|
|
|
Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
Showing 279-280 out of 1137
Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 279
248
MEMORY MANAGEMENT
CHAP. 3
Segment number
Page
number
Offset
Descriptor
segment
Segment
number
Page
number
MULTICS virtual address
Page
table
Page
Word
Offset
Descriptor
Page frame
Figure 3-36.
Conversion of a two-part MULTICS address into a main memory address.
Segment
number
Virtual
page
Page
frame
Comparison
field
Protection
Age
Is this
entry
used?
4
6
12
2
2
1
0
3
1
2
7
2
1
0
12
Read/write
Read only
Read/write
Execute only
Execute only
13
10
2
7
9
1
1
1
0
1
1
Figure 3-37.
A simplified version of the MULTICS TLB.
The existence of two
page sizes made the actual TLB more complicated.
words. Although there are fewer segments, the larger segment size is far more im-
portant, as few programs need more than 1000 segments, but many programs need
large segments. As of x86-64, segmentation is considered obsolete and is no longer
supported, except in legacy mode. Although some vestiges of the old segmentation
Page 280
SEC. 3.7
SEGMENTATION
249
mechanisms are still available in x86-64’s native mode, mostly for compatibility,
they no longer serve the same role and no longer offer true segmentation. The
x86-32, however, still comes equipped with the whole shebang and it is the CPU
we will discuss in this section.
The heart of the x86 virtual memory consists of two tables, called the
LDT
(
Local Descriptor Table
) and the
GDT
(
Global Descriptor Table
). Each pro-
gram has its own LDT, but there is a single GDT, shared by all the programs on the
computer. The LDT describes segments local to each program, including its code,
data, stack, and so on, whereas the GDT describes system segments, including the
operating system itself.
To access a segment, an x86 program first loads a selector for that segment into
one of the machine’s six segment registers. During execution, the CS register holds
the selector for the code segment and the DS register holds the selector for the data
segment. The other segment registers are less important.
Each selector is a 16-bit
number, as shown in Fig. 3-38.
Index
0 = GDT/1 = LDT
Privilege level (0-3)
Bits
13
1
2
Figure 3-38.
An x86 selector.
One of the selector bits tells whether the segment is local or global (i.e., wheth-
er it is in the LDT or GDT).
Thirteen other bits specify the LDT or GDT entry
number, so these tables are each restricted to holding 8K segment descriptors. The
other 2 bits relate to protection, and will be described later.
Descriptor 0 is forbid-
den. It may be safely loaded into a segment register to indicate that the segment
register is not currently available. It causes a trap if used.
At the time a selector is loaded into a segment register, the corresponding de-
scriptor is fetched from the LDT or GDT and stored in microprogram registers, so
it can be accessed quickly.
As depicted in Fig. 3-39, a descriptor consists of 8
bytes, including the segment’s base address, size, and other information.
The format of the selector has been cleverly chosen to make locating the de-
scriptor easy. First either the LDT or GDT is selected, based on selector bit 2.
Then the selector is copied to an internal scratch register, and the 3 low-order bits
set to 0.
Finally, the address of either the LDT or GDT table is added to it, to give
a direct pointer to the descriptor. For example, selector 72 refers to entry 9 in the
GDT, which is located at address GDT + 72.
Let us now trace the steps by which a (selector, offset) pair is converted to a
physical address.
As soon as the microprogram knows which segment register is
Ace your assessments! Get Better Grades
Browse thousands of Study Materials & Solutions from your Favorite Schools
Concordia University
Concordia_University
School:
Operating_Systems
Course:
Great resource for chem class. Had all the past labs and assignments
Leland P.
Santa Clara University
Introducing Study Plan
Using AI Tools to Help you understand and remember your course concepts better and faster than any other resource.
Find the best videos to learn every concept in that course from Youtube and Tiktok without searching.
Save All Relavent Videos & Materials and access anytime and anywhere
Prepare Smart and Guarantee better grades
Know more
Students also viewed documents
lab 18.docx
lab_18.docx
Course
Course
3
Module5QuizSTA2023.d...
Module5QuizSTA2023.docx.docx
Course
Course
10
Week 7 Test Math302....
Week_7_Test_Math302.docx.docx
Course
Course
30
Chapter 1 Assigment ...
Chapter_1_Assigment_Questions.docx.docx
Course
Course
5
Week 4 tests.docx.do...
Week_4_tests.docx.docx
Course
Course
23
Week 6 tests.docx.do...
Week_6_tests.docx.docx
Course
Course
106
