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.