Assembly Journey

/*
|1| Implementation about memcpy 
|2| Implementation about strlen
|3| Implementation about memset 
*/
const char* source = "assembly";
char* destination;

void Implement_memcpy()
{
	
	__asm
	{
		// loop count is 10 
		// movsd is 4 count
		// therefore, the memory has 40bytes.
		push 0xA
		pop ecx
		mov esi, offset source
		mov edi, offset destination
		rep movsd
	}
}

void Implement_strlen()
{
	__asm
	{
		mov ecx, 0xFFFFFFFF
		xor al, al
		mov edi, source
		mov ebx, edi
		repne scasb; repeat will not Equal
		; the scas is must be compare EDI to EAX
		; if edi is not \x00, edi count will increment
		sub edi, ebx
		dec edi; substract '\x00' (null)
	}
}

void Implement_memset()
{
	__asm {
		xor eax, eax
		push 9
		pop ecx; count
		mov edi, esi
		rep stosd
		; the stos is EAX value will store in EDI
		; (d)is dword 4byte
		; Therefore, above instruction is initialize 9X4(36) bytes
	}
}