x86 convert to lower case assembly

Question

This program is to convert a char pointer into lower case. I\'m using Visual Studio 2010.

This is from another question, but much simpler to read and mo

Answer 1

The problem is, that the OR operator like many others don't allow two memory or constant parameters. That means: The OR operator can only have following parameters:

OR register, memory
OR register, register
OR register, constant

The second problem is, that the OR has to store the result to a register, not to memory. Thats why you get an access violation, when the brackets are set. If you remove the brackets, the parameters are ok, but you don't write your lowercase letter to memory, what you intend to do. So use another register, to copy the letter to, and then use OR. For example:

mov eax, 0          ; zero out the result
mov edi, [token]      ; move the token to search for into EDI 
MOV ecx, 0
LOWERCASE_TOKEN:            ;lowercase the token
MOV ebx, [edi]        ;## Copy the value to another register ##
OR ebx, 20h         ;## and compare now the register and the memory ##    
MOV [edi], ebx      ;##Save back the result ##    
INC ecx    
CMP [edi+ecx],0    
JNZ LOWERCASE_TOKEN    
MOV ecx, 0

That should work^^

Answer 2

Your code can alter only the first char (or [edi], 20h) - the EDI does not increment.

EDIT: found this thread with workaround. Try using the 'dl' instead of al.

; move the token address to search for into EDI
; (not the *token, as would be with mov edi, [token])

mov edi, token      

LOWERCASE_TOKEN:            ;lowercase the token
  mov al, [edi]
  ; check for null-terminator here !
  cmp al, 0
  je GET_OUT
  or al, 20h
  mov dl, al
  mov [edi], dl
  inc edi
jmp LOWERCASE_TOKEN
GET_OUT:

Answer 3

I would load the data into a register, manipulate it there, then store the result back to memory.

int make_lower(char* token) {
    __asm {
        mov edi, token
        jmp short start_loop
    top_loop:   
        or al, 20h
        mov [edi], al
        inc edi
    start_loop:
        mov al, [edi]
        test al, al
        jnz top_loop
    }       
}

Note, however, that your conversion to upper-case is somewhat flawed. For example, if the input contains any control characters, it will change them to something else -- but they aren't upper case, and what it converts them to won't be lower case.