diff --git a/keyboard.asm b/keyboard.asm
index dc6ce73..fae8d23 100644
--- a/keyboard.asm
+++ b/keyboard.asm
@@ -2,6 +2,10 @@
 
 ; add events to the event queue if a key was pressed or released
 ; this should only be called by system_vsync_handler
+; inputs:
+; none
+; outputs:
+; r0: non-zero if F12 was pressed, zero otherwise
 keyboard_update:
     ; pop a key from the keyboard queue
     in r0, 0x80000500
@@ -10,7 +14,7 @@ keyboard_update:
 
     ; invoke the debug monitor if F12 was pressed
     cmp r0, 0x58
-    ifz jmp invoke_monitor
+    ifz jmp keyboard_update_end
 
     ; check if this is a make or break scancode
     bts r0, 7
@@ -25,6 +29,7 @@ keyboard_update:
     mov r6, 0
     mov r7, 0
     call new_event
+    mov r0, 0
     jmp keyboard_update_end
 keyboard_update_break_scancode:
     and r0, 0x7F
@@ -37,5 +42,6 @@ keyboard_update_break_scancode:
     mov r6, 0
     mov r7, 0
     call new_event
+    mov r0, 0
 keyboard_update_end:
     ret
diff --git a/vsync.asm b/vsync.asm
index 37c4862..dd8f566 100644
--- a/vsync.asm
+++ b/vsync.asm
@@ -15,6 +15,9 @@ system_vsync_handler:
     call keyboard_update
     cmp.8 [UPDATE_ICON], 0
     ifnz call icon_update
+    ; check if monitor should be started
+    cmp r0, 0
+    ifnz jmp system_vsync_handler_breakpoint
 
     pop r7
     pop r6
@@ -25,3 +28,16 @@ system_vsync_handler:
     pop r1
     pop r0
     reti
+
+system_vsync_handler_breakpoint:
+    pop r7
+    pop r6
+    pop r5
+    pop r4
+    pop r3
+    pop r2
+    pop r1
+    pop r0
+    ; breakpoint handler expects that there is an extra 4 bytes on the stack
+    sub rsp, 4
+    jmp system_breakpoint_handler