(These are actually "M0" and "M1" files)
They are the files used in the SLC 500 family of processors for block transfer functions with I/O and scanner modules that require it. Actually, the 5/03-5/05 processors with newer firmware have actual BTR/BTW instructions, so you don't have to directly manipulate the M files.
You can find a pretty good description of the M files here:
http://www.theautomationbookstore.com/idc/...um013_-en-p.pdf