BCD arithmetic algorithms needed

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2 total messages Started by bretz Sat, 09 Dec 1995 20:14

Author: bretz
Date: Sat, 09 Dec 1995 20:14

23 lines
1109 bytes

Hello,

I am planing to implement a c++ class for bcd arithmetic
and for accurate representation of monetary and the like.

Currently I have an idea for adding packed or unpacked
bcds by adding each digit and saving the decimal carry.
I plan to use the Intel 80x86 bcd instructions (AAD, AAM,
AAS, AAD, DAA and DAS) to implement the arithmetic.

but I have no good idea for subtracting, multiplying and
deviding packed or unpacked bcds.

Can somebody help???

Thank you

Thomas
_____________________________________________________________________
bretz@ibm.net
Thomas Bretz-Rieck
Hamburg, Germany
_____________________________________________________________________

Re: BCD arithmetic algorithms needed

Author: Terje Mathisen
Date: Sat, 09 Dec 1995 22:36

34 lines
1891 bytes

bretz@ibm.net wrote:
> 
> Hello,
> 
> I am planing to implement a c++ class for bcd arithmetic
> and for accurate representation of monetary and the like.
> 
> Currently I have an idea for adding packed or unpacked
> bcds by adding each digit and saving the decimal carry.
> I plan to use the Intel 80x86 bcd instructions (AAD, AAM,
> AAS, AAD, DAA and DAS) to implement the arithmetic.
> 
> but I have no good idea for subtracting, multiplying and
> deviding packed or unpacked bcds.

Unless you're going to do _very_ little arithmetic on your numbers, I 
suggest you use a more efficient way to store your numbers, using a 
higher power of ten as your base.

If you store 9 digits in each 32bit unsigned long, addition and 
subtraction is easy, and you don't have to do nearly as much work to 
implement multiplication either.

Division is a bit harder, but not too bad, you just use the same 
approach you learned in grade school: Use the first few digits of 
divisor and dividend to guess the next digit, multiply back and compare, 
adjusting your guess if needed.

Since each digit is going to be on the order of 1G, this works out very 
well.

-- 
-Terje Mathisen (include std disclaimer) <Terje.Mathisen@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

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