scroll: Start with the scrolling examples

This is still work in progress but at least there's some basic structure
to it.

Signed-off-by: Miquel Sabaté Solà <msabate@suse.com>

1 files changed, 43 insertions, 0 deletions

diff --git a/scroll/include/globals.s b/scroll/include/globals.s
new file mode 100644
index 0000000..d941736
--- /dev/null
+++ b/scroll/include/globals.s
@@ -0,0 +1,43 @@
+;; Resets the background buffer to a zero-sized array.
+.macro CLEAR_BACKGROUND_BUFFER
+    lda #$FF
+    sta Globals::m_background_buffer
+.endmacro
+
+;; TODO: macro PUSH_TO_BACKGROUND_BUFFER_X
+
+;; Global variables used throughout the scrolling examples.
+.scope Globals
+    ;;;
+    ;; Temporary values.
+
+    m_tmp_1  = $90
+    m_tmp_2  = $91
+    m_tmp_3  = $92
+
+    ;; Current value for the scroll on the X axis.
+    m_scroll = $93
+
+    m_background_idx = $19
+
+    ;; TODO
+    m_collisions = $20
+
+    ;; TODO
+    m_background_buffer = $40
+
+    ;; Initialize global variables.
+    .proc init
+        lda #0
+        sta m_tmp_1
+        sta m_tmp_2
+        sta m_tmp_3
+        sta m_scroll
+        sta m_background_idx
+
+        CLEAR_BACKGROUND_BUFFER
+        sta m_collisions
+
+        rts
+    .endproc
+.endscope