Segment descriptor

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In memory addressing for Intel x86 computer architectures, segment descriptors are a part of the segmentation unit, used for translating a logical address to a linear address. Segment descriptors describe the memory segment referred to in the logical address.[1] The segment descriptor (8 bytes long in 80286) contains the following fields:[2]

  1. A segment base address
  2. The segment limit which specifies the segment size
  3. Access rights byte containing the protection mechanism information
  4. Control bits


In X86-64, the code segment descriptor has the following form:[3]

31 24 23 22 21 20 19 16 15 14 13 12 11 10 9 8 7 0
Base Address[31:24] G D L A V L Segment Limit[19:16] P DPL 1 1 C R A Base Address[23:16]
Base Address[15:0] Segment Limit[15:0]

Where the fields stand for:

Base Address 
32 bit starting memory address of the segment
Segment Limit 
20 bit length of the segment. How exactly this should be interpreted depends on other bits of the segment descriptor.
descriptor-privilege level

See also[edit]


  1. ^ Bovet, D.P., & Cesati, M. (2000). Understanding the Linux Kernel (First Edition). O'Reilly & Associates, Inc.
  2. ^ Tabak, Daniel (1995). Advanced Microprocessors. Mcgraw Hill Publishers. p. 149. ISBN 9780070628434. 
  3. ^ AMD64 Architecture Programmer's Manual Volume 2: System Programming (PDF) (Technical report). 2013. p. 80. 
  • Tabak, Daniel. Advanced Microprocessors. McGraw Hill and Co. 
  • Hall, Douglas. Microprocessors and Interfacing. McGraw Hill Publications. 

Further reading[edit]

External links[edit]