Interrupt without LJMP

Timer0 is used for freq. generation at port 3.5 (Portf) of my AT89C4051 @ 24MHz.
Each time the timer overflows the port is toggled.

1st case in asm:
ORG 00BH
tim0_int cpl Portf
reti
2nd case in C51:
void timer0_int (void) interrupt 1 {
Portf = ~Portf;
}
The problem in the second case is that a LJMP is added to the service routine and this consumes extra time.
How do I implement this in C without the LJMP.

Thanks.
Han