Question

I want to check if ‘ep’ is a upper case letter.

So I check it in ascii table, and I found that it is from 0x41 to <0x5A.

I tried to do an if function, that ask if it is a upper case letter. Can some one please tell me how the check this condition in Assembly? I tried to write this, but it of course not correct :

    cmp ep, (>0X40 & <0X5a)
        je letter

Answer 1

You can do this using two checks. Something like this:

cmp ep, 0x40
jl not_capital
cmp ep, 0x54
jg not_letter

# if we arrive here, it's a capital letter

not_letter:
# continue execution flow

Alternatively, you can subtract the lower bound and check if the value is less than the difference, i.e.:

# Note: 0x54 - 0x40 = 0x14
mov ep, <reg>     # replace <reg> with the register of your choice
sub 0x40, <reg>
cmp <reg>, 0x15   # if it's 0-0x14, we're ok. 0x15 or more is bad
jb letter         # we want unsigned check here (jb not jl)

Disclaimer: I don’t know your assembly format (I tried to copy it, though). But don’t trust my syntax to be exact.