nullsound: fix race when handling NMI and INT concurrently

When a Timer interrupts the current sound driver, the interrupt
handler has to save current registers. During this execution,
new interrupts are masked, but a NMI can still occur and interrupt
the interrupt handler.

To avoid losing the state of the original sound driver, one cannot
use shadow registers for both Timer and NMI handlers. Instead,
make the Timer interrupt handler save register on the stack.

Closes #115

Author: dciabrin

nullsound/entrypoint.s

@@ -59,11 +59,7 @@
 ;;;
         .org    0x0038
         di
-        ex      af, af'
-        exx
         call    update_timer_state_tracker
-        exx
-        ex      af', af
         ei
         reti
 

nullsound/timer.s

@@ -60,7 +60,13 @@ init_timer_state_tracker::
         ret
 
 
+;;; Acknowledge and re-arm YM2610 timers
+;;; all registers must be preserved, no shadow register must be
+;;; used as they are currently reserved for processing NMI
+;;; ------
 update_timer_state_tracker::
+        push    af
+        push    bc
         ld      a, #TIMER_CONSUMER_ALL
         ld      (state_timer_tick_reached), a
         ;; keep track of the new interrupt
@@ -89,6 +95,8 @@ update_timer_state_tracker::
         ;; interrupt handler
         call    ym2610_restore_context_port_a
 
+        pop     bc
+        pop     af
         ret