.word $f000
	* = $f000

	scrptr = $8000
	eoscr = $83e7		; last screen address
	lastline = $83c0	; start of last line

	memstart = $0010
	memend = $4000

	addr = $0100
	aoff = $02
	ipass = $03
	pbyte = $04
	scroff = $0605

main:
	sei
	;; clear screen
	ldx #<scrptr
	stx <addr
	ldx #>scrptr
	stx >addr
	ldy #$00
	;; initialize screen pointer
	lda #<scrptr
	sta <scroff
	lda #>scrptr
	sta >scroff
clearloop:
	lda #" "
	sta (<addr),Y
	iny
	cpy #$00
	bne clearloop
	inx
	stx >addr
	cpx #$84
	bne clearloop

	;; print infotext
	ldy #$00
textloop:
	lda infotext,Y
	sta lastline,Y
	iny
	cpy #$20
	bne textloop

	;; set up first pass with $FF
	lda #$00
	sta ipass
	lda #"#"
	ldy #$00
	sta (<scroff),Y
	
pass:
	;; load start address
	ldx #>memstart
	ldy #<memstart
	;; set up screen ptr
	lda #$00
	sta <addr
	stx >addr
	sty aoff
	ldy ipass
	lda passbytes,Y
	sta pbyte
	lda passchars,Y
	sta eoscr

loop:
	ldx >addr
	ldy aoff
	;; break loop when end of DRAM is reached
	cpx #>memend
	bne noskip
	jmp passend

noskip:
	stx eoscr-2		; show current address as chars on screen
	sty eoscr-1
	;; store pass value at the current address
	lda pbyte
	sta (<addr),Y
	;; compare current address to $FF, "continue" if equal
	cmp (<addr),Y
	bne print
	jmp loopend

print:	
	;; memory content was not equal, print

	;; space
	lda #" "
	ldy #$00
	sta (<scroff),Y
	;; increment
	ldy <scroff
	ldx >scroff
	iny
	cpy #$00
	bne nowrap1
	inx
	cpx #$83
nowrap1:	
	cpx #$83
	bne nowrap1a
	cpy #$98
	bne nowrap1a
	ldx #$80
	ldy #$00
nowrap1a:	
	sty <scroff
	stx >scroff

	;; upper nibble of X
	lda >addr
	lsr
	lsr
	lsr
	lsr
	tay
	lda hexchars,Y
	ldy #$00
	sta (<scroff),Y
	;; increment
	ldy <scroff
	ldx >scroff
	iny
	cpy #$00
	bne nowrap2
	inx
nowrap2:	
	cpx #$83
	bne nowrap2a
	cpy #$98
	bne nowrap2a
	ldx #$80
	ldy #$00
nowrap2a:	
	sty <scroff
	stx >scroff

	;; lower nibble of X
	lda >addr
	and #$0f
	tay
	lda hexchars,Y
	ldy #$00
	sta (<scroff),Y
	;; increment
	ldy <scroff
	ldx >scroff
	iny
	cpy #$00
	bne nowrap3
	inx
nowrap3:	
	cpx #$83
	bne nowrap3a
	cpy #$98
	bne nowrap3a
	ldx #$80
	ldy #$00
nowrap3a:	
	sty <scroff
	stx >scroff
	
	;; upper nibble of Y
	lda aoff
	lsr
	lsr
	lsr
	lsr
	tay
	lda hexchars,Y
	ldy #$00
	sta (<scroff),Y
	;; increment
	ldy <scroff
	ldx >scroff
	iny
	cpy #$00
	bne nowrap4
	inx
nowrap4:	
	cpx #$83
	bne nowrap4a
	cpy #$98
	bne nowrap4a
	ldx #$80
	ldy #$00
nowrap4a:	
	sty <scroff
	stx >scroff

	;; lower nibble of Y
	lda aoff
	and #$0f
	tay
	lda hexchars,Y
	ldy #$00
	sta (<scroff),Y
	;; increment
	ldy <scroff
	ldx >scroff
	iny
	cpy #$00
	bne nowrap5
	inx
nowrap5:	
	cpx #$83
	bne nowrap5a
	cpy #$98
	bne nowrap5a
	ldx #$80
	ldy #$00
nowrap5a:	
	sty <scroff
	stx >scroff
	
	;; space
	lda #">"
	ldy #$00
	sta (<scroff),Y
	;; increment
	ldy <scroff
	ldx >scroff
	iny
	cpy #$00
	bne nowrap6
	inx
nowrap6:	
	cpx #$83
	bne nowrap6a
	cpy #$98
	bne nowrap6a
	ldx #$80
	ldy #$00
nowrap6a:	
	sty <scroff
	stx >scroff

	;; upper nibble of pass char
	lda ipass
	asl
	tay
	lda passchars,Y
	ldy #$00
	sta (<scroff),Y
	;; increment
	ldy <scroff
	ldx >scroff
	iny
	cpy #$00
	bne nowrap7
	inx
nowrap7:	
	cpx #$83
	bne nowrap7a
	cpy #$98
	bne nowrap7a
	ldx #$80
	ldy #$00
nowrap7a:	
	sty <scroff
	stx >scroff
	
	;; lower nibble of pass char
	lda ipass
	asl
	tay
	iny
	lda passchars,Y
	ldy #$00
	sta (<scroff),Y
	;; increment
	ldy <scroff
	ldx >scroff
	iny
	cpy #$00
	bne nowrap8
	inx
nowrap8:	
	cpx #$83
	bne nowrap8a
	cpy #$98
	bne nowrap8a
	ldx #$80
	ldy #$00
nowrap8a:	
	sty <scroff
	stx >scroff

	;; upper nibble of actual value
	ldy aoff
	lda (<addr),Y
	lsr
	lsr
	lsr
	lsr
	tay
	lda hexchars,Y
	ldy #$00
	sta (<scroff),Y
	;; increment
	ldy <scroff
	ldx >scroff
	iny
	cpy #$00
	bne nowrap9
	inx
nowrap9:	
	cpx #$83
	bne nowrap9a
	cpy #$98
	bne nowrap9a
	ldx #$80
	ldy #$00
nowrap9a:	
	sty <scroff
	stx >scroff

	;; lower nibble of actual value
	ldy aoff
	lda (<addr),Y
	and #$0f
	tay
	lda hexchars,Y
	ldy #$00
	sta (<scroff),Y
	;; increment
	ldy <scroff
	ldx >scroff
	iny
	cpy #$00
	bne nowrap10
	inx
nowrap10:	
	cpx #$83
	bne nowrap10a
	cpy #$98
	bne nowrap10a
	ldx #$80
	ldy #$00
nowrap10a:	
	sty <scroff
	stx >scroff

	;; print cursor
	lda #"#"
	ldy #$00
	sta (<scroff),Y

loopend:	
	ldx >addr
	ldy aoff
	;; increment memory address
	iny
	cpy #$00
	bne loopend2		; branch on overflow (iny sets Z, but not V)
	inx			; increment y when x overflowed
loopend2:
	stx >addr
	sty aoff
	jmp loop
	
passend:
	ldy ipass
	lda passbytes,Y
	cmp #$7F
	beq eot
	iny
	sty ipass
	jmp pass

	
hexchars:
	.asc "0123456789ABCDEF"
passbytes:
	.byt $FF, $00, $AA, $55, $01, $02, $04, $08, $10, $20, $40, $80, $FE, $FD, $FB, $F7, $EF, $DF, $BF, $7F
passchars:
	.asc "FF00AA550102040810204080FEFDFBF7EFDFBF7F"
infotext:
	.asc " = MOS6502 memtest 2022, s3lph ="

eot:
	;; done, loop forever
	jmp eot


	;; Fill with FF
	* = $fffa
	.dsb (*-eot-3), $ff
	;; 6502 vectors
	* = $fffa
	.byt <main, >main	; NMIV
	.byt <main, >main 	; RESV
	.byt <main, >main 	; IRQV