diff --git a/libsrc/c128/c128-1351.s b/libsrc/c128/c128-1351.s
index afb451eb8..ae8f01f04 100644
--- a/libsrc/c128/c128-1351.s
+++ b/libsrc/c128/c128-1351.s
@@ -269,7 +269,9 @@ IOCTL:  lda     #<MOUSE_ERR_INV_IOCTL     ; We don't support ioclts for now
 
 ;----------------------------------------------------------------------------
 ; IRQ: Irq handler entry point. Called as a subroutine but in IRQ context
-; (so be careful).
+; (so be careful). The routine MUST return carry set if the interrupt has been
+; 'handled' - which means that the interrupt source is gone. Otherwise it
+; MUST return carry clear.
 ;
 
 IRQ:    lda	SID_ADConv1		; Get mouse X movement
@@ -355,10 +357,11 @@ IRQ:    lda	SID_ADConv1		; Get mouse X movement
 ; Move the mouse pointer to the new X pos
 
         tya
-        jmp     CMOVEY
+        jsr     CMOVEY
 
 ; Done
 
+        clc                     ; Interrupt not "handled"
 @SkipY: rts
 
 ; --------------------------------------------------------------------------
diff --git a/libsrc/c128/c128-joymouse.s b/libsrc/c128/c128-joymouse.s
index 161d24c2d..9fad2cf6f 100644
--- a/libsrc/c128/c128-joymouse.s
+++ b/libsrc/c128/c128-joymouse.s
@@ -269,7 +269,9 @@ IOCTL:  lda     #<MOUSE_ERR_INV_IOCTL     ; We don't support ioclts for now
 
 ;----------------------------------------------------------------------------
 ; IRQ: Irq handler entry point. Called as a subroutine but in IRQ context
-; (so be careful).
+; (so be careful). The routine MUST return carry set if the interrupt has been
+; 'handled' - which means that the interrupt source is gone. Otherwise it
+; MUST return carry clear.
 ;
 
 IRQ:	lda	#$7F
@@ -381,9 +383,10 @@ IRQ:	lda	#$7F
 ; Move the mouse pointer to the new X pos
 
         tya
-        jmp     CMOVEY
+        jsr     CMOVEY
 
 ; Done
 
-@SkipY: rts
+@SkipY: clc                             ; Interrupt not "handled"
+        rts
 
diff --git a/libsrc/c64/c64-1351.s b/libsrc/c64/c64-1351.s
index 5fbc8122e..bc849acab 100644
--- a/libsrc/c64/c64-1351.s
+++ b/libsrc/c64/c64-1351.s
@@ -178,7 +178,7 @@ BOX:    ldy     #5
 ; MOVE: Move the mouse to a new position. The position is passed as it comes
 ; from the C program, that is: X on the stack and Y in a/x. The C wrapper will
 ; remove the parameter from the stack on return.
-; No checks are done if the new position is valid (within the bounding box or 
+; No checks are done if the new position is valid (within the bounding box or
 ; the screen). No return code required.
 ;
 
@@ -269,7 +269,9 @@ IOCTL:  lda     #<MOUSE_ERR_INV_IOCTL     ; We don't support ioclts for now
 
 ;----------------------------------------------------------------------------
 ; IRQ: Irq handler entry point. Called as a subroutine but in IRQ context
-; (so be careful).
+; (so be careful). The routine MUST return carry set if the interrupt has been
+; 'handled' - which means that the interrupt source is gone. Otherwise it
+; MUST return carry clear.
 ;
 
 IRQ:    lda	SID_ADConv1		; Get mouse X movement
@@ -355,10 +357,11 @@ IRQ:    lda	SID_ADConv1		; Get mouse X movement
 ; Move the mouse pointer to the new X pos
 
         tya
-        jmp     CMOVEY
+        jsr     CMOVEY                                         
 
 ; Done
 
+        clc                             ; Interrupt not handled
 @SkipY: rts
 
 ; --------------------------------------------------------------------------
diff --git a/libsrc/c64/c64-hitjoy.s b/libsrc/c64/c64-hitjoy.s
index 028d480fe..ae09182dc 100755
--- a/libsrc/c64/c64-hitjoy.s
+++ b/libsrc/c64/c64-hitjoy.s
@@ -79,8 +79,10 @@ UNINSTALL:
         rts
 
 ; ------------------------------------------------------------------------
-; IRQ entry point. Is called from the C layer as a subroutine in the 
-; interrupt.
+; IRQ entry point. Is called from the C layer as a subroutine in the
+; interrupt. The routine MUST return carry set if the interrupt has been
+; 'handled' - which means that the interrupt source is gone. Otherwise it
+; MUST return carry clear.
 
 IRQ:    ; cia 2 setup
 
@@ -156,6 +158,8 @@ fire:
         lda     #0
         sta     $dc04
 
+        ; We do never "handle" the interrupt, we use it just as a timer.
+        clc
         rts
 
 ; ------------------------------------------------------------------------
diff --git a/libsrc/c64/c64-joymouse.s b/libsrc/c64/c64-joymouse.s
index 798eaea0c..fe8bfe249 100644
--- a/libsrc/c64/c64-joymouse.s
+++ b/libsrc/c64/c64-joymouse.s
@@ -269,7 +269,9 @@ IOCTL:  lda     #<MOUSE_ERR_INV_IOCTL     ; We don't support ioclts for now
 
 ;----------------------------------------------------------------------------
 ; IRQ: Irq handler entry point. Called as a subroutine but in IRQ context
-; (so be careful).
+; (so be careful). The routine MUST return carry set if the interrupt has been
+; 'handled' - which means that the interrupt source is gone. Otherwise it
+; MUST return carry clear.
 ;
 
 IRQ:	lda	#$7F
@@ -381,9 +383,10 @@ IRQ:	lda	#$7F
 ; Move the mouse pointer to the new X pos
 
         tya
-        jmp     CMOVEY
+        jsr     CMOVEY
 
 ; Done
 
-@SkipY: rts
+@SkipY: clc                             ; Interrupt not handled
+        rts
 
diff --git a/libsrc/runtime/callirq.s b/libsrc/runtime/callirq.s
index b456ce892..15490e1ab 100644
--- a/libsrc/runtime/callirq.s
+++ b/libsrc/runtime/callirq.s
@@ -16,6 +16,11 @@
 ;      code avoids this by using locking mechanisms, but it's complex and
 ;      has a size and performance penalty.
 ;
+;   3. Special semantics: An interruptor called by callirq must tell by
+;      setting or resetting the carry flag if the interrupt has been handled
+;      (which means that the interrupt is no longer active at the interrupt
+;      source). callirq will call no other interruptors if this happens.
+;
 ; As the normal condes routine, this one has the limitation of 127 table
 ; entries.
 ;
@@ -29,7 +34,9 @@
 
 ; --------------------------------------------------------------------------
 ; Call all IRQ routines. The function needs to use self modifying code and
-; is thereforce placed in the data segment.
+; is thereforce placed in the data segment. It will return carry set if the
+; interrupt was handled and carry clear if not. The caller may choose to
+; ignore this at will.
 ; NOTE: The routine must not be called if the table is empty!
 
 .data
@@ -45,9 +52,9 @@ callirq_y:
         sta     jmpvec+1                ; Modify code below
        	sty    	index+1                 ; Modify code below
 jmpvec: jsr    	$FFFF                   ; Patched at runtime
+        bcs     done                    ; Bail out if interrupt handled
 index: 	ldy    	#$FF                    ; Patched at runtime
        	bne     callirq_y
-        rts
-
+done:   rts