From: Lukas Krickl <lukas@krickl.dev>
Date: Sun, 10 Aug 2025 06:17:24 +0000 (+0200)
Subject: vblank: Added way to overwrite vblank handler at runtime
X-Git-Url: https://git.krickl.dev/?a=commitdiff_plain;h=9b25466a8bd05a8d283d0f4ade8838b7e412428b;p=gbrg%2F.git

vblank: Added way to overwrite vblank handler at runtime

This is done by having 4 bytes of self modifying code in wram.
It offers enough space for a call and a reti instruction.
Currently it is simply hard-set to the default handler, but if a custom handler is ever needed
it can be put there.
---

diff --git a/src/jmp.inc b/src/jmp.inc
index 26d62f5..e5eafac 100644
--- a/src/jmp.inc
+++ b/src/jmp.inc
@@ -33,7 +33,7 @@ sys_farcall:
 ; vblank 0x40
 ;=============
 vec_vblank:
-  call vblank  
+  jp vblank_jp  
   reti
 
 .fill 0, 0x48 - $
diff --git a/src/sys.s b/src/sys.s
index d2cec96..8b89195 100644
--- a/src/sys.s
+++ b/src/sys.s
@@ -97,3 +97,22 @@ reset_unit_obj:
   xor a, a
   ld [unit_sprite], a
   ret
+	
+	; writes default vblank jmp vector
+setup_vblank_jp_smc:
+	ld hl, vblank_jp
+
+	; write instruction
+	ld a, 0xCD ; call 
+	ld [hl+], a
+
+	; write address
+	ld a, vblank LO
+	ld [hl+], a
+	ld a, vblank HI
+	ld [hl+], a
+
+	; write reti
+	ld a, 0xD9 ; reti
+	ld [hl], a
+	ret
diff --git a/src/video.s b/src/video.s
index ddba5e5..5c0cc13 100644
--- a/src/video.s
+++ b/src/video.s
@@ -156,6 +156,9 @@ video_init:
   ld bc, 1024
   ld d, UI_WINDOW_BACKGROUND
   call memset
+
+	; setup vblank jp
+	call setup_vblank_jp_smc
   
   
   ret
diff --git a/src/wram.s b/src/wram.s
index e3c7990..d761532 100644
--- a/src/wram.s
+++ b/src/wram.s
@@ -34,6 +34,13 @@ shadow_ie: .adv 1
   ; status text buffer
 status_text: .adv 32
 
+	; self modifying code
+	; the vblank interrupt jumps here
+	; this should be enough memory for 
+	; a jp + address
+	; and a reti instruction
+vblank_jp: .adv 4 
+
   ; ptr to demo inputs
   ; provides one byte of inputs per frame
   ; if all buttons are held (value is 0xFF) it indicates the end