The project description states that
each single space in the playing field is represented by an unsigned halfword
(section 3.1 Tile Representation).
The public tests for
sw to store entries into the buffer that is then given to the subprocedure to operate on. This strikes me as an oddity. Does it make any difference to using
(In case you are wondering for my motivation to ask this, I keep failing these tests and I am out of my wits to find my mistake.)