Memory Management

1
Memory Management

• Operating Systems - Spring 2003
• Gregory (Grisha) Chokler
• Ittai Abraham
• Zinovi Rabinovich

2
Segmentation

• A program can be divided into segments
• Segments are of variable size
• Segments reflect the logical (modular) structure
  of a program
• Text segment, data segment, stack segment
• Similar to dynamic partitioning except segments
  of the same process can be scattered
• subject to external fragmentation

3
Address translation

• Maximum segment size is always a power of 2
• Process segment table maps segment numbers into
  their base addresses in the memory
• With the maximum segment size of 2r, a logical
  address of lr bits is interpreted as a pair
  (l,r)
• l segment number, r offset within the segment
• l is used as an index into the segment table

4
Hardware support
Virtual Address
Segment Table

Base d
Seg
Offset d
Register
Seg Table Ptr
Segment Table
d
Segment
S

Length Base
Program
Segmentation
Main Memory
5
Segmentation Properties

• Advantages
• Association of protection with the segments
• Sharing of code or data
• Problems
• Dynamic memory allocation with external
  fragmentation

6
Segmentation with paging

• Segmentation
• simplifies protection and sharing, enforce
  modularity, but prone to external fragmentation
• Paging
• transparent, eliminates ext. fragmentation,
  allows for sophisticated memory management
• Segmentation and paging can be combined

7
Address translation
8
Remarks

• The price of paging/segmentation is a
  sophisticated hardware
• But the advantages exceed by far
• Paging decouples address translation and memory
  allocation
• Not all the logical addresses are necessary
  mapped into physical memory at every given moment
  - virtual memory
• Paging and segmentation are often combined to
  benefit from both worlds