I did a “lecture”/“workshop” about this at work today. 16-bit DOS, real mode. 💾 Pretty cool and the audience (devs and sysadmins) seemed quite interested. 🥳

• People used the Intel docs to figure out the instruction encodings.
  
• Then they wrote a little DOS program that exits with a return code and they used uhex in DOSBox to do that. Yes, we wrote a COM file manually, no Assembler involved. (Many of them had never used DOS before.)
  
• DEBUG from FreeDOS was used to single-step through the program, showing what it does.
  
• This gets tedious rather quickly, so we switched to SVED from SvarDOS for writing the rest of the program in Assembly language. nasm worked great for us.
  
• At the end, we switched to BIOS calls instead of DOS syscalls to demonstrate that the same binary COM file works on another OS. Also a good opportunity to talk about bootloaders a little bit.
  
• (I think they even understood the basics of segmentation in the end.)
  

The 8086 / 16-bit real-mode DOS is a great platform to explain a lot of the fundamentals without having to deal with OS semantics or executable file formats.

Now that was a lot of fun. 🥳 It’s very rare that we do something like this, sadly. I love doing this kind of low-level stuff.