-; BOX: Set the mouse bounding box. No checks are done if the mouse is
-; currently inside the box, this is the job of the caller. It is not necessary
-; to validate the parameters, trust the caller and save some code here.
-; No return code required.
+; BOX: Set the mouse bounding box. The parameters are passed as they come from
+; the C program, that is, maxy in a/x and the other parameters on the stack.
+; The C wrapper will remove the parameters from the stack when the driver
+; routine returns.
+; No checks are done if the mouse is currently inside the box, this is the job
+; of the caller. It is not necessary to validate the parameters, trust the
+; caller and save some code here. No return code required.
+
+BOX: ldy #5
+ sei
+ sta YMax
+ stx YMax+1
+
+@L1: lda (sp),y
+ sta XMax,y
+ dey
+ bpl @L1