Add starfield effect

1 files changed, 70 insertions, 0 deletions

diff --git a/game/starfield.asm b/game/starfield.asm
new file mode 100644
index 0000000..e8b8ed1
--- /dev/null
+++ b/game/starfield.asm
@@ -0,0 +1,70 @@
+;
+; Simple starfield effect
+;
+stars_update:
+    ld x, <stars
+    ld a, >stars
+
+stars_update_loop:
+    ld y, [a : x]
+    inc x
+    bo
+    inc a
+    ld b, [a : x]
+    ; is the end of list?
+    cmp b, 0xff
+    bz
+    ret
+
+    push a
+    push x
+
+    ; erase old
+    ld x, 0
+    ld [b : y], x
+    ld x, [sp + 0]
+
+    ; update address
+    inc x
+    bo
+    inc a
+    push x
+    ld x, [a : x]
+stars_update_addr:
+    add y, 128
+    bo
+    inc b
+    cmp b, 0xfe
+    bnz
+    jmp stars_no_wrap
+    cmp y, 0x80
+    bc
+    ld b, 0xbf
+stars_no_wrap:
+    dec x
+    bnz
+    jmp stars_update_addr
+    pop x
+
+    ; draw new: color
+    inc x
+    bo
+    inc a
+    ld x, [a : x]
+    ld [b : y], x
+
+    pop x
+    pop a
+
+    ; save new address
+    ld [a : x], b
+    dec x
+    bo
+    dec a
+    ld [a : x], y
+
+    ; next star
+    add x, 4
+    bo
+    inc a
+    jmp stars_update_loop