%
% ZUSIE FLIPDISC FUN
% By Fredrik Andersson
%
% See comments at the bottom regarding different interesting logic functions you can use.
% Also load 1111 into X, or it won't work!
%
% Variables have the following uses
% A    temporary scratch register
% B   keeps track of direction. Most significant bit is horizontal
%   and least significant is vertical.
% C   the current bit pattern to write to the current column
% X   MUST contain 1111 to address the flipdisc display
% Y   keeps track of the current column

% Set starting column. Whatever will do.
ldl->a h03
mova->y

% Set starting row. Whatever will do.
ldl->c b00100000

% Keep track of direction in b
ldl->b h0


%-----------------------------------------------
% Main loop
%-----------------------------------------------

:again

%-----------------------------------------------
% Horizontal update
%-----------------------------------------------

% If b has the top bit set, then we decrement the horizontal
% position, otherwise we increment it
ldc<-b
sbrs decrementxy
incxy
sbr xydone
:decrementxy
decxy
:xydone

% Now for a little special complication: Memory position 8
% does not map to a flipdisc row. Detect this and do yet another round above.
pushc
movy->c
ldl->a b11111000 % 8 in complementary form
addac
pop->c
sbrz again

%-----------------------------------------------
% Vertical update
%-----------------------------------------------

% If b is non-zero (apart from the top bit),
% we go up, otherwise down
ldl->a b01111111
andab
sbrnz goup

% go down
src->a
mova->c
sbr cdone

% go up
:goup
addcc->a
mova->c

:cdone

%-----------------------------------------------
% Check for changed vertical
% direction from wall hit
%-----------------------------------------------

% If c has the highest bit set, we need to go down in next round
ldc<-c
sbrs change_to_down

% If c has the lowest bit set, we need to go up in next round
src
sbrs change_to_up

sbr done_up_down

:change_to_up
incb->a
mova->b
sbr done_up_down

:change_to_down
decb->a
mova->b

:done_up_down

%-----------------------------------------------
% Check for changed horizontal
% direction from wall hit
%-----------------------------------------------

% If y has the first bit set, we are at the leftmost
% column and must switch to right-going increments

% If y equals 15, we need to switch to left-going decrements
pushc
ldl->a b11110001    % This is 15 in complement form
movy->c
addac
pop->c
sbrz move_left

% If y equals 1, we need to switch to right-going increments
movy->a
deca
sbrz move_right

sbr done

:move_left
pushc
ldl->c b10000000
orbc->a
mova->b
pop->c
sbr done

:move_right
pushc
ldl->c b01111111
andbc->a
mova->b
pop->c


%-----------------------------------------------
% Write out the result to memory,
% i.e. the flipdisc
%-----------------------------------------------

% Note, you can try different operations here
% instead of XOR. Try OR for instance!

:done
rdxy->a
xorac->mxy

sbr again