This is a small language described in Irv Englander's book The Architecture of Computer Hardware and Systems Software (I think I've got that title right!). It is designed as a simple machine code for a hypothetical machine. Its 'bytes' are units of three decimal digits, and these are the form of all code, data, input and output. Like Befunge, it has no built-in distinction between data space and code space, and it can be self-modifying in a similar way.
100
600
100
309
200
410
801
900
700
001
111
I wrote this a while ago when I had the book out of the library, but I can't remember what all the opcodes mean! Maybe I'll be able to crack it someday :-) . But I do remember that it works basically in the same way as the first example on the Befunge quines page.