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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 232
SEC. 3.3
VIRTUAL MEMORY
201
Information the operating system needs to handle page faults is kept in software
tables inside the operating system. The hardware does not need it.
Before getting into more implementation issues, it is worth pointing out again
that what virtual memory fundamentally does is create a new abstraction—the ad-
dress space—which is an abstraction of physical memory, just as a process is an
abstraction of the physical processor (CPU).
Virtual memory can be implemented
by breaking the virtual address space up into pages, and mapping each one onto
some page frame of physical memory or having it (temporarily) unmapped. Thus
this section is bassically about an abstraction created by the operating system and
how that abstraction is managed.
3.3.3 Speeding Up Paging
We have just seen the basics of virtual memory and paging.
It is now time to
go into more detail about possible implementations.
In any paging system, two
major issues must be faced:
1.
The mapping from virtual address to physical address must be fast.
2.
If the virtual address space is large, the page table will be large.
The first point is a consequence of the fact that the virtual-to-physical mapping
must be done on every memory reference. All instructions must ultimately come
from memory and many of them reference operands in memory as well. Conse-
quently, it is necessary to make one, two, or sometimes more page table references
per instruction.
If an instruction execution takes, say, 1 nsec, the page table lookup
must be done in under 0.2 nsec to avoid having the mapping become a major bot-
tleneck.
The second point follows from the fact that all modern computers use virtual
addresses of at least 32 bits, with 64 bits becoming the norm for desktops and lap-
tops. With, say, a 4-KB page size, a 32-bit address space has 1 million pages, and a
64-bit address space has more than you want to contemplate. With 1 million pages
in the virtual address space, the page table must have 1 million entries. And
remember that each process needs its own page table (because it has its own virtual
address space).
The need for large, fast page mapping is a very significant constraint on the
way computers are built. The simplest design (at least conceptually) is to have a
single page table consisting of an array of fast hardware registers, with one entry
for each virtual page, indexed by virtual page number, as shown in Fig. 3-10.
When a process is started up, the operating system loads the registers with the
process’ page table, taken from a copy kept in main memory. During process ex-
ecution, no more memory references are needed for the page table. The advantages
of this method are that it is straightforward and requires no memory references dur-
ing mapping.
A disadvantage is that it is unbearably expensive if the page table is
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