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    SubjectRe: APIC version and 8-bit APIC IDs
    On Fri, 26 Aug 2005, Martin Wilck wrote:

    > Unless I am mistaken, the MP spec does not say that _CPUs_ must start from 0.
    > We had an IO-APIC at 0. The MP spec says that the IDs must be unique (I am
    > told this isn't true any more because an IO APIC and a CPU may have the same
    > ID) and _need not_ be consecutive.

    You are unfortunately mistaken -- the spec is explicit about *local* APIC
    IDs having to start at 0. There are at least two places in the spec that
    refer to that.

    > We tried different setups; one had IO APICs at 0,1,2 and CPUs starting at 16.
    > I can't see that this is forbidden (the reason is that the IO-APICs have only
    > 4-bit APIC ID registers). Anyway we changed it now to have both IO-APICs and
    > CPUs start at 0.

    You can always assign 0, 16, 17, etc. to local APICs and then 1, 2, 3,
    etc. for I/O APICs. Frankly I don't know what the actual justification
    behind the requirement is. Note that the ID of 0 need not necessarily
    belong to the BSP.

    Maciej
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