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

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

• Burroughs large systems descriptors
• Memory segment
• Memory address

References

• Tabak, Daniel (1991). Advanced Microprocessors. McGraw Hill and Co.. ISBN 9780070628076. https://archive.org/details/advancedmicropro00dani. 
• Hall, Douglas. Microprocessors and Interfacing. McGraw Hill Publications. 

Further reading

• Robert R. Collins (August 1998). "The Segment Descriptor Cache". Dr Dobb's Journal. http://www.rcollins.org/ddj/Aug98/Aug98.html. 

External links

• Intel 80386 Reference Programmer's Manual - Segment Translation

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