x=0
y=0
-- how many pixels per 'row' of drawing we want
row_height = 4

function scroll_up(pixels)
    -- each row is 64 bytes in memory (128 pixels / 2 pixels per byte)
    -- so shifting up by 'pixels' rows means skipping 'pixels * 64' bytes
    local row_bytes = 64
    local shift = pixels * row_bytes
    local size = row_bytes * (128 - pixels)

    -- copy screen memory from further down to the top
    memcpy(0x6000, 0x6000 + shift, size)

    -- clear the new bottom rows that just got "pulled up"
    rectfill(0, 128 - pixels, 127, 127, 0)
end

function _init()
    cls()
end

function _update()
    -- draw a single diagonal each frame
    if rnd(1) < 0.5 then
        line(x,   y,   x+row_height, y+row_height, 7)
    else
        line(x+row_height, y, x, y+row_height, 7)
    end

    -- move right by row_height pixels each time
    x += row_height

    -- if we go past the right edge, go to a new "line"
    if x >= 128 then
        x = 0
        y += row_height

        -- if we go past the bottom, scroll the screen up
        if y >= 128 then
            scroll_up(row_height)
            -- reset to bottom "row" so we keep drawing there
            y = 128 - row_height
        end
    end
end

function _draw()
    -- no cls() here, so we keep what we've drawn
end