On Mon, Aug 01, 2005 at 03:04:52PM -0300, luis wrote:
> Hi all,
> 
> I?m looking a rotine in asm to INC ou DEC 2 packed BCD numbers.
> For example: 00000100 10011001 (499) + 1 = 00000101 00000000 (500)

I have conversion routines in my sunrise/sunset outdorr light controller.
You can find it here:

http://www.finitesite.com/d3jsys/clock.asm

The hex2bcd routine converts a hex number to its BCD equivalent. However,
the principles are the same:

1) Each digit needs to be checked to see if it's greater than 9. Digits
greater than 9 need to be compensated by adding/substracting 6.

So for your example (switching to hex for simplicity):

0499
+  1 Increment
---------------
049A Check for overflow in LSD. It's greater than 9 so adjust
+  6
---------------
04A0 Last digit adjusted. Work on next digit. Compensate
  60 
---------------
0400 On the upper nybble you need to check carry. It carried so
 100 You need to increment the low nybble of the high byte
-----------------
0500 You check the low nybble of the high byte to see if it overflowed.

It didn't so you are done.

Now the decrement is interesting. You have to subtract 6 to compensate
and take one away from the succeeding digit. Let's try decrementing
1000.

1000
-  1 Take away one.
-------------------
10FF Compensate for borrow in MSB
-100
-----
0FFF Now start compensating for digits. 
-  6
-----
0FF9   
-060
-----
0F99
-600
-----
0999 No need to do the MSD.

That should be enough to get you going. 

BAJ
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