i  used this code to test whether  the border was drawn correctly. it's easier
than checking the draw-border-routine.

- if  interrupt  10h function 0bh is  used  to  wait for the vertical retrace:
  insert the following code before calling int 10h.

  mov bx,1

- otherwise: insert the following code after switching to mode 13h.

  pusha
  mov ah,0bh
  mov bx,1
  int 10h
  popa

in  this way the physical screen border is activated in blue. so you can check
whether  the border encloses the whole playfield.  the width of the border has
to be one pixel, which can be determined just by taking a glance.

only  one  accepted entry (by nop/wunderwerke)  uses  the trick of filling the
whole  screen instead of drawing only the border. this trick might sound dirty
but it is not strictly forbidden. the rules say:

"draw  the border. the color of this border  can be chosen by you. however, it
must  not  be  0. the border has  to  enclose  the whole screen. therefore the
coordinates of the corners are: (0/0), (319/0), (319/199), (0/199)."

there  is no rule that forbids the playfield  to be in any other color than 0.
so  it's  legal to fill the playfield in  some color, for example the color of
the  border.  the  only thing that matters  is:  hitting  the pixels where the
border  is must have the same effect as  in the other entries. that means: the
snake  has  to die either at once or  after  another step. after the snake has
died, the program quits.

adok.
