.Page
;++
;
;		VfyCksum -- Verify checksum that was just read.
;
; VfyCksum will undo the mess that CreCksum created by xor'ing the
; checksum into the data bytes.  See teh CreCksum description for the
; alogrithm used to understand what is being undone here.
;
; REGISTERS
;  IN
;	All =	Any value
;  OUT
;	All =	Destroyed
;	ccC =	0 if checksum matches
;		1 if checksum does not match
;
; CALLS
;	NONE
;
;--
VfyCksum	.Equ *				; Entry point for all callers
	Lda	#00			; Init temporary checksums to zero
	Sta	TCksm1
	Sta	TCksm2
	Sta	InxPtrl
	Sta	InxPtrH
	Inc	InxPtrH			; Start at page 1
	Ldy	#0F4			; Only last 12 bytes

VfTop	.Equ *
	Asl	A			; Rotate the third checksum byte
	Php				; Save the status bits
	Adc	#00			; Put carry bit into low bit (8 bit rotate)
	Sta	TCksm3			; Save 3rd checksum byte
	Plp				; Restore status bits
	Lda	@InxPtrL,y		; Read 1st byte of 3 byte loop
	Eor	TCksm3			; restore original data
	Sta	@InxPtrL,y
	Adc	TCksm1
	Sta	TCksm1			; And update the checksum
	Iny
	Bne	$20
	Inc	InxPtrH			; Next page

$20	Lda	@InxPtrL,y		; 2nd of 3 byte loop
	Eor	TCksm1			; Restore originl data
	Sta	@InxPtrL,y
	Adc	TCksm2
	Sta	TCksm2			; And update the checksum
	Iny
	Beq	$60			; Last byte to sum

	Lda	@InxPtrL,y		; 3rd byte of 3 byte loop
	Eor	TCksm2			; Restore original data
	Sta	@InxPtrL,y
	Adc	TckSm3			; Update checksum and leave in A
	Iny
	Bne	VfTop			; Not at page boundary, loop
	Inc	InxPtrH			; Next page
	Bne	VfTop			; Branch always taken

$60	Cmp	CkSm2			; 'A' reg has TCksm2
	Bne	$80
	Lda	TCksm3
	Cmp	Cksum3
	Bne	$80
	Lda	TCksm1
	Cmp	Cksum1
	Bne	$80
	Clc
	Rts

$80	Sec
	Rts