explain div weirdness

boot0.s

@@ -59,7 +59,7 @@ bits 16
   setne cl   ; sector - 1 = LBA % (sectors per track)
   inc cl
   xor ah, ah ; zero-extend temp
-  xor dx, dx ; TODO: explain why this is needed for div
+  xor dx, dx ; div by 16-bit register divides dx:ax, so we zero dx
   div bx     ; ah = mod (head), al = div (cylinder)
   
   mov dh, ah ; head