2017-10-04 18:37:32 +00:00
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#include "tchar.h"
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#include <asm.inc>
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PUBLIC _tcslen
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.code
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FUNC _tcslen
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FPO 0, 1, 1, 1, 0, FRAME_FPO
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/* Save edi and eflags (according to the x86 ABI, we don't need to do that
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but since the native function doesn't change the direction flag, we don't
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either */
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push edi
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pushfd
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/* Load the string pointer into edi */
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mov edi, [esp + 12]
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/* Set eax to 0, since we want to compare with 0 */
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xor eax, eax
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/* Set ecx to -1 (i.e. 0xFFFFFFFF) */
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mov ecx, -1
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/* Clear direction flag */
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cld
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/* Now compare the characters until a 0 is found */
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repne _tscas
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/* Calculate the count. For n characters, we do (n + 1) comparisons. Initial
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value of ecx was -1, so end value of ecx is (-1 - (n + 1)) = -(n + 2).
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=> n = -ecx - 2 = ~ecx - 1 */
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not ecx
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lea eax, [ecx - 1]
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/* Restore eflags/edi and return the result */
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popfd
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pop edi
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ret
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ENDFUNC
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END
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/* EOF */
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