diff --git a/libsrc/cbm510/cbm510-stdser.s b/libsrc/cbm510/cbm510-stdser.s
index 59675e0fe..223668fec 100644
--- a/libsrc/cbm510/cbm510-stdser.s
+++ b/libsrc/cbm510/cbm510-stdser.s
@@ -353,7 +353,7 @@ IRQ:    lda     #$0F
         ldy     #ACIA::STATUS
         lda     (acia),y        ; Check ACIA status for receive interrupt
      	and 	#$08
-       	beq    	@L9   		; Jump if no ACIA interrupt
+       	beq    	@L10            ; Jump if no ACIA interrupt
         ldy     #ACIA::DATA
         lda    	(acia),y        ; Get byte from ACIA
     	ldx 	RecvFreeCnt  	; Check if we have free space left
@@ -371,6 +371,7 @@ IRQ:    lda     #$0F
         ldy     #ACIA::CMD
         sta     (acia),y
     	sta 	Stopped
+        sec                     ; Interrupt handled
 
 ; Done, switch back to the execution segment
 
@@ -378,6 +379,13 @@ IRQ:    lda     #$0F
         sta     IndReg
         rts
 
+; No ACIA interrupt
+
+@L10:   clc                     ; Interrupt not handled
+        lda     ExecReg
+        sta     IndReg
+        rts
+
 ;----------------------------------------------------------------------------
 ; Try to send a byte. Internal routine. A = TryHard
 
diff --git a/libsrc/cbm610/cbm610-stdser.s b/libsrc/cbm610/cbm610-stdser.s
index f844b3c2d..74fd480c8 100644
--- a/libsrc/cbm610/cbm610-stdser.s
+++ b/libsrc/cbm610/cbm610-stdser.s
@@ -353,7 +353,7 @@ IRQ:    lda     #$0F
         ldy     #ACIA::STATUS
         lda     (acia),y        ; Check ACIA status for receive interrupt
      	and 	#$08
-       	beq    	@L9   		; Jump if no ACIA interrupt
+       	beq    	@L10            ; Jump if no ACIA interrupt
         ldy     #ACIA::DATA
         lda    	(acia),y        ; Get byte from ACIA
     	ldx 	RecvFreeCnt  	; Check if we have free space left
@@ -371,6 +371,7 @@ IRQ:    lda     #$0F
         ldy     #ACIA::CMD
         sta     (acia),y
     	sta 	Stopped
+        sec                     ; Interrupt handled
 
 ; Done, switch back to the execution segment
 
@@ -378,6 +379,13 @@ IRQ:    lda     #$0F
         sta     IndReg
         rts
 
+; No ACIA interrupt
+
+@L10:   clc                     ; Interrupt not handled
+        lda     ExecReg
+        sta     IndReg
+        rts
+
 ;----------------------------------------------------------------------------
 ; Try to send a byte. Internal routine. A = TryHard
 
diff --git a/libsrc/plus4/plus4-stdser.s b/libsrc/plus4/plus4-stdser.s
index e09c5ab90..53cec2875 100644
--- a/libsrc/plus4/plus4-stdser.s
+++ b/libsrc/plus4/plus4-stdser.s
@@ -352,7 +352,7 @@ IOCTL:  lda     #<SER_ERR_INV_IOCTL     ; We don't support ioclts for now
 
 IRQ:    lda    	ACIA_STATUS     ; Check ACIA status for receive interrupt
      	and 	#$08
-       	beq    	@L9  		; Jump if no ACIA interrupt
+       	beq    	@L10            ; Jump if no ACIA interrupt
         lda 	ACIA_DATA       ; Get byte from ACIA
     	ldx 	RecvFreeCnt  	; Check if we have free space left
        	beq    	@L1    	       	; Jump if no space in receive buffer
@@ -362,13 +362,19 @@ IRQ:    lda    	ACIA_STATUS     ; Check ACIA status for receive interrupt
     	dec 	RecvFreeCnt     ; Decrement free space counter
     	cpx 	#33            	; Check for buffer space low
        	bcc    	@L1            	; Assert flow control if buffer space low
-@L9:    rts
+        rts                     ; Return with carry set (interrupt handled)
 
 ; Assert flow control if buffer space too low
 
 @L1:  	lda 	RtsOff
     	sta 	ACIA_CMD
     	sta 	Stopped
+        sec                     ; Interrupt handled
+        rts
+
+; No ACIA interrupt
+
+@L10:   clc                     ; Interrupt not handled
         rts
 
 ;----------------------------------------------------------------------------