In memory addressing for 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 contains the following fields: [2]
The segment descriptor is 8 bytes long in 80286 and later.
The 80286 segment descriptor has the following form: [3]
bit Off | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | Off |
+7 | Must be zero | +6 | |||||||||||||||
+5 | P | DPL | S = 1 | Type | A | Base23-16 | +4 | ||||||||||
+3 | Base15-0 | +2 | |||||||||||||||
+1 | Limit15-0 | 0 |
The x86 and x86-64 segment descriptor has the following form: [4]
What the fields stand for:
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The S/370, S/370-XA, ESA/370 and ESA/390 segment table entries (STEs) are one word long. All processors running in S/370 mode used the same format, but not all supported the common-segment facility and
With z/Architecture the STE is a doubleword.
The segment-table entry for the S/370 has the following form [5]
0 | 3 | 4 | 7 | 8 | 28 | 29 | 30 | 31 | |||||||||||||||||||||||
TBL | 0000 | Page-Table Origin | P | C | I |
The segment-table entry for the S/370-XA, ESA/370 and ESA/390 has the following form [6]
0 | 1 | 25 | 26 | 27 | 28 | 31 | |||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
0 | Page-Table Origin | I | C | TBL |
In the original z/Architecture, a segment is always subject to paging and the Segment-Table Entry always points to a page table. However, on a model equipped with the Enhanced-DAT Facility 1, the Enhanced-DAT-enablement control (bit 40) o
The segment-table entry for the z/Architecture has the following forms [7]
0 | 31 | ||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Page-Table Origin | |||||||||||||||||||||||||||||||
32 | 53 | 54 | 55 | 57 | 58 | 59 | 60 | 61 | 62 | 63 | |||||||||||||||||||||
Page-Table Origin (continued) | F C | P | I | C S | TT |
0 | 31 | ||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Segment-Frame Absolute Address | |||||||||||||||||||||||||||||||
32 | 43 | 44 | 46 | 47 | 48 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59 | 60 | 61 | 62 | 63 | |||||||||||||
Segment-Frame Absolute Address (continued) | A V | ACC | F | F C | P | I E P | I | C S | TT |