Calling assembly from c

Hello, I have a problem with calling assembly from c.

typedef unsigned char digit; // apn is a pointer to array of digits.

Functions:

add(apn number1,apn number2,apn result)

subtract(apn number1,apn number2,apn result)

multiply(apn number1,apn number2,apn result)

I must call these functions from C and then these funct. must use assembly to add,sub,mul operations. That means i need assembly code for this functions. Numbers are in a char array. Example;

number1="123"//number1 and number2 are digit array.

number2="999"

then when i call add(number1,number2,result) from c

result will be "1122"
May you help me to write assembly code for these functions?(Please remember that numbers are char array) If you give me some links to learn from or explain something about this topic, you will be helped me to write :)