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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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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 228
SEC. 3.3
VIRTUAL MEMORY
197
Virtual
address
space
Physical
memory
address
60K–64K
56K–60K
52K–56K
48K–52K
44K–48K
40K–44K
36K–40K
32K–36K
28K–32K
24K–28K
20K–24K
16K–20K
12K–16K
8K–12K
4K–8K
0K–4K
28K–32K
24K–28K
20K–24K
16K–20K
12K–16K
8K–12K
4K–8K
0K–4K
Virtual page
Page frame
X
X
X
X
7
X
5
X
X
X
3
4
0
6
1
2
Figure 3-9.
The relation between virtual addresses and physical memory ad-
dresses is given by the
page table
.
Every page begins on a multiple of 4096 and
ends 4095 addresses higher, so 4K–8K really means 4096–8191 and 8K to 12K
means 8192–12287.
because virtual address 8192 (in virtual page 2) is mapped onto 24576 (in physical
page frame 6).
As a third example, virtual address 20500 is 20 bytes from the start
of virtual page 5 (virtual addresses 20480 to 24575) and maps onto physical ad-
dress 12288
+
20 = 12308.
By itself, this ability to map the 16 virtual pages onto any of the eight page
frames by setting the MMU’s map appropriately does not solve the problem that
the virtual address space is larger than the physical memory. Since we have only
eight physical page frames, only eight of the virtual pages in Fig. 3-9 are mapped
onto physical memory. The others, shown as a cross in the figure, are not mapped.
In the actual hardware, a
Present/absent bit
keeps track of which pages are physi-
cally present in memory.
What happens if the program references an unmapped address, for example, by
using the instruction
MOV REG,32780
which is byte 12 within virtual page 8 (starting at 32768)?
The MMU notices that
the page is unmapped (indicated by a cross in the figure) and causes the CPU to
Page 229
198
MEMORY MANAGEMENT
CHAP. 3
trap to the operating system. This trap is called a
page fault
.
The operating system
picks a little-used page frame and writes its contents back to the disk (if it is not al-
ready there).
It then fetches (also from the disk) the page that was just referenced
into the page frame just freed, changes the map, and restarts the trapped instruc-
tion.
For example, if the operating system decided to evict page frame 1, it would
load virtual page 8 at physical address 4096 and make two changes to the MMU
map. First, it would mark virtual page 1’s entry as unmapped, to trap any future ac-
cesses to virtual addresses between 4096 and 8191.
Then it would replace the
cross in virtual page 8’s entry with a 1, so that when the trapped instruction is reex-
ecuted, it will map virtual address 32780 to physical address 4108 (4096 + 12).
Now let us look inside the MMU to see how it works and why we have chosen
to use a page size that is a power of 2.
In Fig. 3-10 we see an example of a virtual
address, 8196 (0010000000000100 in binary), being mapped using the MMU map
of Fig. 3-9. The incoming 16-bit virtual address is split into a 4-bit page number
and a 12-bit offset. With 4 bits for the page number, we can have 16 pages, and
with 12 bits for the offset, we can address all 4096 bytes within a page.
The page number is used as an index into the
page table
, yielding the number
of the page frame corresponding to that virtual page.
If the
Present/absent
bit is 0,
a trap to the operating system is caused.
If the bit is 1, the page frame number
found in the page table is copied to the high-order 3 bits of the output register,
along with the 12-bit offset, which is copied unmodified from the incoming virtual
address. Together they form a 15-bit physical address.
The output register is then
put onto the memory bus as the physical memory address.
3.3.2 Page Tables
In a simple implementation, the mapping of virtual addresses onto physical ad-
dresses can be summarized as follows: the virtual address is split into a virtual
page number (high-order bits) and an offset (low-order bits).
For example, with a
16-bit address and a 4-KB page size, the upper 4 bits could specify one of the 16
virtual pages and the lower 12 bits would then specify the byte offset (0 to 4095)
within the selected page. However a split with 3 or 5 or some other number of bits
for the page is also possible. Different splits imply different page sizes.
The virtual page number is used as an index into the page table to find the
entry for that virtual page. From the page table entry, the page frame number (if
any) is found.
The page frame number is attached to the high-order end of the off-
set, replacing the virtual page number, to form a physical address that can be sent
to the memory.
Thus, the purpose of the page table is to map virtual pages onto page frames.
Mathematically speaking, the page table is a function, with the virtual page num-
ber as argument and the physical frame number as result. Using the result of this
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