Well I did a small time measuring routine that I used in my 8051 project before Keil PK51 professional developers package with a very powerful simulator.
The routine was written in C and used timer to get the time. You need an output device to display the result - can be your serial port that I have used.
The routine is calibrated so that when running:

start_timer()

stop_timer()

You get the result 0 cycles.
You can easily modify the routine or write it in assembly language. Routine start_timer () sets timer T0 in 16 bit mode and starts it without effecting the operating mode of timer T1.

Routine stop_timer () stops counting and displays the result.

To use it is very simple:

start_timer();

your_routine_to_measure_execution_time ();

stop_timer ();



void start_timer();
void stop_timer();

void start_timer()
{
TMOD = (TMOD & 0xf0) | 0x01;
TH0 = 0;
TL0 = 0;
TF0 = 0;
TR0 = 1;
}

void stop_timer()
{
TR0 = 0;
if (TF0 == 1)
printf("nTimer T0 overflow.");
else
printf("nTimer T0 = %u cycles.",TH0 * 256 + TL0 - 7);
}