dos33fsprogs/fireworks/vapor_lock.s

	;==============================
	; setup graphics for vapor lock
	;==============================
vapor_lock:

	; Clear Page0
	lda	#$0
	sta	DRAW_PAGE
	lda	#$44
	jsr	clear_gr

	; Make screen half green
	lda	#$11
	ldy	#24
	jsr	clear_page_loop


	;=====================================================
	; attempt vapor lock
	;  by reading the "floating bus" we can see most recently
	;  written value of the display
	; we look for $55 (which is the grey line)
	;=====================================================
	; See:
	;	Have an Apple Split by Bob Bishop
        ;	Softalk, October 1982

	; Challenges: each scan line scans 40 bytes.
	; The blanking happens at the *beginning*
	; So 65 bytes are scanned, starting at adress of the line - 25

	; the scan takes 8 cycles, look for 4 repeats of the value
	; to avoid false positive found if the horiz blanking is mirroring
	; the line (max 3 repeats in that case)

vapor_lock_loop:		; first make sure we have all zeroes
	LDA #$11
zxloop:
	LDX #$04
wiloop:
	CMP $C051
	BNE zxloop
	DEX
	BNE wiloop

	LDA #$44		; now look for our border color (4 times)
zloop:
	LDX #$04
qloop:
	CMP $C051
	BNE zloop
	DEX
	BNE qloop

	rts