diff --git a/asminc/apple2.inc b/asminc/apple2.inc
index 19906ec2f..5ebf73164 100644
--- a/asminc/apple2.inc
+++ b/asminc/apple2.inc
@@ -1,6 +1,6 @@
 
 ;-----------------------------------------------------------------------------
-; Zero page stuff
+; Zero page
 
 WNDLFT  :=      $20     ; Text window left
 WNDWDTH :=      $21     ; Text window width
@@ -31,35 +31,41 @@ PWREDUP :=      $03F4   ; This must be = EOR #$A5 of SOFTEV+1
 KBD     :=      $C000   ; Read keyboard
 KBDSTRB :=      $C010   ; Clear keyboard strobe
 
-; 80 column video switches
+; 80 column video
 CLR80COL:=      $C000   ; Disable 80 column store
 SET80COL:=      $C001   ; Enable 80 column store
 RD80COL :=      $C018   ; >127 if 80 column store enabled
 RD80VID :=      $C01F   ; >127 if 80 column video enabled
 
-; Character set switches
+; Character set
 CLRALTCHAR :=   $C00E   ; Normal Apple II char set
 SETALTCHAR :=   $C00F   ; Norm/inv LC, no flash
 ALTCHARSET :=   $C01E   ; >127 if alt charset switched in
 
-; Language card switches
+; Language card
 RDLCBNK2:=      $C011   ; >127 if LC bank 2 in use
 RDLCRAM :=      $C012   ; >127 if LC is read enabled
 ROMIN   :=      $C081   ; Swap in D000-FFFF ROM
 LCBANK2 :=      $C083   ; Swap in LC bank 2
 LCBANK1 :=      $C08B   ; Swap in LC bank 1
 
-; Video mode switches
-TXTCLR     :=   $C050   ; Display graphics
-TXTSET     :=   $C051   ; Display text
-MIXCLR     :=   $C052   ; Disable 4 lines of text
-MIXSET     :=   $C053   ; Enable 4 lines of text
-LOWSCR     :=   $C054   ; Page 1
-HISCR      :=   $C055   ; Page 2
-LORES      :=   $C056   ; Lores graphics
-HIRES      :=   $C057   ; Hires graphics
-DHIRESON   :=   $C05E   ; Enable double-width graphics
-DHIRESOFF  :=   $C05F   ; Disable double-width graphics
+; Vertical blanking
+RDVBLBAR    :=  $C019   ; >127 if not vertical blanking
+RDVBLMSK    :=  $C041   ; >127 if VBL interrupts enabled
+DISVBL      :=  $C05A   ; Disable VBL interrupts
+ENVBL       :=  $C05B   ; Enable VBL interrupts
+
+; Video mode
+TXTCLR      :=  $C050   ; Display graphics
+TXTSET      :=  $C051   ; Display text
+MIXCLR      :=  $C052   ; Disable 4 lines of text
+MIXSET      :=  $C053   ; Enable 4 lines of text
+LOWSCR      :=  $C054   ; Page 1
+HISCR       :=  $C055   ; Page 2
+LORES       :=  $C056   ; Lores graphics
+HIRES       :=  $C057   ; Hires graphics
+DHIRESON    :=  $C05E   ; Enable double-width graphics
+DHIRESOFF   :=  $C05F   ; Disable double-width graphics
 
 ; Game controller
 TAPEIN  :=      $C060   ; Read casette input / Switch input 3
@@ -73,8 +79,8 @@ PADDL3  :=      $C067   ; Analog input 3
 PTRIG   :=      $C070   ; Analog input reset
 
 ; Input/Output Unit
-IOUDISON   :=   $C07E   ; Disable IOU
-IOUDISOFF  :=   $C07F   ; Enable IOU
+IOUDISON    :=  $C07E   ; Disable IOU
+IOUDISOFF   :=  $C07F   ; Enable IOU
 
 ; Control Your Apple
 CYAREG  :=      $C036   ; Bits 0-3=disk detect 4=shadow all banks 7=fast
diff --git a/doc/apple2enh.sgml b/doc/apple2enh.sgml
index fa70da4ee..2d4381353 100644
--- a/doc/apple2enh.sgml
+++ b/doc/apple2enh.sgml
@@ -332,10 +332,9 @@ usage.
 <item>get_ostype
 <item>rebootafterexit
 <item>ser_apple2_slot
-<item>textframe
-<item>textframexy
 <item>tgi_apple2_mix
 <item>videomode
+<item>waitvsync
 </itemize>
 
 
diff --git a/include/apple2enh.h b/include/apple2enh.h
index 3dd8cffc0..58e0b397f 100644
--- a/include/apple2enh.h
+++ b/include/apple2enh.h
@@ -116,6 +116,9 @@ unsigned __fastcall__ videomode (unsigned mode);
 ** constants.
 */
 
+void waitvsync (void);
+/* Wait for start of next frame */
+
 
 
 /* End of apple2enh.h */
diff --git a/libsrc/apple2/get_ostype.s b/libsrc/apple2/get_ostype.s
index 71b43d174..a1b1eb5be 100644
--- a/libsrc/apple2/get_ostype.s
+++ b/libsrc/apple2/get_ostype.s
@@ -4,7 +4,7 @@
 ; unsigned char get_ostype (void)
 ;
 
-        .constructor    initostype
+        .constructor    initostype, 9
         .export         _get_ostype
 
 ; Identify machine according to:
diff --git a/libsrc/apple2/waitvsync.s b/libsrc/apple2/waitvsync.s
new file mode 100644
index 000000000..a4ab5ebb3
--- /dev/null
+++ b/libsrc/apple2/waitvsync.s
@@ -0,0 +1,60 @@
+;
+; Oliver Schmidt, 2020-06-14
+;
+; void waitvsync (void);
+;
+        .ifdef  __APPLE2ENH__
+
+        .constructor    initvsync
+        .export         _waitvsync
+        .import         _get_ostype
+
+        .include        "apple2.inc"
+
+        .segment        "ONCE"
+
+initvsync:
+        jsr     _get_ostype
+        sta     ostype
+        rts
+
+        .code
+
+_waitvsync:
+        bit     ostype
+        bmi     iigs            ; $8x
+        bvs     iic             ; $4x
+
+:       bit     RDVBLBAR
+        bpl     :-              ; Blanking
+:       bit     RDVBLBAR
+        bmi     :-              ; Drawing
+        rts
+
+        ; Apple IIgs TechNote #40, VBL Signal
+iigs:   bit     RDVBLBAR
+        bmi     iigs            ; Blanking
+:       bit     RDVBLBAR
+        bpl     :-              ; Drawing
+        rts
+
+        ; Apple IIc TechNote #9, Detecting VBL
+iic:    sei
+        sta     IOUDISOFF
+        lda     RDVBLMSK
+        bit     ENVBL
+        bit     PTRIG           ; Reset VBL interrupt flag
+:       bit     RDVBLBAR
+        bpl     :-
+        asl
+        bcs     :+              ; VBL interrupts were already enabled
+        bit     DISVBL
+:       sta     IOUDISON        ; IIc Tech Ref Man: The firmware normally leaves IOUDIS on.
+        cli
+        rts
+
+        .segment        "INIT"
+
+ostype: .res    1
+
+        .endif                  ; __APPLE2ENH__