hi,

The main processor of the system is a 80C32 microcontroller

Well, this chip has no internal program memory. After start, it fetches instructions from external program memory. Indeed, external program memory may be realized as ROM/EEPROM/FLASH/RAM etc.
I assume two posibilities:

1) Your "firmware ROM" is constantly mapped into program memory address space from 0x0000 and your external program RAM is downloaded somewhere above. If so then I may suggest you to look at manual of the firmware - what does it say about interrupts` vectors and how may user utilize them. Probably it defines fixed addresses in the program RAM for user Interrupt Service Routines. Last choise is just looking at ROM binary contenys to see what happens at address 0x23.

2) Your "firmware ROM" is bootloader shadow one. It support external program RAM download and then somehow (a switch, button etc) it disappears from the program memory space. Then MCU resets and starts to execute your program from 0x0000. In this case you have controled all interrupts yourself and I see no problem how to implement serial interrupt routine - just do it with standard way at address 0x23.

Regards,
Oleg