Earlier quoted context omitted.
Windows 9x/XP did not come with C#, Visual C++ or Visual Basic. Likewise, classic macOS did not come with any programming or scripting languages. In this respect, they were equally “shitty”, and your solution would have been unavailable for both.
Everything from DOS 5.x onwards (including Windows 9x and NT) came with QBasic, previous versions of DOS had GW-BASIC. Not the greatest tools out of the box, but still better than nothing.
MS/PC-DOS shipped with debug.com since pretty much the first versions. It is a perfectly usable assembler where you can type a short loop that receives characters from the serial port and outputs them to a file. Similar things have been done many times to salvage hosed systems where a little more than copy file COM1: was required.
With the Internet today, you probably don't even need to remember the instruction names, you can probably find a printout of a program that does this.