still new to DOS on this level and didn’t know that all segment registers are initialized to the same values, apparently, so copying CS to DS was not necessary. Too lazy to update the screenshot. File size shrinks by 4 bytes.)