Apply a correction on enemy movement for PAL

We cannot just "inc"/"dec" for enemy movement, but we have to apply an
overstep on PAL every 5 frames in order to be equivalent to the NTSC
version. This has been abstracted away on a macro that just does
"inc"/"dec" on NTSC, but does this proper "adc"/"sbc" instructions with
a helper memory region which is updated every 5 frames.

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

1 files changed, 12 insertions, 0 deletions

diff --git a/src/driver.s b/src/driver.s
index 4f0e501..3d9eb53 100644
--- a/src/driver.s
+++ b/src/driver.s
@@ -562,15 +562,22 @@
         .proc pal_handler
             ;; Check if 5 frames have passed since last counter reset.
             lda Driver::zp_pal_counter
+            beq @reset_movement_arg
             cmp #4
             beq @do_handle
 
+        @player_timer_reset:
             ;; Nope! Reset the player's step on PAL and increase the counter.
             lda #1
             sta Player::zp_step_on_pal
             inc Driver::zp_pal_counter
             bne @end
 
+        @reset_movement_arg:
+            ;; Restore the enemy movement to the same value as NTSC.
+            dec Enemies::zp_movement_arg
+            jmp @player_timer_reset
+
         @do_handle:
             ;; Increase the step just for this frame and reset the counter.
             lda Player::zp_step_on_pal
@@ -579,6 +586,11 @@
             sta Player::zp_step_on_pal
             lda #0
             sta Driver::zp_pal_counter
+
+            ;; Increase the movement arg for this frame. This way we catch up to
+            ;; the NTSC real velocity on screen.
+            inc Enemies::zp_movement_arg
+
         @end:
             rts
         .endproc