xor al,0x20
See explanation
This instruction swaps the case of ASCII letters.
This is one of the oldest and well-known ASCII tricks: the ASCII symbols are arranged such as the only difference between a lower-case and an upper-case latin letter is the 6th bit: set to 1 for lower-case, set to 0 for upper-case.
A quick look at the 8-bit binary representation of a few ASCII letters:
a = 01100001
A = 01000001
b = 01100010
B = 01000010
Notice the difference? Just the 6th bit between lower and upper case.
So, the trick is to do a XOR
between al (the lowest order byte of rax) and 0x20, and here we go,
'a' becomes 'A' and 'A' becomes 'a'.
This instruction lets you go back and forth between upper and lower case, but
there's more. As you have probably already imagined you can implement tolower
by setting the 6th bit with a binary OR, and implement toupper by clearing
the 6th bit with an AND NOT. In C:
int
tolower(int c) {
return c | 0x20;
}
int
toupper(int c) {
return c & ~0x20;
}
Or, in assembly:
tolower:
or al,0x20
...
toupper:
and al,0xdf