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    SubjectRe: ioremap_nocache problem?
    Timur Tabi <> writes:
    > ** Reply to message from David Wragg <> on 24 Jan 2001
    > 00:50:20 +0000
    > > (x86 processors with PAT and IA64 can set write-combining through
    > >page flags. x86 processors with MTRRs but not PAT would need a more
    > >elaborate implementation for write-combining.)
    > What is PAT? I desperately need to figure out how to turn on write
    > combining on a per-page level. I thought I had to use MTRRs, but now
    > you're saying I can use this "PAT" thing instead. Please explain!

    PAT is basically the MTRR memory types on a per-page basis. It adds a
    new flag bit to the x86 page table entry, then that bit together with
    the PCD and PWT bits is used to do a look-up in an 8-entry table that
    gives the effective memory type (the table is set through an MSR).
    All the details are in the Intel x86 manual, volume 3
    <URL:> (at the end
    of chapter 9).

    Quite a lot of the x86 CPUs out there support PAT: The PII except the
    first couple of models, the Celeron except the first model, the PIII,
    all PII and PIII Xeons, the P4, all AMD K7 models. I'm guessing, but
    I suspect that the majority of x86 CPUs supporting write combining in
    any form that have been made also support PAT.

    I wish Intel had put PAT in the PPro, rather than messing everyone
    around with MTRRs (MTRRs are good for BIOS writers, but a pain for
    everyone else).

    David Wragg
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