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DBM bit in descriptor

Dear all:

As described in armv8 reference manual, DBM means " Dirty Bit Modifier".

My understanding is:

     If DBM =1, it means that the page or block is dirty.

     If DBM =0, it means the page or block is not dirty.

Is that rignt?

Parents
  • Not exactly.

    When mapping in a "clean" write-able page the OS would set the attributes to AP="read-only" and DBM==1.  On the first write, the hardware automatically updates AP from "read-only" to "read-write".

    DBM is therefore telling the processor how to understand the AP bits.

    Note: If you tried to write a page with AP="read-only" and DBM==0, then you'd get an abort.

    For software, you can tell whether the page is dirty or clean by looking at the AP bits.  Still "read-only" = clean, while "read-write" means its been written to.

Reply
  • Not exactly.

    When mapping in a "clean" write-able page the OS would set the attributes to AP="read-only" and DBM==1.  On the first write, the hardware automatically updates AP from "read-only" to "read-write".

    DBM is therefore telling the processor how to understand the AP bits.

    Note: If you tried to write a page with AP="read-only" and DBM==0, then you'd get an abort.

    For software, you can tell whether the page is dirty or clean by looking at the AP bits.  Still "read-only" = clean, while "read-write" means its been written to.

Children