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Search google , you will find thousands of answers!!

 

you need to first understand "binary" and "binary coded decimal". when you do it is easy. read up on both.

 

i know that BCD is a sub group of binary numbers from 0-9

so i dont know how to convert 1111 to BCD code (its impossible)
or 0011 to BCD (it stays the same)

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I suppose you have to convert binary to decimal and then convert it to BCD. So 1111 would become 0001 0101.

 

BCD is just a method of using 4-bit values to represent a digit of a decimal number. The 4-digit codes, 1010,1011,1100,1101, and 1111 are invalid because they would translate to 10-15, which are not decimal numbers.

You can see that BCD is more inefficient that binary, but easier to directly read.

1111 is invalid as a BCD number directly, but as binary it is equal to decimal 15, which will require two BCD digits 0001 and 0101.

 

Number system conversion should have been covered in your classes. Her is a link for a tutorial - http://www.permadi.com/tutorial/numHexToBin/index.html.