My first microcomputer, a TRS-80 Model I with 16 KB RAM and a 500 bits/second cassette tape drive, was a gift from my father in 1979. I gradually expanded it with an Expansion Interface, an additional 32 KB of RAM, dual floppy drives, and an RS-232C serial port.
In late 1980, working with a licensed clinical psychologist, I started writing my first commercial software, which generated reports on the Minnesota Multiphasic Personality Inventory (MMPI) and was sold only to licensed psychologists. It was finished and successfully marketed from 1981 through 1984 or 1985 when the business was sold. We developed and marketed about a dozen specialty programs for licensed psychologists over those years.
Initially all development work was on that TRS-80, but later it was done on my second computer, an IBM PC (Model 5150). All of the software was ported to or from MS-DOS, TRS-DOS, Apple DOS, and CP/M, and sold for all four platforms.