Hello Micheal
The program is as follows:
ORG 0000H
SJMP MAIN

ORG 000BH
CPL P1.0
MOV TH0,#0FFH
MOV TL0,#0E6H
RETI

ORG 0030H
MAIN:
MOV SP,#50H
MOV TMOD,#01H
MOV IE,#82H
MOV TH0,#0FFH
MOV TL0,#0E6H
SETB TR0

LOOP: SJMP LOOP

maximum overhead can be 1 -2 microsecond
I am getting overhead of 4 microseconds

Pravin