* Mark Hounschell [Wed, 05 Dec 2001 14:35:14 -0500]:
Then why does my dual 800Mz intel pIII box show 17, and my 2 dual 1.7Gz intel p4 zeons show 19? I recently read in some PCI spec book that there were 256 available and the REAL limitation was in the vendor bios's. In fact we are considering having a custom bios done for us for these dual 1.7 boxes and they say they can support 256 irq's with it. ???
Dual boards have a so called APIC (Advanced Programmable Interrupt Controller), these offer 256 IRQs at maximum. The kernel makes use of them for instance to reroute shared IRQs so that each device has its own. There's nothing wrong with devices sharing an IRQ, but it's easier for the kernel to identify the source of the IRQ. UP boards normally don't offer an APIC, so you're limited to the classic 15 IRQs. Now some UP boards do feature an APIC, that's why there's an option in the kernel configuration to enable it. Philipp -- Penguins to save the dinosaurs -- Handelsblatt on Linux for S/390