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BCD

Posted on 2002-06-13
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Last Modified: 2013-12-03
Does anyone know of a BCD (Binary coded decimal) library available for C?
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Question by:philsmicronet
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Expert Comment

by:jkr
ID: 7075616
Hmm, I cannot see the need for a lib, as BCD is pretty straight foward:

The Binary Coded Decimal, or BCD code, is used as the direct code to communicate decimal numbers using
a binary code. Recall that the decimal numbers are 0 to 9.  BCD code is used to directly translate these
decimal numbers using a 4-bit code.
Here are the decimal equivalent 4-bit BCD codes;
 

Decimal BCD
0 0000
1 0001
2 0010
3 0011
4 0100
5 0101
6 0110
7 0111
8 1000
9 1001

 
Notice that each single decimal digit has an equivalent 4-bit BCD code.

Converting Decimal to BCD

This conversion simply requires you to represent each decimal digit with its 4-bit BCD code.

Eg.  38210 = (310 = 0011BCD), (810 = 1000BCD) and (210 = 0010BCD)  so;
       the result is 001110000010BCD   and dropping the leading zeros = 1110000010BCD

Converting BCD to Decimal

This is just grouping every 4 bits of BDC code (starting at the LSD) and representing each 4-bit code
with its equivalent decimal number.

Eg.  100100101000BCD = (1000 = 8), (0010 = 2) and (1001 = 9)   so;

        the decimal equivalent is 92810
 


Nevertheless, check out this one: http://www.1cplusplusstreet.com/vb/scripts/ShowCode.asp?txtCodeId=625&lngWId=3 ("BCD math")
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Author Comment

by:philsmicronet
ID: 7076777
What about if you need to add, subtract, multiply etv etc
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Expert Comment

by:jkr
ID: 7076783
That is why I provided the link :o)
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Author Comment

by:philsmicronet
ID: 7076885
This allows you to convert from BCD to double and back, however, if you do this you are going to still have the same rounding problems that you would normally have with floating point arithmetic. I would have thought that generic BCD add, subtract etc etc would be required.
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Expert Comment

by:robert_marquardt
ID: 7077536
"BCD math library C source" on Goolge gives you scores of links.
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Expert Comment

by:fl0yd
ID: 7080533
@ robert_marquardt:
True, google gives you scores of links -- but 'quantity' is spelled differently from 'quality'.

@ philsmicronet:
Basically, any x86-processor supports BCD's. I didn't find a link with a BCD math library. However, this might be useful to you:
http://www.intel.com/design/intarch/techinfo/Pentium/instsum.htm
Search for "Packed BCD Adjustment Instructions"/"Unpacked BCD Adjustment Instructions" -- it deals with double/single-digit BCD's only, but can easily extended to multi-digit-BCD's. It is 16-bit code so don't expect any high-performance-operations. Anyway, here are some ASM-code-snippets:

Addition [using aaa = "ascii adjust after addition"]:
    mov ax,9        ;ax = 0x0009
    mov bx,3        ;bx = 0x0003
    add al,bl       ;ax = 0x000c = 12d
    aaa             ;ax = 0x0102

Multiply [using aam = "ascii adjust after multiply"]:
    mov al,5        ;al = 0x0005
    mov bl,7        ;bl = 0x0007
    mul bl          ;ax = al *  bl = 0x0023 = 35d
    aam             ;ax = 0x0305

Hope that provides at least some help to you. Regards,
.f
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Accepted Solution

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jimbucci earned 100 total points
ID: 7126696
Sounds like a Data Structures I project I did many years ago.  The code is in Pascal, but I was able to add and subtract BCD numbers.

1st of all - is this homework?

If not, and if I can find the code, would you be interested in it?

Jim
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Author Comment

by:philsmicronet
ID: 7126762
No, its just aprogram I am working on.
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Expert Comment

by:jkr
ID: 9715016
Wow, that was really a quick accept - so, now, would you be so kind and explain why you accepted an answer that reads " if I can find the code, would you be interested in it?" over the others that had good suggestions? As for my part, this is not really motivating to help you in the future.
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Expert Comment

by:jimbucci
ID: 9715086
I'm puzzled as well.  Must have been a mouse click error.  
Jim
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Author Comment

by:philsmicronet
ID: 9717577
Oooops. Was going through my old questions that I had not accepted (only two) and selected the wrong comment.
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