safer NMI handling

c64/basic/wedge_tape.s

@@ -56,7 +56,10 @@ wedge_arrow_L:
 wedge_arrow_H:
 
 	jsr tape_head_align
-	jmp basic_main_loop
+
+	jsr print_return
+	ldx #B_ERR_BREAK
+	jmp do_basic_error
 
 #endif
 

c64/kernal/interrupts/ea81.return_from_interrupt.s

@@ -20,4 +20,8 @@ return_from_interrupt:
 	plx_trash_a
 	pla
 
+	// FALLTROUGH
+
+return_from_interrupt_rti:
+
 	rti

c64/kernal/interrupts/hw_entry_nmi.s

@@ -5,17 +5,16 @@
 
 hw_entry_nmi:
 
-	sei // do not allow IRQ to interfere, see https://www.c64-wiki.com/wiki/Interrupt
+	sei                      // do not allow IRQ to interfere, see https://www.c64-wiki.com/wiki/Interrupt
 
 #if CONFIG_CPU_MOS_6502 && CONFIG_BCD_SAFE_INTERRUPTS
-	cld // clear decimal flag to allow using it without disabling interrupts
+	cld                      // clear decimal flag to allow using it without disabling interrupts
 #endif
 
 	// Call interrupt routine (only if initialised)
 	pha
-	lda NMINV
-	ora NMINV+1
-	beq !+
+	lda NMINV+1              // consider zeropage NMI address as uninitialized vector
+	beq !+                   // this allows safer interrupt vector modifications 
 	pla
 	jmp (NMINV)
 !:

c64/kernal/interrupts/nmi_lock.s

@@ -0,0 +1,60 @@
+// #LAYOUT# STD *        #TAKE
+// #LAYOUT# *   KERNAL_0 #TAKE
+// #LAYOUT# *   *        #IGNORE
+
+//
+// Routine to lock NMI interrupts, using a trick described here:
+// - https://codebase64.org/doku.php?id=base:nmi_lock
+//
+
+
+#if CONFIG_TAPE_HEAD_ALIGN
+
+
+// XXX consider broader usage of this method to make IO safer
+
+
+nmi_lock:
+
+	// We will have to change NMI vector to point empty routine,
+	// which will not acknowledge the interrupt - thus preventing
+	// the NMI to be triggered again
+	
+	// Our code considers zeropage address invalid, so put
+	// it there as a preparation
+
+	ldx #$00
+	stx NMINV+1
+
+	// We can now safelly replace the vector
+
+	lda #<return_from_interrupt_rti
+	sta NMINV+0
+	lda #>return_from_interrupt_rti
+	sta NMINV+1
+
+	// Stop timer A
+
+    stx CIA2_CRA // $DD0E
+
+    // Set timer A to 0, after starting NMI will occur immediately
+
+	stx CIA2_TIMALO // $DD04
+	stx CIA2_TIMAHI // $DD05
+        
+	// Set timer A as source for NMI
+
+    lda #$81
+    sta CIA2_ICR // $DD0D
+
+    // Start timer A to trigger the NMI
+
+    lda #$01
+    sta CIA2_CRA // $DD0E
+
+    // All done. Unmaskable interrupts are now masked.
+
+    rts
+
+
+#endif

c64/kernal/memory/vector_real.s

@@ -23,6 +23,9 @@ vector_real:
 	sty MEMUSS + 1
 	stx MEMUSS + 0
 	
+	lda #$00
+	sta NMINV + 1            // our routine considers this a disabled interrupt 
+
 	// Select routine variant - store or restore vectors
 	ldy #$1F
 	bcc vector_restore
@@ -30,7 +33,7 @@ vector_real:
 	lda CINV, y
 	sta (MEMUSS), y
 
-	bcs vector_end_loop // branch always
+	bcs vector_end_loop      // branch always
 	
 vector_restore:
 	lda (MEMUSS), y

c64/kernal/panic.s

@@ -22,11 +22,18 @@ panic:
 	jsr setup_vicii
 	jsr clear_screen
 
-	// Disable NMIs - our routine checks for NULL vector
+	// Disable NMIs
+
+#if CONFIG_TAPE_HEAD_ALIGN
+
+	jsr nmi_lock
+
+#else
 
 	lda #$00
-	sta NMINV+0
-	sta NMINV+1
+	sta NMINV + 1            // our routine does not allow zeropage vectors
+
+#endif
 
 	// Display KERNAL PANIC message
 

c64/kernal/tape/load_tape_normal.s

@@ -45,6 +45,9 @@ load_tape_normal:
 	// Start playing
 	jsr tape_common_prepare_cia
 	jsr tape_ask_play
+#if CONFIG_TAPE_HEAD_ALIGN
+	jsr tape_prepare_reading
+#endif
 
 	// FALLTROUGH
 

c64/kernal/tape/load_tape_turbo.s

@@ -32,6 +32,9 @@ load_tape_auto:
 	// Start playing
 	jsr tape_common_prepare_cia
 	jsr tape_ask_play
+#if CONFIG_TAPE_HEAD_ALIGN
+	jsr tape_prepare_reading
+#endif
 
 	// FALLTROUGH
 

c64/kernal/tape/tape_common.s

@@ -104,6 +104,17 @@ tape_wait_first_pulse:
 
 tape_wait_play_done:
 
+	lda #$01
+	sta CAS1                           // set the interlock
+
+#if CONFIG_TAPE_HEAD_ALIGN
+
+	rts
+
+tape_prepare_reading:
+
+#endif // CONFIG_TAPE_HEAD_ALIGN
+
 #if CONFIG_MB_MEGA_65
 
 	// Display confirmation
@@ -115,9 +126,6 @@ tape_wait_play_done:
 
 	// Prepare for reading
 
-	lda #$01
-	sta CAS1                           // set the interlock
-
 	jsr print_return
 	jmp tape_screen_off_motor_on
 

c64/kernal/tape/tape_head_align.s

@@ -12,9 +12,7 @@
 
 // XXX for Mega65 it can be done more easily, by just disabling the badlines
 
-// XXX consider "PRESS PLAY ON TAPE" before start, with option to stop
 // XXX document CONFIG_TAPE_HEAD_ALIGN option
-// XXX block NMIs
 // XXX fix Mega65 build compilation issues
 
 
@@ -50,12 +48,22 @@ tape_head_align:
 
 #else
 
+	// Make sure user really wants to launch the tool
+
+	jsr tape_ask_play
+
 	// Disable interrupts, set proper I/O values
 
 	jsr CLALL
+
 	sei
+
 	jsr IOINIT
-	jsr cint_legacy
+	jsr CINT
+
+	jsr nmi_lock
+
+	// Prepare CIA timers
 
 	ldx #$02                  // set timer A to 3 ticks
 	jsr tape_common_prepare_cia_by_x
@@ -111,8 +119,6 @@ tape_head_align:
 	dey
 	bne !-
 
-
-
 	// Set graphics mode
 
 	lda #$28                           // graphics start at $2000, color at $0800
@@ -252,7 +258,6 @@ tape_head_align_drawn:
 	beq tape_head_align_loop_1
 
 
-
 tape_head_align_quit:
 
 	// It makes no sense to return to system, as we already destroyed