// #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