Provide an example on multiscreen scrolling

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

1 files changed, 98 insertions, 0 deletions

diff --git a/scroll/include/driver.s b/scroll/include/driver.s
new file mode 100644
index 0000000..abb3f5e
--- /dev/null
+++ b/scroll/include/driver.s
@@ -0,0 +1,98 @@
+;; The runtime of the game.
+.scope Driver
+    ;; Index of the current level.
+    zp_level = $77
+
+    ;; Whenever "Select" is pressed, it's used to count how many cycles have to
+    ;; pass before allowing another level switch.
+    zp_counter = $97
+
+    ;; Value that will be set for the `zp_counter` whenever the "Select" button
+    ;; is pressed for the first time.
+    SELECT_COUNTER_VALUE = $20
+
+    ;; Initialize global variables and other variables which have an impact on
+    ;; the runtime of the game.
+    .proc init
+        lda #0
+        sta Globals::zp_arg0
+        sta Globals::zp_arg1
+        sta Globals::zp_arg2
+        sta Globals::zp_arg3
+        sta Globals::zp_arg4
+        sta Globals::zp_tmp0
+        sta Globals::zp_tmp1
+        sta Globals::zp_tmp2
+        sta Globals::zp_tmp3
+        sta Globals::zp_idx
+        sta Globals::zp_flags
+        sta zp_level
+        sta zp_counter
+
+        rts
+    .endproc
+
+    ;; Function to be called at each iteration of the game loop. It loads any
+    ;; pending background columns and checks for level selection.
+    .proc update
+        ;; Are we actually loading a new level?
+        lda #%00100000
+        and Globals::zp_flags
+        beq @new_level
+
+        ;; Is the user actually allowed to hit the 'Select' button?
+        lda zp_counter
+        beq @check_select
+        dec zp_counter
+        jmp @check_column
+
+    @check_select:
+        ;; If `select` is pressed, then go for a new level. Otherwise check
+        ;; whether there is a pending column to be loaded.
+        lda #Joypad::BUTTON_SELECT
+        and Joypad::m_buttons1
+        beq @check_column
+
+        ;; Set the counter.
+        lda #SELECT_COUNTER_VALUE
+        sta zp_counter
+
+        ;; 'Select' was pressed and we weren't loading another level. Hence,
+        ;; increase the level index and load it.
+        inc zp_level
+    @new_level:
+        ;; Check the level index.
+        ldx zp_level
+        cpx #$02
+        bne :+
+        ldx #$00
+    :
+        ;; And load the computed level.
+        stx zp_level
+        jsr Background::load_level_x
+        beq :+
+
+        ;; Level could be rendered, enable back the PPU.
+        lda PPU::zp_mask
+        sta PPU::MASK
+
+        ;; Set the player back to its position.
+        RESET_PLAYER_POSITION
+    :
+        rts
+
+    @check_column:
+        ;; Do nothing if the `column` flag is not set.
+        lda #%00010000
+        and Globals::zp_flags
+        beq @end
+
+        ;; Continue loading the pending column.
+        jsr Background::load_column
+        beq @end
+        JAL Background::prepare_next_column
+
+    @end:
+        rts
+    .endproc
+.endscope