From 681ba9adf948ebdc909b08979c76251053af7a9e Mon Sep 17 00:00:00 2001
From: "Juan J. Martinez" <jjm@usebox.net>
Date: Thu, 11 May 2023 20:57:55 +0100
Subject: Add starfield effect

---
 game/starfield.asm | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100644 game/starfield.asm

(limited to 'game/starfield.asm')

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
-- 
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