This routine compares a players score string, pointed to by the X register, to the current hi score, pointed by the U register. If the player's score is higher than the currently saved hi score, then the player's score will be copied into the hi score buffer pointed to by the U register.
X-reg points to a player's score stringX-reg points to double-sized coordinate pair
U-reg points to the high score string (usually $CBEB?)
X-reg, U-reg, D-reg trashed