Mandelbrot in BASIC

A bit more nostalgia, this time BBCSDL and a slightly dodgy Mandelbrot generator:

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MODE 0
ITERATIONS = 50
LIMIT = 50
DEF FN_mag(a,b) = SQR((a * a) + (b * b))
DEF FN_add_a(a,c) = a + c
DEF FN_add_b(b,d) = b +d
DEF FN_mult_a(a,b,c,d) = (a*c) - (b*d)
DEF FN_mult_b(a,b,c,d) = (a * d) + (b * c)

FOR a = -2 TO 2 STEP 0.01
  FOR b = -2 TO 2 STEP 0.01
    z_a = 0
    z_b = 0
    FOR iter = 0 TO ITERATIONS
      orig_a = z_a
      z_a = FN_mult_a(z_a, z_b, z_a, z_b)
      z_b = FN_mult_b(orig_a, z_b, orig_a, z_b)
      z_a = FN_add_a(z_a, a)
      z_b = FN_add_b(z_b, b)
      IF FN_mag(z_a, z_b)>LIMIT THEN EXIT FOR
    NEXT
    point_x = (a * 320) + 640
    point_y = (b * 256) + 512
    MOVE point_x, point_y
    IF FN_mag(z_a,z_b)>LIMIT THEN
      GCOL 0
    ELSE
      GCOL 1
    ENDIF
    DRAW point_x, point_y
  NEXT
NEXT

And the output:

slightly grainy mandelbrot