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• SS references the descriptor table for the stack segment
Flat Segmentation Model
In the ﬂat segmentation model, all segments are mapped to the entire 32-bit physical address
space of the computer. At least two segments are required, one for code and one for data. Each
segment is deﬁned by a segment descriptor, a 64-bit integer stored in a table known as the global
descriptor table (GDT). Figure 2–9 shows a segment descriptor whose base address ﬁeld points
to the ﬁrst available location in memory (00000000). In this ﬁgure, the segment limit is 0040.
The access ﬁeld contains bits that determine how the segment can be used. All modern operating
systems based on x86 architecture use the ﬂat segmentation model. For More BS-IT Books, Notes & Assignments visit: www.bsit.zxq.net 46 Chapter 2 • x86
Downloaded From: www.bsit.zxq.net Processor Architecture Figure 2–9 Flat Segmentation Model.
not used Segment descriptor, in the
Global Descriptor Table FFFFFFFF
(4 GByte) 00040000
limit access 00000000 0040 ---- physical RAM base address 00000000 Multi-Segment Model
In the multi-segment model, each task or program is given its own table of segment descriptors,
called a local descriptor table (LDT). Each descriptor points to a segment, which can be distinct
from all segments used by other processes. Each segment has its own address space. In
Figure 2–10, each entry in the LDT points to a different segment in memory. Each segment
descriptor speciﬁes the exact size of its segment. For example, the segment beginning at 3000
has size 2000 hexadecimal, which is computed as (0002
1000 hexadecimal). The segment
beginning at 8000 has size A000 hexadecimal.
Figure 2–10 Multi-Segment Model.
RAM Local Descriptor Table 26000
0010 00008000 000A 00003000 0002 access 8000 3000 Paging
x86 processors support paging, a feature that permits segments to be divided into 4,096-byte
blocks of memory called pages. Paging permits the total memory used by all programs running at the For More BS-IT Books, No...
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This note was uploaded on 11/29/2013 for the course CSE 451 taught by Professor Hussein during the Winter '13 term at Fatih Üniversitesi.
- Winter '13