Subversion Repositories HelenOS-doc

          <primary>address space</primary>

    <para>last bits about address spaces</para>

        <para>Every task in the operating system has it's own view of the

      <para>Modern processor architectures optimize TLB utilization by

        virtual memory. When performing context switch between different

      associating TLB entries with address spaces through assigning

        tasks, the kernel must switch the address space mapping as well. As

      identification numbers to them. In HelenOS, the term ASID, originally

        modern processors perform very aggressive caching of virtual mappings,

      taken from the mips32 terminology, is used to refer to the address space

        flushing the complete TLB on every context switch would be very

      identification number. The advantage of having ASIDs is that TLB does

        inefficient. To avoid such performance penalty, some architectures

      not have to be invalidated on thread context switch as long as ASIDs are

        introduce an address space identifier, which allows storing several

      unique. Unfotunatelly, architectures supported by HelenOS use all

        different mappings inside TLB.</para>

      different widths of ASID numbers<footnote>

        <para>HelenOS kernel can take advantage of this hardware support by

          <para>amd64 and ia32 don't use similar abstraction at all, mips32

        having an ASID abstraction. I.e. on ia64 kernel ASID is derived from

          has 8-bit ASIDs and ia64 can have ASIDs between 18 to 24 bits

        RID (region identifier) and on the mips32 kernel ASID is actually the

        </footnote> out of which none is sufficient. The amd64 and ia32

        hardware identifier. As expected, this ASID information record is the

      architectures cannot make use of ASIDs as their TLB doesn't recognize

        part of <emphasis>as_t</emphasis> structure.</para>

      such an abstraction. Other architectures have support for ASIDs, but for

        <para>Due to the hardware limitations, hardware ASID has limited

      instance ppc32 doesn't make use of them in the current version of

        length from 8 bits on ia64 to 24 bits on mips32, which makes it

      HelenOS. The rest of the architectures does use ASIDs. However, even on

        impossible to use it as unique address space identifier for all tasks

        </footnote> is insufficient to provide a unique integer identifier to

        running in the system. In such situations special ASID stealing

      all address spaces that might hypothetically coexist in the running

        algoritm is used, which takes ASID from inactive task and assigns it

      system. The situation on mips32 is even worse: the architecture has only

        to the active task.</para>

      256 unique identifiers.</para>

        <para>

      strategy. When the system initializes, a FIFO queue<footnote>

          <classname>ASID stealing algoritm here.</classname>

          <para>Note that architecture-specific measures are taken to avoid

        </para>

          grouped to form one HelenOS ASID.</para>

      </section>

      be address spaces that are not active on any processor.</para>