Juan,

You obviously have the idea, but some of your terminology is a bit loose.
As William obviously hasn't got the idea at all, we need to get these things clear:


BCD = Binary-Coded Decimal; therefore, by definition the digits can only ever go from 0-9.
In binary, that's 0000-1010 - so four bits are required for each BCD digit.

Thus an eight-bit byte can accomodate two BCD digits - this is sometimes known as packed BCD.


No, that's wrong.

First, we need to be clear that ASCII has nothing to do with numbers. ASCII is a numerical code for representing characters for use in printers, on displays, etc.
ASCII is not used for arithmetic!

In ASCII, the numerical value thirty-two represents a 'space' character; the numerical value sixty-five represents an uppercase 'A' character; the numerical value ninety-nine represents a lowercase 'c' character; the numerical value fifty represents a character '2'; etc, etc,...

We can write those numerical values in various formats, including binary, decimal, and hexadecimal - so the above paragraph can be re-stated as:

            Decimal        'C' Hexadecimal Binary         ASCII-Coded
Number      Representation Representation  Representation Character
----------- -------------- --------------  -------------- -----------
thirty-two       32             0x20         00100000       ' '
sixty-five       65             0x41         01000001       'A'
ninety-nine      99             0x63         01100011       'c'
fifty            50             0x32         00110010       '2'

Now, if you look at the ASCII codes for the digit characters '0' to '9', you should spot a simple relationsip between the numerical value of the ASCII code, and the digit that it represents:

          ASCII
Character Code
--------- -----
  '0'      0x30
  '1'      0x31
  '2'      0x32
  '3'      0x33
  '4'      0x34
  '5'      0x35
  '6'      0x36
  '7'      0x37
  '8'      0x38
  '9'      0x39

Thus, to convert a numerical value 0-9 into its corresponding ASCII-code value, you simply add (not "AND") 0x30.

For multiple digits, you need to apply this conversion to each digit.

For a review of the basic principles of radix notation (different number bases), take a look at the excellent MathForum article cited recently by David Mars:

http://www.8052.com/forum/read.phtml?id=94177

In fact, while you're there, it'd be well worth looking through the other MathForum articles on the subject...