"Over time, programs were written with MS-DOS as their primary target, and they started to realize that they could use environment variables as a way to store configuration data. In the ensuing chaos of the marketplace, two environment variables emerged as the front-runners for specifying where temporary files should go: TEMP and TMP."

And before that there are a few paragraphs describing the migration of applications from i8080/Z80 based CP/M towards x86 based DOS via mechanical translation.