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Old 08 September 2020, 10:48   #1
JoeJoe
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div and mul of BCD or Hex?

Hello coder,

I am currently working on the display and calculation of scores. I have already read the thread "BCD Arithmetic - howto^". The simple adding and subtracting is no problem. But I have to divide the score by 50. Also the points can be increased by graduation 3*90 or 5*240 etc.
Now to my problem. For dividing and multiplying with BCD there are no 1:1 commands. Multiplying can be avoided by using LookUp tables and adding. But for x:50 there is no alternative.
Of course I can calculate everything in hex and convert the hex values to decimal for the display. But this eats a lot of cycles.
How can I solve the problem without overloading the computer.

Thanks in advance
JoeJoe

@Admins: please move to right section, sorry

Last edited by JoeJoe; 08 September 2020 at 11:44. Reason: wrong section
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Old 08 September 2020, 11:13   #2
meynaf
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You can convert the divide into a multiply.
First, add the number to itself to do *2.
Then, remove the two last digits to do /100.
You will have achieved /50.
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Old 08 September 2020, 11:18   #3
alkis
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/50 can be achieved by /100 (dropping last byte) and add the number to itself (so /100*2)
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Old 08 September 2020, 11:46   #4
JoeJoe
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thanks for your replies
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Old 08 September 2020, 13:11   #5
8bitbubsy
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Quote:
Originally Posted by meynaf View Post
Then, remove the two last digits to do /100.
How is this achieved in a fast way when working with binary?
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Old 08 September 2020, 13:18   #6
meynaf
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How is this achieved in a fast way when working with binary?
Who cares ? This thread is about BCD, not binary.
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Old 08 September 2020, 14:10   #7
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That's right, but we don't want to be like that

I do this with the reciprocal method. x/50 = x*(1/50)
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Old 08 September 2020, 16:07   #8
Thomas Richter
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Quote:
Originally Posted by JoeJoe View Post
I am currently working on the display and calculation of scores. I have already read the thread "BCD Arithmetic - howto^". The simple adding and subtracting is no problem. But I have to divide the score by 50. Also the points can be increased by graduation 3*90 or 5*240 etc.
Now to my problem. For dividing and multiplying with BCD there are no 1:1 commands. Multiplying can be avoided by using LookUp tables and adding. But for x:50 there is no alternative.

:50 is simple enough (already answered by others), but general multiplication or division in BCD is slow, in general. To multply, use the pen-and-paper algorithm of multiplying digit by digit, then add the partial products by a BCD add. Similarly, a BCD division follows the pen-and-paper algorithm of continuous subtraction until an overflow, and keeping the number of times a number was subtracted, then carry over the remainder, and continue with the next digit.


All these algorithms are slow...
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Old 08 September 2020, 16:09   #9
Thomas Richter
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How is this achieved in a fast way when working with binary?

If you have no division instruction, the same way as in BCD, except that each digit can be either 0 or 1, which simplifies continuous subtraction to a single compare. If you do, well, use the division instruction.
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