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Last active Dec 15, 2015
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_f proc near
input = dword ptr 8
push ebp ;Stack winding , remember where we came from.
mov ebp, esp ;Stack winding , update the current stack locations.
movzx eax, byte ptr [ebp+input] ; Move the byte value @ the address [ebp + input] and extend it if needed into the eax register
; eax now contains the the first 8 bytes of the stack for the current function which is your first parameter passed to the function
lea edx, [eax-61h] ; This subtracts 61h from eax (the parameter passed in ) and sets edx to the result.
; Assuming we passed in the letter 'a' , this would result in 61h - 61h == 0 in edx
cmp dl, 19h ; Compare the lower register of edx to 19h , if the value is less than 19h then we
; we know we are dealing with lower case letters. Otherwise this is an upper case letter or some other char which means we can ignore it.
ja short loc_80483F2 ; If the dl is greater than 19h then jump to loc_80483F2 and exit the function as nothing needs to be done.
sub eax, 20h ; Subtract 20h from eax , this convert a lower case char to upper case. This is a very effecient way to change the
; the case because of the following , 'a' == 61h and 'A' == 41h , so by subtracting 20h from any letter char , we will be left
; with the upper case value.
pop ebp ;Stack unwinding
retn ; Pop the instruction pointer and jump to it , this exits the function and returns to the caller.
_f endp
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