How do I write an ISR greater than 8 bytes which is triggered by INT0?
Your code jumps to the ISR which ends in a RETI:
org 0003H jmp isr ; ... isr: ; any instruction you need reti
If you program in C, this will be handled for you automatically. For example, Keil-C51 places ISRs right at the start address if there is enough room, or jumps to it if not.