2--x86 Processor

Figure 29 shows a segment descriptor whose base

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Unformatted text preview: or the data segment • 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 base 00026000 limit 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.

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