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SubjectRe: large memory support for x86
Timur Tabi wrote:
> I understand that a normal virtual address (i.e. a pointer) can only address a
> single 32-bit (4GB) memory block. My point was that by also using more than
> one 16-bit selector, you can have multiple 4GB areas. So for instance,
> 1000:00000000 can point to one physical address, and 1001:00000000 can point a
> different physical address.
> Yes, this means that you need to use multiple selectors in order to access more
> than 4GB of virtual space.

If you were to try to overflow the linear address you would either get a
fault or a wraparound. It won't work like the old DOS highmem tricks.

> According to section 3.8 of Intel's P3 manual, Volume 3, enabling the PAE
> increases the size of the page table entries to 64 bits. There are other
> changes, such as extended the 20-bit page directory base address to 27 bits.
> All this means that a virtual address (selector:offset) can point to a physical
> address larger than 32 bits.
> Frankly, the whole thing makes my head hurt.

Section 3.9.1, quote:
"No matter which 36-bit addressing feature is used (PAE or 36-bit PSE),
the linear address space of the processor remains at 32 bits.
Applications must partition the address space of their work loads across
multiple operating system processes to take advantage of the additonal
physical memory provided in the system."


Brian Gerst
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