PCI has four interrupt lines. The logic of leaving off the fourth PCI slot is that it would have to share with the ATi chip. Interrupt sharing was always part of the standard, but early implementations weren't great at it. Whether it will work at all depends on whether the RageII+ likes to share (not been able to confirm that either way); if it does, whatever you put in the fourth PCI slot also needs to play nice.

dionb wrote on 2025-04-29, 12:43:

PCI has four interrupt lines. The logic of leaving off the fourth PCI slot is that it would have to share with the ATi chip. Interrupt sharing was always part of the standard, but early implementations weren't great at it. Whether it will work at all depends on whether the RageII+ likes to share (not been able to confirm that either way); if it does, whatever you put in the fourth PCI slot also needs to play nice.

Thank you for that! I did a quick search too and see no mention anywhere of whether or not the ATI GPU will support interrupt sharing. Is this the same as sharing an IRQ? I know that, at least from what I have read on forums, you can't disable the ATI Rage in the bios settings and it just cantons as a secondary graphics adapter unless you disable it in device manager in Windows which I assume could still result in errors on the DOS side of things maybe. I guess it's worth a shot. It wouldn't hurt anything if I get issues and then just decide to leave the slot empty and not desolder it correct? Maybe I could desolder the ATI chip but that sounds drastic for just the ability to add a PCI LAN card haha.

9646gt wrote on 2025-04-29, 13:48:

dionb wrote on 2025-04-29, 12:43:

PCI has four interrupt lines. The logic of leaving off the fourth PCI slot is that it would have to share with the ATi chip. Interrupt sharing was always part of the standard, but early implementations weren't great at it. Whether it will work at all depends on whether the RageII+ likes to share (not been able to confirm that either way); if it does, whatever you put in the fourth PCI slot also needs to play nice.

Thank you for that! I did a quick search too and see no mention anywhere of whether or not the ATI GPU will support interrupt sharing. Is this the same as sharing an IRQ?

Not quite, IRQs are really ISA resources - but sharing a PCI interrupt will lead to the devices sharing an IRQ too.

I know that, at least from what I have read on forums, you can't disable the ATI Rage in the bios settings and it just cantons as a secondary graphics adapter unless you disable it in device manager in Windows which I assume could still result in errors on the DOS side of things maybe.

Not so much DOS as hardware side of things. Your OS can choose to ignore it, but the hardware is there, communicating on the PCI bus and potenially getting mixed up by signals from another device, or mixing its signals up.

I guess it's worth a shot. It wouldn't hurt anything if I get issues and then just decide to leave the slot empty and not desolder it correct? Maybe I could desolder the ATI chip but that sounds drastic for just the ability to add a PCI LAN card haha.

Desoldering the chip will certainly remove the problem - although you'd probably also have to find the VGA BIOS and desolder/remove that too. I don't see how disabling/removing the onboard VGA will win you a PCI slot, as that will require you to use a PCI VGA card.

If you don't break or short-circuit anything while soldering in the slot it won't hury anything to try adding a card though. Worst-case the system will just hang when you do but boot correctly once you remove it.

As for the NIC - why not just use ISA? If you're talking DOS/Win9x the added value of PCI is minimal to negative and it's going to be slow anyway.