Segment descriptor
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]
- A segment base address
- The segment limit which specifies the segment size
- Access rights byte containing the protection mechanism information
- Control bits
x86-64
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.
- DPL
- descriptor-privilege level
See also
- Burroughs large systems descriptors
- Memory segment
- Memory address
References
- ↑ Bovet, D.P., & Cesati, M. (2000). Understanding the Linux Kernel (First Edition). O'Reilly & Associates, Inc.
- ↑ Tabak, Daniel (1995). Advanced Microprocessors. Mcgraw Hill Publishers. p. 149. ISBN 9780070628434.
- ↑ MD64 Architecture Programmer's Manual Volume 2: System Programming (PDF) (Technical report). 2013. p. 88.
- Tabak, Daniel. Advanced Microprocessors. McGraw Hill and Co.
- Hall, Douglas. Microprocessors and Interfacing. McGraw Hill Publications.
Further reading
- Robert R. Collins (August 1998). "The Segment Descriptor Cache". Dr Dobb's Journal.