APIC? PIC?

Fred, I upgraded the bios in the wife’s machine. Lo and behold, there is a new parameter APIC OR PIC. I’ll save the horror I encountered by seeing the NIC occupying IRQ19 (yup, 19, not a typo) and the sound card on IRQ 18.

I didn’t start building computers yesterday, nor even last month, but I never saw APIC, OR PIC and I have only known Interrupt Vectors to range from 0 to 15.

Since I don’t smoke anything illegal, I am wondering if Bill Gates Crew has done something to windows2000 that wasn’t clearly covered when I did my research on migrating from an MSDN version of WIN98SE…. any Ideas? Best wishes, Bruce Apple

It’s not Microsoft; it’s actually a hardware thing, started by IBM and Intel.

Ready for some alphabet soup? PIC is "Programmable Interrupt Controller;" APIC is "Advanced Programmable Interrupt Controller."

The core idea behind both is simply to provide a way around an ancient limitation of the classic "AT" style PC architecture, which IBM developed in 1983 or so. Then, PCs were small and had few built-in or add-on devices. The AT-class design allowed for 15 unique "interrupt request" (IRQ) lines, by which devices could call for the attention of ("interrupt") the central processor. Then, 15 IRQs seemed like plenty.

But it wasn’t nearly enough, and after just a few years, the manual juggling of IRQs became a necessary black art among users who wanted to expand their PCs. It was ugly.

This problem was partially alleviated with the addition of PCI-based ("Peripheral Component Interconnect"), Plug-And-Play PCs, which could automatically juggle IRQ assignments and— better still— even share certain single IRQs among several different components. The PIC is part of the hardware that enables IRQ reassignment and sharing on these PCs.

The APIC supercedes the PIC. Some APICs provide support for reasonably exotic applications such as multiprocessing (more than one CPU in a system); but virtually all current PCs have some kind of APIC chipset built-in. Newer OSes, including Win2000 and XP, explicitly support various APIC capabilities.

For the most part, it’s an FYI kind of thing— good to know about, but not something you have to fiddle with. That is, unless there’s a problem. For example, there’s a known issue involving XP and some APIC chipsets, including the Intel 815x and the VIA 686B. If you have the problem, you’ll probably know about it, because— among other things— your video will be choppy and horrible. See, for example, http://support.microsoft.com/default.aspx?scid=kb;en-us;Q310718 

But for the most part, PIC and APIC are terms you can just tuck away in the back of your mind— you’ll probably never have to deal with them directly.

More info:
IRQ: http://www.techweb.com/encyclopedia/defineterm?term=irq
PCI: http://www.techweb.com/encyclopedia/defineterm?term=pci
PIC: http://www.techweb.com/encyclopedia/defineterm?term=pic
APIC: http://www.techweb.com/encyclopedia/defineterm?term=apic
Win2K and APICs: http://www.microsoft.com/hwdev/platform/proc/IO-APIC.asp



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Fred Langa

About Fred Langa

Fred Langa is senior editor. His LangaList Newsletter merged with Windows Secrets on Nov. 16, 2006. Prior to that, Fred was editor of Byte Magazine (1987 to 1991) and editorial director of CMP Media (1991 to 1996), overseeing Windows Magazine and others.