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Re: AMD64-generic doesn't see all 4GB RAM?

Jo Shields wrote:
32-bit OSen have a theoretical limit of 4GiB addressable memory.
Wrong. IA-32 processors have had 36 address lines since PAE was added. That's what CONFIG_HIGHMEM64G does for IA-32 kernels, BTW. It turns on PAE support.

However, there's an additional limitation as all expansion cards (e.g.
graphics cards) need to have their memory regions mapped within that
4GiB area, lowering the overall amount of "system" memory you can
This is also wrong. PCI devices with 64-bit BARs can obviously use a 64-bit address. However, most devices have a 32-bit BAR, requiring that the address be below the 4GiB boundary. Obviously then, physically memory has to remapped to accommodate[1].

64-bit OSen don't have that limitation,
No, they still do. The width of the virtual address space has nothing to do with physical addressing limitations. If the PCI device has a 32-bit BAR, then the mapping for its I/O space must occur below 4GiB. If RAM wants to be there, then it must be remapped, regardless of whether your OS is 32-bits or 64-bits or 1024-bits virtual address space.

It's worth nothing that no AMD64 processor on the market has 64-bit physical lines or 64-bits of v.a.s.

Current processors have 40 physical lines (some early EM64T have 36). The maximum limit is 52-bits. Virtual address space is fixed at 48-bits presently but can be extended to the 64-bit limit.

Remember that limitations on physical addressing usually have physical reasons. So if a remapping option exists, it's likely for a damn good physical reason that the OS has nothing to do with whatsoever. Nor has any control over.


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