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, 28 insertions, 0 deletions

diff --git a/src/driver.s b/src/driver.s
new file mode 100644
index 0000000..521c335
--- /dev/null
+++ b/src/driver.s
@@ -0,0 +1,28 @@
+.segment "CODE"
+
+.scope Driver
+    .proc switch
+        ;; Get the assets ready for the main screen. That is, make sure that the
+        ;; palettes and such are as desired since the title screen needed
+        ;; another setup.
+        jsr Assets::prepare_for_main_screen
+
+        ;; Switch to the other base nametable.
+        lda #%10001010
+        sta PPU::zp_control
+
+        ;; Mark the state of the game as "game". That is, the player has
+        ;; started. Also set the `ppu` flag so the PPU control update takes
+        ;; place.
+        lda #%01000001
+        ora Globals::zp_flags
+        sta Globals::zp_flags
+
+        rts
+    .endproc
+
+    .proc update
+        ;; TODO
+        rts
+    .endproc
+.endscope