I haven't seen this posted anywhere, so hopefully this information is useful to some people. The RCX ROM interrupt vectors (0x6..0x48) point to ISRs in ROM that then use another interrupt vector table in RAM (0xfd92..0xfdbe) to call the real ISRs. The RAM vector table is initialized to 0x46a which just contains a "rts" instruction (basically an unhandled interrupt). Later on some of the vectors are overwritten with "real" ISR addresses, all of which reside in ROM. It doesn't appear that the firmware touches this table, so my hunch is that any interrupts that don't have ISRs registered by the ROM are unused, unless there is some indirect way the firmware writes to this table.
I made a table of interrupts and handlers:
| Name | ROM Vector | RAM Vector | ROM Handler | Description |
|---|---|---|---|---|
| NMI | 0x0006 | 0xfd92 | None | Non Maskable Interrupt |
| IRQ0 | 0x0008 | 0xfd94 | 0x1ab8 | Interrupt 0 (button?) |
| IRQ1 | 0x000a | 0xfd96 | 0x294a | Interrupt 1 (button?) |
| IRQ2 | 0x000c | 0xfd98 | None | Interrupt 2 |
| ICIA | 0x0018 | 0xfd9a | None | 16 bit Timer - Input Capture A |
| ICIB | 0x001a | 0xfd9c | None | 16 bit Timer - Input Capture B |
| ICIC | 0x001c | 0xfd9e | None | 16 bit Timer - Input Capture C |
| ICID | 0x001e | 0xfda0 | None | 16 bit Timer - Input Capture D |
| OCIA | 0x0020 | 0xfda2 | 0x36ba | 16 bit Timer - Output Compare A |
| OCIB | 0x0022 | 0xfda4 | None | 16 bit Timer - Output Compare B |
| FOVI | 0x0024 | 0xfda6 | None | 16 bit Timer - Overflow |
| CMI0A | 0x0026 | 0xfda8 | None | 8 bit Timer 0 - Compare Match A |
| CMI0B | 0x0028 | 0xfdaa | None | 8 bit Timer 0 - Compare Match B |
| OVI0 | 0x002a | 0xfdac | None | 8 bit Timer 0 - Overflow |
| CMI1A | 0x002c | 0xfdae | None | 8 bit Timer 1 - Compare Match A |
| CMI1B | 0x002e | 0xfdb0 | None | 8 bit Timer 1 - Compare Match B |
| OVI1 | 0x0030 | 0xfdb2 | None | 8 bit Timer 1 - Overflow |
| ERI | 0x0036 | 0xfdb4 | 0x30a4 | Serial Receive Error |
| RXI | 0x0038 | 0xfdb6 | 0x2c10 | Serial Receive End |
| TXI | 0x003a | 0xfdb8 | 0x2a9c | Serial TDR Empty |
| TEI | 0x003c | 0xfdba | 0x2a84 | Serial TSR Empty |
| ADI | 0x0046 | 0xfdbc | 0x3b74 | A/D Conversion End |
| WOVF | 0x0048 | 0xfdbe | None | Watchdog Timer Overflow |