On Wed, 23 Feb 2000, Theodore Y. Ts'o wrote:
> It's not that the APIC is disabled on most UP motherboards; it's not
> there at all. APIC's are expensive, so on motherboards where it's not
> necessary (since all OS's have to have 8259 PIC support anyway) most UP
> chipsets only provide a PIC which 8259 compatible. It's not that you
> *can't* put an APIC on a UP machine, it's just that in practice most MB
> manufacturer's don't, for cost reasons.
Hmm, that's partially true, actually. There is no I/O APIC on UP boards
but since P54C, all CPUs contain an onchip local APIC. There should be no
problem with using the local APIC configured for the virtual wire mode
even if there is no other APIC in the system. Even wiring is exactly the
same. But the local APIC can be disabled on RESET in Pentium chips and by
software means on P6 CPUs. Once disabled it cannot be enabled till the
next RESET.
AFAIK, there is no problem with the inter-APIC bus dangling (which would
be happening in this case) as long as one does not try to send IPIs.
So all these systems have a local APIC with it's timer and some other
nice features but the manufacturer chose the easy way and disabled the
unit upon startup. The "difficult" way would require BIOS switching into
the protected mode and writing a few 32-bit words into the APIC to get
setup right -- what's the deal?... But who said manufacturers are clever?
-- + Maciej W. Rozycki, Technical University of Gdansk, Poland + +--------------------------------------------------------------+ + e-mail: macro@ds2.pg.gda.pl, PGP key available +- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Tue Feb 29 2000 - 21:00:10 EST