reactos/lib/sdk/crt/string/i386/tcslen.inc

   1
   2 #include "tchar.h"
   3 #include <asm.inc>
   4
   5 PUBLIC _tcslen
   6 .code
   7
   8 FUNC _tcslen
   9     FPO 0, 1, 1, 1, 0, FRAME_FPO
  10
  11     /* Save edi and eflags (according to the x86 ABI, we don't need to do that
  12        but since the native function doesn't change the direction flag, we don't
  13        either */
  14     push edi
  15     pushfd
  16
  17     /* Load the string pointer into edi */
  18     mov edi, [esp + 12]
  19
  20     /* Set eax to 0, since we want to compare with 0 */
  21     xor eax, eax
  22
  23     /* Set ecx to -1 (i.e. 0xFFFFFFFF) */
  24     mov ecx, -1
  25
  26     /* Clear direction flag */
  27     cld
  28
  29     /* Now compare the characters until a 0 is found */
  30     repne _tscas
  31
  32     /* Calculate the count. For n characters, we do (n + 1) comparisons. Initial
  33        value of ecx was -1, so end value of ecx is (-1 - (n + 1)) = -(n + 2).
  34        => n = -ecx - 2 = ~ecx - 1 */
  35     not ecx
  36     lea eax, [ecx - 1]
  37
  38     /* Restore eflags/edi and return the result */
  39     popfd
  40     pop edi
  41     ret
  42 ENDFUNC
  43
  44 END
  45 /* EOF */