The polling method, half variance technique, in its shortest form. PAL/NTSC.

;64tass format

stirq	.byte $a5	;subroutine
        .byte $ea
        .byte $a9 	;for NTSC, change this to $ea
        .byte $ea
	ldy #$07    
-       dey
        bne -
        inx
        rts

        ldx #$28	;main start is here!
-       cpx $d012
        bne -
        
        jsr stirq+1 	;40 cycles
        cpx $d012
        beq +
        nop
        nop

+       jsr stirq+1 	;40 cycles
        cpx $d012
        beq +
        bit $ea

+       jsr stirq  	;41 cycles
        cpx $d012
        bne +
        
+       nop      	;stable raster here