I was a junior programmer on a team that built software, and managed the operations for a company with 27 offices across the continental US, with something like 2,000 different PC's deployed on the network in various locations around the country.

A new version of the business software we built was being deployed, and it required a proper shutdown/reboot of the system after installation. At that time in history, this reboot was done manually, by sub-contractors and fellow work colleagues who were flown from Burbank to New Hampshire to make sure the machines - which the clients weren't allowed to touch - were successfully rebooted after the update.

This cost tens of thousands in fees - subcontractors and plane tickets and whatnot - and my boss was sick of it .. he was a mainframe guy and hated these new DOS machines that didn't behave properly, having replaced dumb terminals with great fanfare, because per him, upper management just had to have The Newest Shit™ without thinking about it.

He tasked me to do the 'impossible' - work out how to safely reboot all these DOS PC's after a software update. If I could do it, I'd get a fat bonus - 1/3rd of the plane fare/travel budget that was on the block.

It was the easiest US$10K I ever made, looking (something like) this:

    C:\>debug.com
    a 
    JMP FFFF
    s 5
    n C:\reboot.com
    w
    q

I used to do this for fun to show how much of an old timer I was. Here's the detail from a different thread on HN 22 days ago[2]

How to create hello.com using debug in MS-DOS.

  debug

  a 100
  mov ah,9
  mov dx,120
  int 21
  mov ax,4c00
  int 21

  e 120 "Hello, World!" 0d 0a 24
  r cx 
  80
  n hello.com
  w
  q

  hello

[1] https://www.pcjs.org/software/pcx86/sys/dos/microsoft/6.22/

[2] https://news.ycombinator.com/item?id=41475874

As far as I remember, when a COM program is executed¹, DS is set to CS, so it's not necessary to manually set it.

¹=confirmed here: https://stackoverflow.com/a/13663859.

I remember writing a pretty complicated "guess the number" program in 16-bit DOS assembler in the 90's for a college course. It used colour and everything and I spent a number of long nights getting tasm to do what I wanted on my 486 I had at the time. Except the colour didn't work on the college computers because the DOS prompt was differently configured (I might have been using PC DOS, and college had MS DOS.) Fun times.

Writing assembly programs for DOS was incredibly easy: BIOS calls via interrupts, so you could already do a lot. Fixed video memory address... and the BIOS in order to enter the video modes without poking the video card directly. High level and low level mixed together.

I wrote something in x86 assembly a few years back... trying to write a TSR that reclaims half of its memory before going resident was an interesting experience...

I was playing that a lot when I'm 15. I, however, used LEA instead of MOV, as suggested by the book I read. Miss those days.

Hope one day we can build FreeDOS with 100% libre tools.