====== Hex String to Integer ======


This is a very simple routine that converts a string containing a hexadecimal number to the corresponding integer. The string can be any length (up to 128 chars) and the routine will find its end (null-terminated) and start working from the last digit backwards, as the 6502 is backwards (little-endian). The "bmi l4" instruction makes sure the routine exits safely, if a zero-length string is passed in, and everything else should be obvious.

<code>
; hexadecimal string to integer conversion routine by FMan/Tropyx

	!to "strtoint.prg",cbm	; use ACME to assemble this

; very straight-forward, does no error checking on the input values,
; converts a string containing a hexadecimal number of any length

	tmp = $63		; temporary variable
	str = $64		; pointer to the null-terminated string
	out = $1f00		; address of the output buffer

	*=$2000

	ldy #0			; strlen
l0	lda (str),y
	beq l1
	iny
	bpl l0
l1	dey			; set Y to point to the last character of
	bmi l4			; the string (least significant digit)
	ldx #0
l2	lda (str),y		; read digit for low nybble of this byte
	jsr hex
	dey
	bmi l3
	sta tmp			; store it temporarily
	lda (str),y
	jsr hex			; convert the high nybble of this byte and
	asl			; slide it to occupy the corresponding bits
	asl
	asl
	asl
	ora tmp			; add the low nybble stored earlier
l3	sta out,x		; write the resulting byte out
	inx
	dey
	bpl l2			; loop until there is no more input
l4	rts

hex	sec			; subroutine that converts a single hex digit
	sbc #$30		; in Akku to the corresponding numeric value
	cmp #$a
	bcc h0
	sec
	sbc #7
h0	rts
</code>