Hex to BCD conversion (0-99) Part Trois
Entry posted by Omegamatrix
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I've covered some Hex to Decimal conversions like 0-255 and 0-65535 already in my blog, and have had done some previous solutions to 0-99 as well. Today I was thinking about converting a single byte from a hex value to BCD (0-99) once again. I came up with this routine which is the shortest, simplest, and fastest yet.
;Hex2Bcd (good 0-99)
;22 bytes, 26 cycles
tay ;2 @2
lsr ;2 @4
lsr ;2 @6
lsr ;2 @8
lsr ;2 @10
tax ;2 @12
tya ;2 @14
sed ;2 @16
clc ;2 @18
adc #0 ;2 @20
adc BcdTab,X ;4 @24
cld ;2 @26
BcdTab:
.byte $00,$06,$12,$18,$24,$30,$36
The code can be modified to preserve Y by using PHA/PLA, but that will cost you more cycles of course.
This routine is quite similar to the one I left in the comments of Hex to Decimal (0-99) part deux. I was able to eliminate the AND #$0F, and that made all the difference. In the code the ADC #0 corrects the low nibble to BCD if the low nibbles value is greater than nine. Looking at the BcdTab you can see that it increases in BCD values of six. This is used to correct the final value with the high nibble, and that is pretty much it. 
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