How do I convert a string representing a signed hex int into its signed int Doubleword number in 80x86?

Question

So I am taking an assembly language course and I am stuck on this problem from the book: Using the windows 32 console (so I have an io.h to use), I am supposed to t

Answer 1

If you need to convert a character (for simplicity, say, in upper case) representing a hex digit into the value of that digit you need to do this:

IF char >= 'A'
  value = char - 'A' + 10
ELSE
  value = char - '0'
ENDIF

If you need to do the reverse, you do the reverse:

IF value >= 10
  char = value - 10 + 'A'
ELSE
  char = value + '0'
ENDIF

Here you exploit the fact that the ASCII characters 0 through 9 have consecutive ASCII codes and so do the ASCII characters A through F.