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, 27 insertions, 0 deletions

diff --git a/src/vectors.s b/src/vectors.s
index 2edc6a6..b06f404 100644
--- a/src/vectors.s
+++ b/src/vectors.s
@@ -78,16 +78,43 @@
     tya
     pha
 
+    ;; Sprite DMA.
     lda #$00
     sta OAM::ADDRESS
     lda #$02
     sta OAM::DMA
 
+    ;; TODO: some actions here will depend on the status of the game...
+
+    ;; Decrease title timer.
+    lda Title::zp_title_timer
+    beq :+
+    dec Title::zp_title_timer
+:
+
+    ;; Should we update PPU registers?
+    bit Globals::zp_flags
+    bvc @scroll
+
+    ;; Zero out the `ppu` flag.
+    lda #%10111111
+    and Globals::zp_flags
+    sta Globals::zp_flags
+
+    bit PPU::STATUS
+
+    ;; Update the PPU control register with the shadowed value.
+    lda PPU::zp_control
+    sta PPU::CONTROL
+
+@scroll:
+    ;; Always reset the scroll just in case.
     bit PPU::STATUS
     lda #$00
     sta PPU::SCROLL
     sta PPU::SCROLL
 
+    ;; Unblock the main code.
     lda #%01111111
     and Globals::zp_flags
     sta Globals::zp_flags