A ring buffer (or "circular buffer"), as the name suggests, is a buffer that behaves as if its elements were in a ring.

You have a read pointer (or index) and a write pointer (or index);
Each time you write (or read) a byte, you advance the write (or read) pointer;
When a pointer is at the end of the buffer, advancing it makes it "wrap around" back to the start of the buffer - hence it looks like the elements are in a ring.

Ring buffers can be implemented just as easily in assembler as in 'C'