Operating Systems Lecture 12: Paging and Segmentation - Prof. B. L. Barnett, Study notes of Operating Systems

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CMSC 321:CMSC 321:

Operating Systems

Lecture 12

Paging and SegmentationPaging and Segmentation

Paging

• Page:

titi

i^

t^

ll^

l fi

d

i^

h

k

• process : partition into small, equal fixed-size chunks
  • Frame:
    • RAM : partition into page-sized chunks

      • Page size == Frame size• Page size == Frame size• Fragmentation:

no external

internal only in last page of a process

Address Translation in Paging System

Data Structures NeededData Structures Needed

• Page table

– one per process– associate page w/ frame– pointer to table in PCB

• Frame table

– track free/occupied frames

Advantages of Paging

• Don’t need contiguous space in RAM• Reduces IF, eliminates EF• Abstract user view of memory from physical• Allow more processes in RAM• Allow more processes in RAM• Protection (bits w/ each page)• Ability to share reentrant code

never changes during execution

• never changes during execution

Reentrant Code

ProblemProblem

• each reference requires two accesses

• 1. index into page table to get frame– 2. then get actual data

l^

d b

f^

t^

f 2

• mem access slowed by factor of 2• solution: translation look aside buffer (TLB)• solution: translation look-aside buffer (TLB)

• high-speed associative cache– contains MRU page table entries

contains MRU page table entries

• very fast, expensive

T

LB

• TLB entries compared simultaneously
  • associative memory: fast!

associative memory: fast!

• TLB Hit: entry is in TLB

f^

t i

d

i kl

• frame # retrieved quickly
  • TLB Miss: entry not in TLB
    • 1. access page table to get frame #– 2. if frame in memory, add to TLB
      • may have to replace… LRU?
        • 3. if frame not in memory, page fault
          • I/O request to pull from disk• causes process to block• eventually update TLB

Segmentation

g

• User view of memory:
  • various data structures

procedures

various data structures, procedures

• no real ordering– don’t care where in RAM these reside
  • Segment: partition of memory
    • similar to page, except…

f

• size of segments can vary– segment may be dynamic
  • Usually handled by the compiler

Advantages

• Facilitates natural programming
  • think in terms of data structures, methods, etc.
    • Easy to handle growing data structures
      • rarely know size a priori

rarely know size a priori

• Programs can be altered/recompiled independently

no need to relink/reload entire set

• no need to relink/reload entire set
  • Facilitates sharing among processes
    • put data in commonly accessible segment– e.g., graphics libraries
      • Facilitates protection
        • one segment for methods, one for arrays, etc.

Segment Table

• Typically one per process• Table can be any length• Must be in RAM to be accessed• Each entry contains:

base address of segment in RAM

• base address of segment in RAM– length of segment in RAM
  • Note: process not partitioned like in paging

Example of Segmentation

Translating an Address

• extract leftmost

n

bits of logical address

• extract leftmost

n

bits of logical address

• use resulting # as index into segment table

gives starting physical address of segment

•^

compare offset (rightmost

m

bits) to length

if >, invalid address!

•^

physical address = base + offsetphysical address

base + offset

•^

e.g.: address 4848

in binary: 0001 | 001011110000

(logical address)

in binary: 0001 | 001011110000

(logical address)

compare offset with length in entry 1 of segment table;then add offset to base from entry 1 of segment table

then add offset to base from entry 1 of segment table

Address Translation