Use specific variables for cached player's tile coordinates

Before this commit we were using the zp_argX variables, but this can go
wrong, and more so considering other functions like background collision
checking also use these variables. All in all, since we have plenty of
RAM to spare, let's just allocate a new set of bytes just for that.

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

3 files changed, 16 insertions, 9 deletions

diff --git a/.nasm/memory.txt b/.nasm/memory.txt
index 23ba954..1e5caad 100644
--- a/.nasm/memory.txt
+++ b/.nasm/memory.txt
@@ -34,9 +34,11 @@ $37: zp_next_enemy_cycle
 $38: zp_flags
 $40: zp_screen_y
 $41-$42: zp_position_y
+$43: zp_player_tile_y
 $44: zp_velocity_y
 $45: zp_screen_x
 $46-$47: zp_position_x
+$48: zp_player_tile_x
 $49: zp_velocity_x
 $50: zp_state
 $51: zp_walk_counter
@@ -77,4 +79,4 @@ $4016: m_joypad
 $4017: m_frame_counter
 
 --- Summary (in bytes) ---
-- Internal RAM: 415/2048 (20.26%)
+- Internal RAM: 417/2048 (20.36%)
diff --git a/src/driver.s b/src/driver.s
index f747c4b..632af84 100644
--- a/src/driver.s
+++ b/src/driver.s
@@ -195,8 +195,6 @@
         lda #BLINKING_TIME
         sta Driver::zp_blink_timer
 
-        ;; TODO: items falling down.
-
     @no_update:
         rts
 
diff --git a/src/items.s b/src/items.s
index e90ae82..447b874 100644
--- a/src/items.s
+++ b/src/items.s
@@ -17,6 +17,13 @@
     zp_player_screen_x = $45    ; asan:ignore
     PLAYER_WAIST  = $0C
 
+    ;; The tile coordinates are also cached during the update()
+    ;; function. Reserve them into their own memory regions to avoid surprises
+    ;; by using the 'Globals::zp_arg0' and 'Globals::zp_arg1' variables which
+    ;; are also being used by the background check.
+    zp_player_tile_y = $43
+    zp_player_tile_x = $48
+
     ;; Maximum amount of items allowed on screen at the same time.
     POOL_CAPACITY = 3
 
@@ -341,14 +348,14 @@
         lsr
         lsr
         lsr
-        sta Globals::zp_arg0
+        sta Items::zp_player_tile_y
         lda zp_player_screen_x
         lsr
         lsr
         lsr
         clc
         adc #1
-        sta Globals::zp_arg1
+        sta Items::zp_player_tile_x
 
     @loop:
         ;; This index will be valid throughout the iteration so different
@@ -735,22 +742,22 @@
 
         ;; Check for the Y tile coordinate. If it's not the same on either the
         ;; upper or the bottom parts of the item, then it's a no.
-        cmp Globals::zp_arg0
+        cmp Items::zp_player_tile_y
         beq @check_x
         clc
         adc #1
-        cmp Globals::zp_arg0
+        cmp Items::zp_player_tile_y
         bne @no
 
     @check_x:
         ;; If the Y tile coordinate checks out, let's narrow it down to the X
         ;; coordinate.
         lda Items::zp_current_tiles + 1, x
-        cmp Globals::zp_arg1
+        cmp Items::zp_player_tile_x
         beq @yes
         clc
         adc #1
-        cmp Globals::zp_arg1
+        cmp Items::zp_player_tile_x
         bne @no
 
     @yes: