I recently found out that if there was no diskette inserted, the IBM 5150 started directly in Basic. Just like a home computer.
Now I’m wondering whether you could only use the cassette port in Basic back then?
To put it another way, could you save a DOS program on a cassette?
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No. Because their is no DOS (DOS = Disk Operating System).
But you can load non-BASIC programs off the cassette. But I’ve never seen that personally. At the time, cassettes were on the way out and everyone wanted disk drives.
But if you bought a cassette BASIC machine,how did you upgrade to disk?
DOS is already there when I start from a floppy disk.
The question is, is there a cassette command in DOS?
Ooooo… That’s an interesting question.
But my first response is “Why?” 
Looking at my BASIC for DOS 2.11 (earliest that I have), there’s no CLOAD/CSAVE commands and the LOAD/SAVE commands only reference the disk drives. So I think we can safely say that by DOS 2.11 days, there was no market or support for the cassette.
There may have been a version of BASIC with DOS 1.0 that allowed cassette use from DOS, but I’ve never seen it.
As a contrast, looking at my Commodore PET and TRS-80 Model I, both have commands for disk and cassette access at the same time.
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I’ve seen suggestions that there may be a CAS1: device to load/save from, but can’t confirm.
If you must, there’s 5150CAXX, which adds tape load/save to MS-DOS on a 5150.
PC-BASIC - a Python-based BASICA emulator - has tape support, mainly to do things with BASICODE
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Just dropping this in here…
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