example:
                        ...
                        mov edi,example_test
                        mov esi,edi
                        mov edx,esi ; ESI == EDI == EDX == example_test
                        call strcmp
                        cmp dword ecx,00 ; if ECX is not '0' it means non-of the strings are equal
                        jnz str_err ; then throw an error if ECX => '1'
                        ...
    strlen:
                        mov eax,edx ; copy example_test to EAX
                        @@:
                        add eax,01 ; INC EAX
                        cmp byte [eax],00 ; then compare if NULL terminator found
                        jnz @B ; if not found then loop
                        sub eax,edx ; otherwise subtract to get the length
                        ret
    strcmp:
                        call strlen
                        sub eax,01 ; DEC EAX to remove NULL terminator from count, we dont want to compare '0's
                        mov ecx,eax ; mov length to ECX , used for loop
                        repz cmpsb ; repeat while they are equal and while ECX is not '0'
                        ret
    str_err:
                        ; some code to throw error

example_test db 'fasm',0 ; since both ESI and EDI have the same address then both strings are equal and ecx will be '0'    

So, you say your strcmp has three parameters (passed in registers)? Which should be passed in EDX?

Will the convention change if you decide to change the implementation by removing the useless call to strlen from the procedure?

Why would you even need strlen as part of strcmp? Just xor ecx, ecx / dec ecx.

Comparing ECX with zero is not the way to check for equality. ZF is. ECX might be zero after REP-loop no matter which was the result of the last comparison.

P.S. If you decide to use string instructions for your task, feel free to use scasb instead of branching in strlen impementation.

DimonSoft wrote:

So, you say your strcmp has three parameters (passed in registers)? Which should be passed in EDX?

it doesnt have to be like that, this is how i wrote it in my program.
because i keep my string addresses in ESI and EDI, if i want to get the length i would pass edx the address and call strlen. (or pass edx the address i want to compare to, and call strcmp)

DimonSoft wrote:

Will the convention change if you decide to change the implementation by removing the useless call to strlen from the procedure?

i can write different strcmp algorithm, which takes at least couple lines more.

DimonSoft wrote:

Why would you even need strlen as part of strcmp? Just xor ecx, ecx / dec ecx.

as said above, i can change it.
"xor ecx,ecx"? seriously? how would REPZ repeat then?
"dec ecx"? then i have to get the length. (unless i want to compare 3-5 byte or more/less not the whole string)

DimonSoft wrote:

Comparing ECX with zero is not the way to check for equality. ZF is. ECX might be zero after REP-loop no matter which was the result of the last comparison.

ecx will never be zero, UNTIL the last char is compared. (in case it was not '0' then string X isnt = to string Y, and ECX = NUM OF CHARS NOT COMPARED)
** please note ECX = NUM OF CHARS NOT COMPARED it depends on the length of what you are comparing with, a constant-string or from input.
* yes i know i can use jz jnz but what remains in ecx do matter.

abcd, 0
abc, 0    

DimonSoft wrote:

P.S. If you decide to use string instructions for your task, feel free to use scasb instead of branching in strlen impementation.

i would use scasb for other purposes. but thank you anyway.

(O(N) instead of O(N * L / 2), where N is the number of commands, L is an average length of the command).

proc CmpStr uses ecx esi edi,len,str1,str2

  cld
  xor eax, eax
  mov ecx,[len]
  mov esi,[str1]
  mov edi,[str2]
  repe cmpsb
  sete al
  ret

endp
    

Multi_ConcatenationA:
        pop rdx
        mov r8,rsi
        mov r9,rdi
        pop rdi
        pop rsi

@@:     lodsb           ;       lodsw
        stosb           ;       stosw
        test al,al      ;       test ax,ax
        jnz @B

        pop rsi
        dec rdi
        test rsi,rsi
        jnz @B

        mov rsi,r8
        mov rdi,r9
        jmp rdx