How to convert char to digit in Assembly Irvine?

Question

I want to convert user's input character to digit(decimal) in Assembly MASM and Irvine library.

`CALL ReadChar ; store user input in AL register -> c ; user input character C

some code here to convert AL data to decimal 12

`

Answer 1

read the char
subtract a '0' from it
if after that, it's larger than 9,
and it with 0xBF (to lowercase 'A'-'F' to 'a'-'f')
and subtract 7 again.
et voilá, there's your value