Multi-level page tables - hierarchical paging
Solution 1
Since page table must fit in a page, page table size is 2048 bytes and each entry is 4 bytes thus a table holds 2048/4=512 entries. To address 512 entries it requires log2(512)=9 bits. The total number of bits available to encode the entry for each page level is 64-log2(2048)=53 bits (the number of bits of address space minus the page offset bits). Thus the total number of levels required is 53/9=6 (rounded up).
The x86-64 default page table size is 4096 bytes, each page table must fit in a page and a page table entry is 8 bytes. Current CPUs only implement 48 bits of virtual address space. How many page table levels are required?
Solution 2
- Logical Address bit=64,
- Number of page will be= 2^64/2048 = 2^64/2^11 = 2^53
- Pages we have entry sine of page table= 4 Byte ,
- Number of Entry in 1 Page will be= 2048/4=>512,
- bit To represent one Entry=Log(512)=9bit,
- and bit for Page is= 53bit
- Therefore Number of Level =53/9=>6 Level Page Table
Comments
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Bobby S almost 2 years
Example question from a past operating system final, how do I calculate this kind of question?
A computer has a 64-bit virtual address space and 2048-byte pages. A page table entry takes 4 bytes. A multi-level page table is used because each table must be contained within a page. How many levels are required?
How would I calculate this?
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Bobby S about 13 years4096/8 = 512 entries; To address 512 entries log2(512) = 9 bits. The total number of bits to encode each page level is 48 - log2(4096) = 36 bits. Thus the total number of levels required is 36/9 = 4.
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Bobby S about 13 yearsSince the 2048 is in bytes, shouldn't it be converted to bits, which would make it 50/9 = 6 rounded up?
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bsantos about 13 yearsNo, because you what to know how many bits are required to address 2048 bytes and not bits.
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Hans over 10 yearsI might be answering in 2014, but big THANKS!!
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Jeff Hammond over 8 yearsThe 2048 is for 2K pages? What OS uses those? Don't you want to use 4K pages here?