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 and later) 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
See also: Global descriptor table

Structure

The x86 and x86-64 segment descriptor has the following form:[3]

Where the fields stand for:

Base Address
Starting memory address of the segment. Its length is 32 bits and it is created from the lower part bits 16 to 31, and the upper part bits 0 to 7, followed by bits 24 to 31.
Segment Limit
Its length is 20 bits and is created from the lower part bits 0 to 15 and the upper part bits 16 to 19. It defines the address of the last accessible data. The length is one more than the value stored here. How exactly this should be interpreted depends on the Granularity bit of the segment descriptor.
G=Granularity
If clear, the limit is in units of bytes, with a maximum of 220 bytes. If set, the limit is in units of 4096-byte pages, for a maximum of 232 bytes.
D/B
D = Default operand size : If clear, this is a 16-bit code segment; if set, this is a 32-bit segment.
B = Big: If set, the maximum offset size for a data segment is increased to 32-bit 0xffffffff. Otherwise it's the 16-bit max 0x0000ffff. Essentially the same meaning as "D".
L=Long
If set, this is a 64-bit segment (and D must be zero), and code in this segment uses the 64-bit instruction encoding. "L" cannot be set at the same time as "D" aka "B". (Bit 21 in the image)
AVL=Available
For software use, not used by hardware (Bit 20 in the image with the label A)
P=Present
If clear, a "segment not present" exception is generated on any reference to this segment
DPL=Descriptor privilege level
Privilege level (ring) required to access this descriptor
S=System Segment
If clear, this is system segment, if 1, this is Code/Data segment.
Type
If bit 11 set, this is a code segment descriptor. If clear, this is a data/stack segment descriptor, which has "D" replaced by "B", "C" replaced by "E" and "R" replaced by "W". This is in fact a special case of the 2-bit type field, where the preceding bit 12 cleared as "0" refers to more internal system descriptors, for LDT, LSS, and gates.
C=Conforming
Code in this segment may be called from less-privileged levels.
E=Expand-Down
If clear, the segment expands from base address up to base+limit. If set, it expands from maximum offset down to limit, a behavior usually used for stacks.
R=Readable
If clear, the segment may be executed but not read from.
W=Writable
If clear, the data segment may be read but not written to.
A=Accessed
This bit is set to 1 by hardware when the segment is accessed, and cleared by software.

See also

References
  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. Archived from the original (PDF) on 2018-02-18.
  • Tabak, Daniel (1991). Advanced Microprocessors. McGraw Hill and Co. ISBN 9780070628076.
  • Hall, Douglas. Microprocessors and Interfacing. McGraw Hill Publications.

Further reading


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