> 
> The AT also had the 10 MB disk.  Back when I had an AT, I ran Xenix
> System III on it along with the MS C compiler, and was able to create
> console-mode programs to run on everyone else's MS-DOS machines.
> It's hard to remember/believe that Xenix was a Microsoft product before
> DOS was.

I had an Xenix running on my AT as well.

> 
> I would say even RT-11 is somewhere between executive and OS.  It could
> run foreground tasks (hence the name Real Time) if properly sysgenned,
> and it had a decent kernel API that you didn't have to bypass.

I remember the FB (Foreground/Background) version that had more flexibility, even so, it didn’t preempt any running job.   My second paying computer job was writing database software for an RT-11 system.    This was a port of a 370 mainframe application to do lab test management at Hopkins hospital.   This was after the two guys who were tasked with porting it to the Series-1 were having a hard time with it.    Being the wizkid, the IBM guys brought me a 3101 Ascii terminal and asked if I could do anything with it and I connected it to the RT system in lieu of the ADM3 I had been using.