Segmentation divides memory into logical segments. Each process has code and data segments, and the OS has its own segments. Segmentation uses virtual addresses and disk to make memory appear larger. Segments are variable in size, which can lead to external fragmentation. Most systems implement both segmentation and paging, where segments exist in virtual address space and paging translates virtual addresses to physical addresses.

1. Segmentation
CS 537 – Introduction to Operating Systems

2. Segmentation
• Segmentation is a technique for breaking memory
up into logical pieces
• Each “piece” is a grouping of related information
– data segments for each process
– code segments for each process
– data segments for the OS
– etc.
• Like paging, use virtual addresses and use disk to
make memory look bigger than it really is
• Segmentation can be implemented with or without
paging

3. Segmentation
P1 data P2 code
print
function
P2 data
P1 code
OS Code OS data OS stack
logical address space

4. Addressing Segments
• Let’s first assume no paging in the system
• User generates logical addresses
• These addresses consist of a segment number and
an offset into the segment
• Use segment number to index into a table
• Table contains the physical address of the start of
the segment
– often called the base address
• Add the offset to the base and generate the
physical address
– before doing this, check the offset against a limit
– the limit is the size of the segment

5. Addressing Segments
limit base
S o
logical address
segment table
yes Physical Address
o < limit +
no
error

6. Segmentation Hardware
• Sounds very similar to paging
• Big difference – segments can be variable in size
• As with paging, to be effective hardware must be
used to translate logical address
• Most systems provide segment registers
• If a reference isn’t found in one of the segment
registers
– trap to operating system
– OS does lookup in segment table and loads new
segment descriptor into the register
– return control to the user and resume
• Again, similar to paging

7. Protection and Sharing
• Like page tables, each process usually gets
its own segment table
• Unlike page tables, there usually exists a
global segment table for everyone
– this, however, is usually used by OS
• Access rights for segment are usually
included in table entry
• Multiple processes can share a segment

8. Protection and Sharing
limit base 200
0 1000 200 1200
1 3500 9000
2000
LDT limit base
2300
0 500 2500 2500
1 9000 20000 3000
GDT 9000
limit base
0 1000 200
1 300 2000
12500
LDT

9. Segmentation Issues
• Entire segment is either in memory or on
disk
• Variable sized segments leads to external
fragmentation in memory
• Must find a space big enough to place
segment into
• May need to swap out some segments to
bring a new segment in

10. Segmentation with Paging
• Most architectures support segmentation and
paging
• Basic idea,
– segments exist in virtual address space
– base address in segment descriptor table is a virtual
address
– use paging mechanism to translate this virtual address
into a physical address
• Now an entire segment does not have to be in
memory at one time
– only the part of the segment that we need will be in
memory

11. Linear Address
• The base address gotten from the segment
descriptor table is concatenated with the
offset
• This new address is often referred to as a
linear address
• This is the address that is translated by the
paging hardware

12. virtual address from user
descriptor offset
limit base +
segment table
linear address directory page offset
page frame
PT directory page table
directory base