Add a title and a main screens

This commit adds the skeleton for having a title and a main screen. For
now the title menu doesn't do much, as the selection is simply ignored,
but at least it already knows how to cycle between these two states.

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

1 files changed, 100 insertions, 0 deletions

diff --git a/src/title.s b/src/title.s
new file mode 100644
index 0000000..db627ae
--- /dev/null
+++ b/src/title.s
@@ -0,0 +1,100 @@
+.segment "CODE"
+
+;; All the functions and variables which are related to the title screen.
+.scope Title
+    ;; Y and X coordinates for the sprite that guides the player on the menu.
+    SPRITE_Y_POSITION0 = $A7
+    SPRITE_Y_POSITION1 = $BF
+    SPRITE_X_POSITION  = $40
+
+    ;; The title has a timer as a delay between joypad presses from the player.
+    TIMER_INIT_VALUE = 20
+    zp_title_timer = $30
+
+    ;; Initialize all the elements for the title screen.
+    .proc init
+        lda #0
+        sta zp_title_timer
+
+        ;; Initialize the sprite that guides the player on the menu.
+        lda #SPRITE_Y_POSITION0
+        sta $200
+        lda #$30
+        sta $201
+        lda #$00
+        sta $202
+        lda #SPRITE_X_POSITION
+        sta $203
+
+        rts
+    .endproc
+
+    ;; Checks the pressed buttons from the joypad and moves the sprite for the
+    ;; menu accordingly.
+    ;;
+    ;; Returns 1 if the player hit start and the game can start, 0 otherwise.
+    .proc update
+        lda zp_title_timer
+        bne @end
+
+        lda #Joypad::BUTTON_UP
+        and Joypad::zp_buttons1
+        beq @check_down
+
+        lda #SPRITE_Y_POSITION0
+        cmp $200
+        beq @end
+        sta $200
+        jmp @set_timer_and_end
+
+    @check_down:
+        lda #Joypad::BUTTON_DOWN
+        and Joypad::zp_buttons1
+        beq @check_select
+
+        lda #SPRITE_Y_POSITION1
+        cmp $200
+        beq @end
+        sta $200
+        jmp @set_timer_and_end
+
+    @check_select:
+        lda #Joypad::BUTTON_SELECT
+        and Joypad::zp_buttons1
+        beq @check_start
+
+        lda #SPRITE_Y_POSITION0
+        cmp $200
+        beq @down
+        sta $200
+        jmp @set_timer_and_end
+
+    @check_start:
+        lda #Joypad::BUTTON_START
+        and Joypad::zp_buttons1
+        beq @end
+        JAL start
+
+    @down:
+        lda #SPRITE_Y_POSITION1
+        sta $200
+
+    @set_timer_and_end:
+        lda #TIMER_INIT_VALUE
+        sta zp_title_timer
+    @end:
+        lda #0
+        rts
+    .endproc
+
+    ;; Save the selection from the menu (TODO), hide all elements from the title
+    ;; screen, and return always 1.
+    .proc start
+        ;; Hide the sprite from the menu.
+        lda #$EF
+        sta $200
+
+        lda #1
+        rts
+    .endproc
+.endscope