Initial iteration of enemy movement

For now only the basic algorithm has been written, but the framework for
adding the rest has also been written down.

Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/src/prng.s b/src/prng.s
index d62392e..41f5f5d 100644
--- a/src/prng.s
+++ b/src/prng.s
@@ -12,6 +12,8 @@
     ;; Updates the 'a' register with the next random number set after the
     ;; current value of `zp_rand`, while also making sure that it is a valid
     ;; screen Y coordinate.
+    ;;
+    ;; NOTE: the 'y' register is preserved.
     .proc random_valid_y_coordinate
         ;; Get the new random number and store it right away.
         ldx zp_rand