More Hexadecimal to Decimal Conversion by Andrew Jacobs
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Hexadecimal to Decimal Conversion

These are equivalent routines to Garth Wilson's Hexadecimal to Decimal routines, eliminating the lookup tables in exchange for a little slower execution speed. A discussion of hexadecimal-to-decimal routines can be found in this thread on the 6502.org Forum.

First is the 8-bit version. Start with the input number in accumulator:

; Convert an 8 bit binary value to BCD
;
; This function converts an 8 bit binary value into a 16 bit BCD. It
; works by transferring one bit a time from the source and adding it
; into a BCD value that is being doubled on each iteration. As all the
; arithmetic is being done in BCD the result is a binary to decimal
; conversion.  All conversions take 311 clock cycles.
;
; For example the conversion of a $96 would look like this:
;
; BIN = $96 -> BIN' = $2C C = 1 | BCD $0000 x2 + C -> BCD' $0001
; BIN = $2C -> BIN' = $58 C = 0 | BCD $0001 x2 + C -> BCD' $0002
; BIN = $58 -> BIN' = $B0 C = 0 | BCD $0002 x2 + C -> BCD' $0004
; BIN = $B0 -> BIN' = $60 C = 1 | BCD $0004 x2 + C -> BCD' $0009
; BIN = $60 -> BIN' = $C0 C = 0 | BCD $0009 x2 + C -> BCD' $0018
; BIN = $C0 -> BIN' = $80 C = 1 | BCD $0018 x2 + C -> BCD' $0037
; BIN = $80 -> BIN' = $00 C = 1 | BCD $0037 x2 + C -> BCD' $0075
; BIN = $00 -> BIN' = $00 C = 0 | BCD $0075 x2 + C -> BCD' $0150
;
; This technique is very similar to Garth Wilson's, but does away with
; the look up table for powers of two and much simpler than the approach
; used by Lance Leventhal in his books (e.g. subtracting out 1000s, 100s,
; 10s and 1s).
;
; Andrew Jacobs, 28-Feb-2004

		.ORG $0200

BINBCD8:	SED		; Switch to decimal mode
		LDA #0		; Ensure the result is clear
		STA BCD+0
		STA BCD+1
		LDX #8		; The number of source bits

CNVBIT:		ASL BIN		; Shift out one bit
		LDA BCD+0	; And add into result
		ADC BCD+0
		STA BCD+0
		LDA BCD+1	; propagating any carry
		ADC BCD+1
		STA BCD+1
		DEX		; And repeat for next bit
		BNE CNVBIT
		CLD		; Back to binary

		BRK		; All Done.

; A test value to be converted

		.ORG $0300

BIN		.DB  234
BCD		.DS  2

; Convert an 16 bit binary value to BCD
;
; This function converts a 16 bit binary value into a 24 bit BCD. It
; works by transferring one bit a time from the source and adding it
; into a BCD value that is being doubled on each iteration. As all the
; arithmetic is being done in BCD the result is a binary to decimal
; conversion. All conversions take 915 clock cycles.
;
; See BINBCD8 for more details of its operation.
;
; Andrew Jacobs, 28-Feb-2004

		.ORG $0200

BINBCD16:	SED		; Switch to decimal mode
		LDA #0		; Ensure the result is clear
		STA BCD+0
		STA BCD+1
		STA BCD+2
		LDX #16		; The number of source bits

CNVBIT:		ASL BIN+0	; Shift out one bit
		ROL BIN+1
		LDA BCD+0	; And add into result
		ADC BCD+0
		STA BCD+0
		LDA BCD+1	; propagating any carry
		ADC BCD+1
		STA BCD+1
		LDA BCD+2	; ... thru whole result
		ADC BCD+2
		STA BCD+2
		DEX		; And repeat for next bit
		BNE CNVBIT
		CLD		; Back to binary

		BRK		; All Done.

; A test value to be converted

		.ORG $0300

BIN		.DW  12345
BCD		.DS  3