Re: Segments

[David Edelsohn <dje%watson.ibm.com@localhost>]

>>>>> Bill Studenmund writes:

Bill> Could someone please explain to me the rational behind how the
Bill> Power{,PC,2} architecture handles segments? I've been studying both the
Bill> Mindshare book, and the 601 manual (I haven't raided the Motorola document
Bill> site recently :-(  ), and it seems "weird." They just seem to break up the
Bill> memory space for little gain.

        The POWER/PowerPC virtual address space is not limited to
32-bits.  The upper bits of the 32-bit address select segments which
extend the address space.  The upper address bits actually are a sort of
cache of virtual addresses that can be selected depending on how the
segment registers are loaded.

        For instance, AIX has all open files mapped into different
segments and swaps them into the addressing range as they are accessed.
Unlike other systems, files do not need to be mapped and unmapped into
small address range windows.

        It is not breaking up the address space but providing a much
larger effective address range that can accessed in the system as a
whole. 

David