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A CPU with 5-level paging is available now

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Philipp Oppermann
2020-01-31 11:10:09 +01:00
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blog/content/second-edition/posts/08-paging-introduction/index.md
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@@ -141,7 +141,7 @@ The page table index for level is derived directly from the virtual address:
![Bits 0–12 are the page offset, bits 12–21 the level 1 index, bits 21–30 the level 2 index, bits 30–39 the level 3 index, and bits 39–48 the level 4 index](x86_64-table-indices-from-address.svg) ![Bits 0–12 are the page offset, bits 12–21 the level 1 index, bits 21–30 the level 2 index, bits 30–39 the level 3 index, and bits 39–48 the level 4 index](x86_64-table-indices-from-address.svg)
We see that each table index consists of 9 bits, which makes sense because each table has 2^9 = 512 entries. The lowest 12 bits are the offset in the 4KiB page (2^12 bytes = 4KiB). Bits 48 to 64 are discarded, which means that x86_64 is not really 64-bit since it only supports 48-bit addresses. There are plans to extend the address size to 57 bits through a [5-level page table], but no processor that supports this feature exists yet. We see that each table index consists of 9 bits, which makes sense because each table has 2^9 = 512 entries. The lowest 12 bits are the offset in the 4KiB page (2^12 bytes = 4KiB). Bits 48 to 64 are discarded, which means that x86_64 is not really 64-bit since it only supports 48-bit addresses.
[5-level page table]: https://en.wikipedia.org/wiki/Intel_5-level_paging [5-level page table]: https://en.wikipedia.org/wiki/Intel_5-level_paging
@@ -149,6 +149,10 @@ Even though bits 48 to 64 are discarded, they can't be set to arbitrary values.
[sign extension in two's complement]: https://en.wikipedia.org/wiki/Two's_complement#Sign_extension [sign extension in two's complement]: https://en.wikipedia.org/wiki/Two's_complement#Sign_extension
It's worth noting that the recent "Ice Lake" Intel CPUs optionally support [5-level page tables] to extends virtual addresses from 48-bit to 57-bit. Given that optimizing our kernel for a specific CPU does not make sense at this stage, we will only work with standard 4-level page tables in this post.
[5-level page tables]: https://en.wikipedia.org/wiki/Intel_5-level_paging
### Example Translation ### Example Translation
Let's go through an example to understand how the translation process works in detail: Let's go through an example to understand how the translation process works in detail: