DOS APPEND
https://www.os2museum.com/wp/dos-append/Is it really that irrelevant? I mean, if you look past the specifics (directories, interrupts, DOS versions), this seems to be implementing the idea of bringing something into scope, in particular bringing it into scope from the outside, to modify the behavior of the consumer (here, assembler) without modifying the consumer itself. Today, we'd do the equivalent with `ln -sr ../../inc ../inc`.
I'd argue the general idea remains very important and useful today, though it's definitely not obvious looking back from the future what this was what APPEND was going for.
I, too, remember the trifecta of APPEND, JOIN, and SUBST. And while I always thought they were interesting, I was also wondering for most of them when I would ever use that. At the time, DOS versions and hence applications for it that don’t know subdirectories didn’t cross my mind, as my first DOS version was 2.11, I think.
Even windows 3.x and 95 (surpisingly) ran faster with smartdrv preloaded. 95’s default cache for some reason was worse than smartdrv and literally produced harder sounds on my hdds.
The second favorite was a TSR NG viewer, can’t remember the name.
Came in pretty handy when I wanted to share a folder with Remote Desktop, but it would only let me select whole drives.
Made a SUBST drive letter for that folder, worked like a charm!
Nitpick, but DEC never made a mainframe. Their products like the PDP-11 were considered minicomputers (even though the CPU was the size of a fridge) to distinguish them from IBM’s mainframes and medium sized computers.