Lecture 18 Paging

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Lecture 18Paging

• Page Table Structure

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Problem Very Large Logical Address Space

• Consider a system with 32-bit logical address
  space
• Page size 4K bytes (212)
• Then, a page table may consists of up to 1
  million entries (232/212)
• Each entry consists of 4 bytes, each process may
  need up to 4 megabytes of physical address space
  for the table page alone

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A two-level page-table scheme

• A logical address (on 32-bit machine with 4K page
  size) is divided into
• a page number consisting of 20 bits.
• a page offset consisting of 12 bits.
• Since the page table is paged, the page number is
  further divided into
• a 10-bit page number.
• a 10-bit page offset.
• Thus, a logical address is as follows
• where pi is an index into the outer page table,
  and p2 is the displacement within the page of the
  outer page table.

From Operating System Concepts. Silbershatz
Galvin. Addison Wesley
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Two-Level Page-Table Scheme
From Operating System Concepts. Silbershatz
Galvin. Addison Wesley.
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A Problem

• Usually, a process has a page table associated
  with it
• The page table has one entry for each page that
  the process is using
• Each page table may consists of millions of
  entries
• These tables may consume large amounts of
  physical memory, which is requested just to keep
  track of how the other physical memory is being
  used

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Inverted Page Table

• An inverted page table has one entry for each
  real page (frame) of memory
• Each entry consists
• Virtual address of the page stored in that real
  memory location
• Information about the process that owns that page

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Inverted Page Table Architecture
From Operating System Concepts. Silbershatz
Galvin. Addison Wesley.
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Inverted Page Table Structure
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Shared Pages

• Shared code
• One copy of read-only (reentrant) code shared
  among processes (i.e., text editors, compilers,
  window systems).
• Shared code must appear in same location in the
  logical address space of all processes.
• Private code and data
• Each process keeps a separate copy of the code
  and data.
• The pages for the private code and data can
  appear anywhere in the logical address space.

From Operating System Concepts. Silbershatz
Galvin. Addison Wesley
10
Shared Pages
From Operating System Concepts. Silbershatz
Galvin. Addison Wesley