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.
The segment descriptor (8 bytes long in 80286 and later) contains the following fields:
Structure.
The x86 and x86-64 segment descriptor has the following form:
Where the fields stand for:
 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".
References.
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