Workaround for "phantom" key presses in the C128 "joystick" mouse driver.

libsrc/c128/mouseref.s

@@ -4,6 +4,8 @@
 ; 2013-07-25, Greg King
 ;
 
+        .include        "c128.inc"
+
         .export         mouse_libref, _pen_adjuster
 
         .data
@@ -21,3 +23,70 @@ mouse_libref:                   ; generic label for mouse-kernel
 ;
 _pen_adjuster:
         .addr   $0000
+        .addr   IRQStub2
+callback:                       ; callback into mouse driver after ROM IRQ handler has been run
+        .addr   $0000           ; (filled in by mouse driver)
+jmp_rom_hdlr:                   ; "trampoline" to jump to ROM IRQ handler
+        .addr   $0000           ; (filled in by mouse driver)
+
+
+.segment        "LOWCODE"
+
+; Called from irq.s when it thinks it chains to the original handler.
+; ROM is banked in again. In order to call the callback we have to
+; bank it out one more time.
+
+IRQStub2:
+
+; Call ROM handler and prepare stack so that it will return to us.
+
+        ; setup fake IRQ stack frame which will return to "IRQCont"
+        lda     #>@IRQCont
+        pha
+        lda     #<@IRQCont
+        pha
+        php
+
+        ; mimic the contents saved on the stack by the ROM IRQ entry handler
+        pha                     ; A
+        pha                     ; X
+        pha                     ; Y
+        lda     #MMU_CFG_CC65   ; MMU configuration which will be active after the ROM handler returns
+        pha
+
+        ; push address of ROM handler on stack and jump to it
+        lda     jmp_rom_hdlr+1
+        pha
+        lda     jmp_rom_hdlr
+        pha
+        rts                     ; jump to ROM handler
+
+        ; our MMU configuration byte we pushed on the stack before (MMU_CFG_CC65) is now active
+
+@IRQCont:
+
+        ; call mouse driver callback routine
+        lda     #>(@IRQCont2-1)
+        pha
+        lda     #<(@IRQCont2-1)
+        pha
+        lda     callback+1
+        pha
+        lda     callback
+        pha
+        rts                     ; jump to callback routine
+
+@IRQCont2:
+        
+        ; return from interrupt
+        ; We could just jump to $FF33, but since I don't know whether this address is valid in all
+        ; ROM versions, duplicate that code here.
+
+        pla
+        sta     MMU_CR          ; MMU configuration register
+        pla
+        tay
+        pla
+        tax
+        pla
+        rti