The interrupt vectors are fixed at 8 byte intervals beginning at 0003h (IE0). There are some who will argue that location 0000h is also an interrupt vector (Reset interrupt). Although the code space between vectors is only 8 bytes, by simply placing an AJMP to your ISR elsewhere like you have done allows you to have an ISR as long as you like. However, it is very important to keep the ISR as brief as possible so that it will be ready to handle the next interrupt. It is also good practice to place a RETI at all the unused vectors so that if an unexpected interrupt occurs from another source your program won't run off into the enchanted forest.

Hal