Oh, you’re right, it’s right there in the link you shared–it was built in to MS-DOS, but only from version 6 on. I must have misremembered it as paid because it was something we didn’t have, and then later we did.

Ah, yeah, I think that may actually have been a paid program. It was something folks were willing to pay not to have to do, because, as I say, it was surprisingly tricky to manage the memory below 640K.

IIRC the application was just “edit.com”, as in “edit autoexec.bat”. The different kinds of memory were expanded memory, extended memory, and the high memory area; high memory was useful regardless which of the other two you were using, and those two were for the most part kind of interchangeable. You also typically had to mess with config.sys, which handled some things like the mouse driver. It was really common to have specific floppy disks that had only those two files on them (well, and were set to be bootable), so that if you needed a particular configuration for some game–maybe you didn’t load the CD-ROM driver, since that took up a lot of precious low-memory kilobytes–you could leave your normal setup alone and just stick your custom boot disk in for that program. Some programs were really tricky to make enough room for, even if you had a ton of RAM, because that privileged low ram area was so hard to manage.