X86 NASM Assembly converting lower to upper and upper to lowercase characters

Question

As i am pretty new to assembly, i have a few questions in regards to how i should convert from a lowercase to an uppercase if the user enters an uppercase letter or vice ver

Answer 1

If you only support ASCII, then you can force lowercase using an OR 0x20

  or   eax, 0x20

Similarly, you can transform a letter to uppercase by clearing that bit:

  and  eax, 0xBF   ; or use ~0x20

And as nneonneo mentioned, the character case can be swapped using the XOR instruction:

  xor  eax, 0x20

That only works if eax is between 'a' and 'z' or 'A' and 'Z', so you'd have to compare and make sure you are in the range:

  cmp  eax, 'a'
  jl   .not-lower
  cmp  eax, 'z'
  jg   .not-lower
  or   eax, 0x20
.not-lower:

I used nasm syntax. You may want to make sure the jl and jg are correct too...

If you need to transform any international character, then that's a lot more complicated unless you can call a libc tolower() or toupper() function that accept Unicode characters.

---

As a fair question: why would it work? (asked by kuhaku)

ASCII characters (also ISO-8859-1) have the basic uppercase characters defined between 0x41 and 0x5A and the lowercase characters between 0x61 and 0x7A.

To force 4 into 6 and 5 into 7, you force bit 5 (0x20) to be set.

To go to uppercase, you do the opposite, you remove bit 5 so it becomes zero.