Reading a number using INT 21h (DOS) & 8086 assmebly

Question

I need to prompt to user a msg that tells him to write a number , then I store this number and do some operation on it After searching in INT 21h I found this :

Answer 1

Once you've got the string you have to convert it to number. The problem is, you have to code your own procedure to do that. This is the one I usually use (written in C though):

int strToNum(char *s) {
    int len = strlen(s), res = 0, mul = 0;
    char *ptr = s + len;

    while(ptr >= s)
        res += (*ptr-- - '0') * (int)pow(10.0, mul++);

    return res;
}

Here's the explanation. First of all, *ptr-- - '0' gets the integer representation of a number (so that '9' - '0' = 9, then it decremenst ptr so that it points to the previous char. Once we know that number, we have to raise it to a power of 10. For example, suppose the input is '357', what the code does is:

('7' - '0' = 7) * 10 ^ 0 =   7 +
('5' - '0' = 5) * 10 ^ 1 =  50 +
('3' - '0' = 3) * 10 ^ 2 = 300 = 
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                           357