BCD issue

[easmith]

This is a dumb question I'm sure , but I am having trouble with a BCD calculation.

 

I am using the Collect Tutorial 2 digit score display , which uses BCD encoded values.

 

In the past , all of the numbers I have used have been obtainable by 

 

sed

clc 

  lda numberbcd

   adc #10   ( some fixed value) 

 

   sta numberbcd

 

 

But now I am trying to subtract two variables and store as BCD. No combination I have tried works.   ( having both BCD before arithmetic or converting after ) 

 

Any help would be appreciated !

 

 

ES

[RevEng]

44 minutes ago, easmith said:

  sed

  clc 

  lda numberbcd

   adc #10   ( some fixed value) 

 

   sta numberbcd

 

Not exactly your question, but I figured I'd point out that for BCD operations you use the hex representations of the decimal numbers. So "ADC #$10" instead of "ADC 10". The latter will mess up your BCD math for any operand greater than 9. (since $0=0, $1=1, ..., $9=9)

 

Subtraction with BCD math isn't any more complicated than addition (use SEC instead of CLC), so I'm wondering if your problem isn't with the issue I've mentioned above.

[Nukey Shay]

Yup...gotta be $hex values only.   Decimal mode then treats $00 - $99 as base 10 values 0 - 99.  You also cannot use any other method other than ADC and SBC when doing your math (unless you know for a fact that it won't result in a carry or borrow from either column).

[easmith]

What about subtracting RAM variables x and y ?

[Nukey Shay]

Variables, Rom data, registers, doesn't matter.  They need to contain numeric digits only when being used for BCD.  Those that hold values only up to 9 could be safe in certain cases when BCD is not invoked (when it's guaranteed that a carry or borrow isn't going to happen to/from the 1's column in the result).

[Nukey Shay]

If X or Y do contain $hex values, you'd need to convert them to decimal before BCD arithmetic.  It might be easier to use a conversion table in Rom for such a case.  It's wiser just not to use those specific variables for anything BUT decimal mode, so no conversion is necessary.