Converting a program and it uses the 'mod' function. i know it is not in Atari basic and i don't know how to adapt it to Atari.
clip from program:
390 IF D >= 0 THEN
400 X = X + (D - 2 MOD 2)
410 Y = Y + (D - 1 MOD 2)
420 MAZE(X + Y * MAZEWIDTH) = 1
430 X = X + (D - 2 MOD 2)
440 Y = Y + (D - 1 MOD 2)
450 MAZE(X + Y * MAZEWIDTH) = 1