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Merge pull request #634 from leocassarani/patch-1

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Introduction to Paging: fix a couple of small typos
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Philipp Oppermann
2019-07-04 22:51:28 +02:00
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parent a40b65a77d 41c9efd734
commit 85b83aed32
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blog/content/second-edition/posts/08-paging-introduction/index.md
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@@ -135,7 +135,7 @@ Now that we know how paging and multilevel page tables works, we can look at how
## Paging on x86_64
The x86_64 architecture uses a 4-level page table and a page size of 4KiB. Each page table, independent of the level, has a fixed sized of 512 entries. Each entry has a size of 8 bytes, so each table is 512 * 8B = 4KiB large and thus fits exactly into one page.
The x86_64 architecture uses a 4-level page table and a page size of 4KiB. Each page table, independent of the level, has a fixed size of 512 entries. Each entry has a size of 8 bytes, so each table is 512 * 8B = 4KiB large and thus fits exactly into one page.
The page table index for level is derived directly from the virtual address:
@@ -145,7 +145,7 @@ We see that each table index consists of 9 bits, which makes sense because each
[5-level page table]: https://en.wikipedia.org/wiki/Intel_5-level_paging
Even though bits 48 to 64 are discarded, they can't be set to arbitrary values. Instead all bits in this range have to be copies of bit 47 in order to keep addresses unique and allow future extensions like the 5-level page table. This is called _sign-extension_ because it's very similar to the [sign extension in two's complement]. When a address is not correctly sign-extended, the CPU throws an exception.
Even though bits 48 to 64 are discarded, they can't be set to arbitrary values. Instead all bits in this range have to be copies of bit 47 in order to keep addresses unique and allow future extensions like the 5-level page table. This is called _sign-extension_ because it's very similar to the [sign extension in two's complement]. When an address is not correctly sign-extended, the CPU throws an exception.
[sign extension in two's complement]: https://en.wikipedia.org/wiki/Two's_complement#Sign_extension