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Billy Holmes posted <43972B9D.1040605@××××××.net>, excerpted below, on |
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Wed, 07 Dec 2005 13:36:13 -0500: |
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> Duncan wrote: |
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>> The current AMD spec says the CPUs offer 40-bit physical memory |
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>> addressing, 48-bit virtual memory addressing. So, 256 TB virtual |
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>> memory, but only a terabyte physical memory, in a flat-address |
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>> configuration. |
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> |
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> thanks Duncan! that's good to know :) |
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> |
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> can you imagine the RAID array needed for the SWAP partition to get that |
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> 256 TB of virtual memory? |
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I'm /definitely/ not sure on this, hopefully someone else will correct me |
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if I'm wrong, but I /believe/ "virtual address space" or "virtual memory" |
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in this case means something other than swap. I /believe/ swap would |
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still be part of the physical memory address space. |
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/If/ I understand it correctly, the "virtual" in this case refers not to |
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swap, but to the implementation where in 32-bit flat address usage, the |
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address space was 4 gig, but that was divided between the kernel (using |
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the upper addresses) and userland (using the lower addresses). PCI device |
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addresses were traditionally mapped into the top of the upper space, 3.5-4 |
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gig, but those were "virtual" addresses in the sense that it didn't mean |
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the entire 4 gig was mapped -- there could be and normally were holes in |
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the mapping, so folks didn't have to run a swap large enough to make up |
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the difference between physical memory and 4 gig. |
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Again, /if/ this reading is correct, the total of swap and physical memory |
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cannot exceed a terabyte (40 bits addressable) in the flat memory address |
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scheme, but that memory can be mapped anywhere within a 256 TB virtual |
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memory space. Thus, the kernel /could/ still use the top of that virtual |
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space, letting userland use the bottom of it, if the OS folks decided to |
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do it that way. (In practice, I believe at least a portion of kernel |
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space is still mapped much lower, for compatibility reasons. I |
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/definitely/ know that the 3.5-4 GB space remains reserved for 32-bit PCI |
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compatibility, because certain PCI devices can't handle anything above |
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that. I know that, having read about the problems folks may have getting |
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the computer to recognize all their memory, when they have 4 gig or more, |
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because the PCI space at the top of the 4 gig overwrites the real memory |
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at that location, if the BIOS hasn't been updated and configured to avoid |
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it by mapping the otherwise overwritten physical memory addresses up above |
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four gigs.) |
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If that's an incorrect understanding, as I said, hopefully someone |
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informed enough as to the true situation will correct my error. If it's |
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correct, and someone informed as to the fact can verify, I'd definitely |
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appreciate that, too. |
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Anyway, in this case, it VERY MUCH depends on what one's definition of |
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"virtual" is! <g> (For non-USians, this is an attempt at humor, |
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being an oblique reference to the Clinton Monicagate political scandal |
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here in the US a few years ago.) |
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-- |
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Duncan - List replies preferred. No HTML msgs. |
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"Every nonfree program has a lord, a master -- |
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and if you use the program, he is your master." Richard Stallman in |
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http://www.linuxdevcenter.com/pub/a/linux/2004/12/22/rms_interview.html |
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-- |
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