I noticed I can program it without reseting uC. I guess while it is performing my code, it waits for "UUUUUUUU"'s and enters isp mode.

here is some data which i sniffed from serial communication:

my program in keil c:

#include <p89v51rx2.h>

main()
{
 while (1)
     {
      P0=0;P1=0;P2=0;
     };
}

comm. when programming chip:

UUUUUUUUUUUUUUUUUUUUUUUUUUUUUU:020000050000F9:020000050001F8:020000050700F2:020000050700F2:00000007F9:10000000020003787FE4F6D8FD758107020000FF47:00000001FF:05000004000000FF00F8

I noticed there is no "reset and go" command (copied from datasheet of uC):

0B Reset and run user code
:xxxxxx0Bcc
Where:
xxxxxx = required field but value is a ‘don’t care’
0B = Reset and run user code
cc = checksum
Example:
:0000000BF5