> So they introduced text and binary modes for files on DOS
It actually long predates DOS
C stdio is descended from Mike Lesk’s “portable IO package” (original release circa 1973). Bell Labs ported their C compiler from Unix to Honeywell GCOS and IBM S/370 mainframes. Mainframes handle text files very differently from how Unix systems do-it is much more complex than simply changing the newline character. So in Lesk’s package, the mode parameter to copen() told you whether the file was text or binary. copen() was renamed to fopen(), and the character to indicate binary mode was changed from “i” to “b”, and hence stdio
stdio has always had text-vs-binary file distinction, on some platforms (such as Unix) it has always been a no-op, on others it hasn’t