Summary

• Paging partitions virtual and physical memory into fixed-size pages.
• Superpages in x86-64 omit last-level page table for up to 2 MiB pages.
• 64b Intel page table entry allows for bigger virtual addresses.
• Future advancements may include hardware bounds checking for security.

MMU Goals

• Relocation for independence from physical address
  • Why?: Program may not load in the same location every time, swap may no reload to same location..
• Protection with hardware checks
• Sharing when needed

Mechanisms

• Segments: Variable-length blocks of memory, programmer-visible and controllable
• Pages: Fixed-length blocks of memory, invisible to programs
• Mechanisms can be combined

Segmentation

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• Divide process into unequal size segments with different values for base, length, and protection
• Programmer must be aware of maximum segment size
  • e.g., 64 KiB on 8086/8088!
• Introduced in 1960s with systems like Burroughs B5000 and Multics
• Intel 8086/8088 used fixed length, overlapping segments like…
  • CS: code segment
  • DS: data segment
  • SS: stack segment
  • ES: extra segment
• x64 architectures simplifies to $CS = DS = SS = ES = 0-2^{64}$
  • FS, GS still work

Paging

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• Partition virtual memory into fixed-size pages and…
• …physical memory into frames of the same size
• Don’t need to be stored in contiguous frames

Important Differences

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Segmentation VS Paging

• Size:
  • Segments are blocks of memory with variable lengths
  • Pages are fixed length
• Visibility:
  • Segments are visible to programs
  • Pages are not