added first CPU specific optimizations

c64/kernal/interrupts/ea81.return_from_interrupt.s

@@ -11,10 +11,16 @@ return_from_interrupt:
 
 	// Restore registers and return
 	// Sequence according to Compute's Mapping the 64 p73
+
+#if CONFIG_CPU_MOS_6502
 	pla
 	tay
 	pla
 	tax
+#else
+	ply
+	plx
+#endif
 	pla
 	rti
 

c64/kernal/interrupts/fe47.default_nmi_handler.s

@@ -6,10 +6,15 @@ default_nmi_handler:
 
 	// Save registers, sequence according to Compute's Mapping the Commodore 64, page 73
 	pha
+#if CONFIG_CPU_MOS_6502
 	txa
 	pha
 	tya
 	pha
+#else
+	phx
+	phy
+#endif
 
 	// XXX: RS-232 support is not implemented
 

c64/kernal/interrupts/irq_handler.s

@@ -10,12 +10,19 @@ irq_handler:
 
 	// Save registers, sequence according to Compute's Mapping the Commodore 64, page 73
 	pha
+#if CONFIG_CPU_MOS_6502
 	txa
 	pha
 	tya
 	pha
+#else
+	phx
+	phy
+#endif
 
-	cld // clear decimal flag, to prevent possible problems
+#if CONFIG_CPU_MOS_6502 && CONFIG_BCD_SAFE_INTERRUPTS
+	cld // clear decimal flag to allow using it without disabling interrupts
+#endif
 
 	// Check if caused by BRK
 	tsx

c64/kernal/interrupts/nmi_handler.s

@@ -2,7 +2,10 @@
 nmi_handler:
 
 	sei // don't allow IRQ to interfere, see https://www.c64-wiki.com/wiki/Interrupt
-	cld // clear decimal flag, to prevent possible problems
+
+#if CONFIG_CPU_MOS_6502 && CONFIG_BCD_SAFE_INTERRUPTS
+	cld // clear decimal flag to allow using it without disabling interrupts
+#endif
 
 	// Call interrupt routine (only if initialised)
 	pha