[OS]Virtual memory

Virtual Storage (Memory)

• Non-continuous allocation

Address Mapping

• Continuous allocation
  • Relative address
  • Relocation
• Non-continuous allocation
  • Virtual address = relative address
    • Logical address
  • Real address = abosolute(physical)
• Address mapping
  • Virtual address -> real address

Block Mapping

• Virtual address : v = (b,d)
  • b = block number
  • d = displacement(offset) in a block
• Blck map table(BMT)

Virtual Storage Methods

• Paging system
• Segmentation system
• Hybrid paging/segmentation system

Paging system

• Terminologies
  • Page
  • Page frame
• Simple and Efficient
  • Segmentation
• No external fragmentation
  • Internal fragmentation possible

    Address Mapping

• Virtual address : v = (p,d)
  • p : page number
  • d : displacement(offset)
• Address mapping
  • PMT(age Map Table)
• Address mapping mechanism
  • Direct mapping
  • Associative mapping
    • TLB(Translation Look-aside Buffer)
  • Hybrid direct/associative mapping

Direct Mapping

• Problem
  • performance degradation
• Solution
  • Associative mapping(TLB)
• Hierarchical paging
• Hashed page table
• Inverted page table

Associative Mapping

• PMT in TLB(Translation Look-aside Buffer)
  • Associative high-speed memory
• Low overhead, high speed
• Expensive hardware

Hybrid Direct/Associative Mapping

• Use Locality

Memory Management

• page
  • Page frame
• Frame table
  • One entry per page frame
• Configuration
  • Allocated/available field
  • PID field
  • Link field : For free list
  • AV : Free list headeentry

Page Sharing

• Non-continuous allocation
• Procedure pages
  • Pure code (reenter code)
• Data page
  • Read-only data
  • Read-write data

Segmentation System

• Segment
  • ex) stack, heap, .ain procedure, shared lib, Etc.
• Distinction
  • Segmet sharing/protection
  • High overhead for Address mapping
  • No internal fragmentation
    • External fragmentation possible

Address mapping

• Virtual address: v = (s,d)
  • s : segment number
  • d : displacement in a segment
• Segment Map Table (SMT)
• Address mapping mechanism

Hybrid paging/Segmentation System

Address mapping

• Virtual address : v = (s, p, d)
  • s : segment number
  • p : page number
  • d : offset in a page
• Use SMT and PMT
• Address mapping
  • Dirct, associated etc.

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Reference

Virtural memory