x86 Assembly: wcsncpy Procedure

Use MASM for Visual C++ Express Edition 2005 to compile this procedure.

The wcsncpy_asm function copies the initial count characters of source to destination and returns a string to the end of destination. If count is less than or equal to the length of source, a null character is not appended automatically to the copied string. If count is greater than the length of source, destination is padded with null characters up to length count.

The return value is the end of the text in destination.

TITLE 'extern "C" wchar_t *wcsncpy_asm(wchar_t *destination, const wchar_t *source, unsigned count);'

.386P

.model FLAT

PUBLIC	_wcsncpy_asm

_TEXT	SEGMENT
_wcsncpy_asm PROC NEAR

	push  esi

	mov  esi, DWORD PTR [esp+16] ; count
	test esi, esi
	je   SHORT label4
	mov  ecx, DWORD PTR [esp+12] ; source
	mov  edx, DWORD PTR [esp+8] ; destination

label1:

	mov  ax, WORD PTR [ecx]
	mov  WORD PTR [edx], ax
	add  edx, 2
	add  ecx, 2
	test ax, ax
	je   SHORT label3 ; count exceeded the length of source so copy zeros into destination from this point foward

	dec  esi

	jne  SHORT label1

label2:

	lea  eax, DWORD PTR [edx-2] ; return pointer to the end of the text in destination
	pop  esi

	ret  0

label3:

	test esi, esi
	je   SHORT label2

	dec  esi
	je   SHORT label2
	push edi
	xor  eax, eax
	mov  ecx, esi
	shr  ecx, 1 ; divide by two
	mov  edi, edx
	rep  stosd ; DWORD, store contents of EAX in memory at the offset pointed to by EDI, increment EDI; repeat while ECX > 0
	adc  ecx, ecx
	rep  stosw ; WORD, store contents of AX in memory at the offset pointed to by EDI, increment EDI; repeat while ECX > 0
	pop  edi
	lea  eax, DWORD PTR [edx-2] ; return pointer to the end of the text in destination
	pop  esi

	ret  0

label4:

	lea  eax, DWORD PTR [edx-2] ; return pointer to the end of the text in destination
	pop  esi

	ret  0

_wcsncpy_asm ENDP
_TEXT	ENDS
END

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