Hi all,

Attached is 2 versions of code that convert 2 digit BCD to 8 bit binary...

I'm trying to understand the algorithm behind these codes...
I just need to understand oNe of them...
Can anyone explain in details the algorithm of one of them in a big paragraph??
Also I wana know how is the input represented in the code
Regards,

 

All it's doing is (10 X the left 4 bits)+(the right 4 bits). They have just been optimised compared to what someone might write as a first attempt.

 

You can do it with a very small amount of code as successive subtraction, like this;

// converts two 4bit BCD numbers to 2 decimal digits, orig BCD is
// in numb, and numb is destroyed during the conversion
digit1 = 0;
while(1)
  if(numb & 0xF0)  
  {
    digit1++;
    numb -= 16;
  }
  else break;
}
digit0 = numb;

 

thx The RB but i needed it in assembly language...

 

To understand it, you could work it by hand, following what the assembler does.

I used to do it in graph paper where every square was a bit from a register.

After some time I managed to write my own division routine which I still use today. (16F family at that time, now ported to the 18F family - PIC micros)

It is all here

http://cablemodem.fibertel.com.ar/atferrari/

under PICs

 

Oh, maybe your doubts are much more basic:

Do you know what "packed" does mean?

Binary coded decimal is well explained in Wikipedia. Once you understand it, going step by step through the code should be not difficult.

 

thx The RB but i needed it in assembly language...

That's why I typed it in the simplest crudest possible form, so it would be effortless to convert to ASM or BASIC etc.

If you are confused by the C code's peculiarities, here is a more "standard math" way of typing it;

// converts two 4bit BCD numbers to 2 decimal digits, orig BCD is
// in numb, and numb is destroyed during the conversion
digit1 = 0;
while(numb >= 16)   // loop WHILE numb is greater or equal to 16
{
    digit1 = (digit1 + 1);    // add 1 to digit1
    numb = (numb - 16);   // subtract 16 from numb
}
digit0 = numb;

 

What you posted is assembler already!