6502 Shorts - BCD to binary

6502 Shorts - BCD to binary. By Lee Davison
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Introduction.

I'd not thought about this until recently when I needed it. Here's what I came up with....

The code.

I don't think this will win any prizes but it works (which is always a bonus). The byte temp can be anywhere but is probably best in page 0.
	TAX				; copy BCD value
	AND	#$F0			; mask top nibble
	LSR				; /2 (/16*8)
	STA	temp			; save it
	LSR				; /4 (/16*4)
	LSR				; /8 (/16*2)
	ADC	temp			; add /2 (carry always clear)
					; ((n/16*8)+(n/16*2) = (n/16*10))
	STA	temp			; save it
	TXA				; get original back
	AND	#$0F			; mask low nibble
	ADC	temp			; add shifted (carry always clear)
This is seventeen bytes long (assuming temp is in page 0) which makes it just about the right size for this section.
There is no error checking and it is up to the calling routine to ensure that the source byte is a valid BCD byte. If it isn't nothing bad happens, you just get the sum of the low nibble and the high nibble times ten.

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Last page update: 27th January, 2003.