Something

include 'win32ax.inc'

.data
	string1	db	'test',0
	string2	rb	256

.code

rvsString:
	invoke	lstrlen, string1
	mov	ecx, eax
	xor	edx, edx

rvsLoop:
	dec	ecx
	mov	bl, byte [string1+ecx]
	mov	byte [string2+edx], bl
	inc	edx
	cmp	ecx, 0
	jne	rvsLoop
	mov	byte [string2+edx], 0

rvsEnd:
	invoke	MessageBox, 0, string2, "rvsString Test", 0
	invoke	ExitProcess, 0

.end rvsString