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, 142 insertions, 0 deletions

diff --git a/src/assets.s b/src/assets.s
new file mode 100644
index 0000000..8e839d3
--- /dev/null
+++ b/src/assets.s
@@ -0,0 +1,142 @@
+.segment "CODE"
+
+;; The game is so simple that it can fit in both nametables. This scope has all
+;; the functions and data that initializes and resets all of them properly.
+.scope Assets
+    ;; Initialize palettes and nametables.
+    .proc init
+        jsr init_palettes
+
+        ;; Load the title screen on the first nametable.
+        lda #.lobyte(title_screen)
+        sta Globals::zp_arg0
+        lda #.hibyte(title_screen)
+        sta Globals::zp_arg1
+        ldx #$20
+        jsr load_screen_x
+
+        ;; Ensure that the title looks as expected.
+        jsr prepare_for_title_screen
+
+        ;; Load the main screen on the second nametable.
+        lda #.lobyte(main_screen)
+        sta Globals::zp_arg0
+        lda #.hibyte(main_screen)
+        sta Globals::zp_arg1
+        ldx #$28
+        jsr load_screen_x
+
+        rts
+    .endproc
+
+    ;; Load the 1KB worth of screen data located via the 16-bit pointer on
+    ;; Globals::zp_arg{0,1}. Set the `x` register to the high byte of the
+    ;; nametable to be used.
+    .proc load_screen_x
+        bit PPU::STATUS
+
+        stx PPU::ADDRESS
+        lda #$00
+        sta PPU::ADDRESS
+
+        ldy #0
+        ldx #4
+    @loop:
+        lda (Globals::zp_arg0), y
+        sta PPU::DATA
+        iny
+        bne @loop
+
+        dex
+        beq @end
+
+        inc Globals::zp_arg1
+        jmp @loop
+
+    @end:
+        rts
+    .endproc
+
+    ;; Performs all the needed tricks in order to get the first nametable as
+    ;; expected.
+    .proc prepare_for_title_screen
+        bit PPU::STATUS
+
+        lda #$23
+        sta $2006
+        lda #$C8
+        sta $2006
+
+        ldx #$10
+    @upper_title_bar_loop:
+        lda #%10101010
+        sta $2007
+        dex
+        bne @upper_title_bar_loop
+
+        ;; TODO: store back palettes after game over.
+
+        rts
+    .endproc
+
+    ;; Performs all the needed tricks in order to get the second nametable as
+    ;; expected.
+    .proc prepare_for_main_screen
+        ;; TODO: second palette for background should be:
+        ;; .byte $0F, $14, $2C, $28
+        ;;
+        ;; TODO: first palette for the foreground should be:
+        ;; .byte $0F, $16, $10, $30
+        rts
+    .endproc
+
+    ;; Copies all the palettes for our game into the proper PPU address.
+    .proc init_palettes
+        lda #$3F
+        sta PPU::ADDRESS
+        lda #$00
+        sta PPU::ADDRESS
+
+        ldx #0
+    @load_palettes_loop:
+        lda palettes, x
+        sta PPU::DATA
+        inx
+        cpx #$20
+        bne @load_palettes_loop
+        rts
+    palettes:
+        ;; Background
+        ;; 0: score
+        .byte $0F, $30, $2C, $28
+        ;; 1: floating platforms
+        .byte $0F, $2C, $30, $2A
+        ;; 2: ground
+        .byte $0F, $28, $2C, $16
+        ;; 3: ship
+        .byte $0F, $16, $30, $00
+
+        ;; TODO: fuel tank needs color $24
+        ;; Foreground
+        ;; 0: player & ship
+        .byte $0F, $30, $10, $30
+        ;; 1: enemy 1 & bonuses
+        .byte $0F, $16, $2C, $2A
+        ;; 2: enemy 2, fuel & bonuses
+        .byte $0F, $16, $14, $28
+        ;; 3: SUSE easter egg
+        .byte $0F, $16, $00, $2B
+    .endproc
+
+    ;; Having 2KB for screen data is quite wasteful, but since it's such a
+    ;; simple game, I have so much space left in the cartridge that I can go
+    ;; bananas with it. This helps out loading the screen as we can go faster
+    ;; and it is less error prone.
+    title_screen:   
+        .incbin "../assets/title.nam"
+    main_screen:   
+        .incbin "../assets/main.nam"
+.endscope
+
+.segment "CHARS"
+    .incbin "../assets/jetpac.chr"