Code snippets for my flicker issue.

FlickerExample.txt

static void px_ppu_sync_off(){
	px_mask &= ~PX_MASK_RENDER_ENABLE;
	px_wait_nmi();
	
	// Without these I get a few garbage lines at the top of the screen
	waitvsync();
	waitvsync();
}

static void px_ppu_sync_on(){
	px_mask |= PX_MASK_RENDER_ENABLE;
	px_wait_nmi();
}

*************************************************
// Screen changes all look like this.

// Buffer the background color change to be run in NMI
px_buffer_set_color(0, CLR_BLACK);
	
px_ppu_sync_off(); {
	// Update whole screen here...
} px_ppu_sync_on();

*************************************************
// My NMI routines look like this:

.proc px_nmi
	; Interrupt enter.
	pha
	txa
	pha
	tya
	pha

	lda px_nmi_ready
	beq @skip_frame
	
	lda px_mask
	sta PPU_MASK
	
	lda px_ctrl
	sta PPU_CTRL
	
	; Invoke OAM DMA copy.
	lda #>OAM
	sta APU_SPR_DMA
	
	; Execute the display list buffer.
	jsr _px_buffer_exec
	
	; Reset PPU Address
	lda #0
	sta PPU_VRAM_ADDR
	sta PPU_VRAM_ADDR
	
	; Set the scroll registers.
	lda PX_scroll_x+0
	sta PPU_SCROLL
	div240_quick PX_scroll_y
	sta PPU_SCROLL
	; Note: Y-scroll remainder is in x.
	
	; Set nametable base address.
	lda PX_scroll_x+1
	lsr a
	txa
	rol a
	and #$03
	ora px_ctrl
	sta PPU_CTRL
	
	@skip_frame:
	
	jsr FamiToneUpdate
	
	; Reset ready flag.
	lda #0
	sta px_nmi_ready
	
	; Interrupt exit.
	pla
	tay
	pla
	tax
	pla
	
	rti
.endproc

.export _px_wait_nmi
.proc _px_wait_nmi
	lda #1
	sta px_nmi_ready
	:	lda px_nmi_ready
		bne :-
	
	inc px_ticks
	rts
.endproc