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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 217
186
MEMORY MANAGEMENT
CHAP. 3
3.2.1 The Notion of an Address Space
Two problems have to be solved to allow multiple applications to be in memo-
ry at the same time without interfering with each other: protection and relocation.
We looked at a primitive solution to the former used on the IBM 360: label chunks
of memory with a protection key and compare the key of the executing process to
that of every memory word fetched. However, this approach by itself does not
solve the latter problem, although it can be solved by relocating programs as they
are loaded, but this is a slow and complicated solution.
A better solution is to invent a new abstraction for memory: the address space.
Just as the process concept creates a kind of abstract CPU to run programs, the ad-
dress space creates a kind of abstract memory for programs to live in. An
address
space
is the set of addresses that a process can use to address memory. Each proc-
ess has its own address space, independent of those belonging to other processes
(except in some special circumstances where processes want to share their address
spaces).
The concept of an address space is very general and occurs in many contexts.
Consider telephone numbers.
In the United States and many other countries, a
local telephone number is usually a 7-digit number. The address space for tele-
phone numbers thus runs from 0,000,000 to 9,999,999, although some numbers,
such as those beginning with 000 are not used. With the growth of smartphones,
modems, and fax machines, this space is becoming too small, in which case more
digits have to be used. The address space for I/O ports on the x86 runs from 0 to
16383. IPv4 addresses are 32-bit numbers, so their address space runs from 0 to
2
32
−
1 (again, with some reserved numbers).
Address spaces do not have to be numeric. The set of
.com
Internet domains is
also an address space.
This address space consists of all the strings of length 2 to
63 characters that can be made using letters, numbers, and hyphens, followed by
.com
.
By now you should get the idea.
It is fairly simple.
Somewhat harder is how to give each program its own address space, so ad-
dress 28 in one program means a different physical location than address 28 in an-
other program. Below we will discuss a simple way that used to be common but
has fallen into disuse due to the ability to put much more complicated (and better)
schemes on modern CPU chips.
Base and Limit Registers
This simple solution uses a particularly simple version of
dynamic relocation
.
What it does is map each process’ address space onto a different part of physical
memory in a simple way. The classical solution, which was used on machines
ranging from the CDC 6600 (the world’s first supercomputer) to the Intel 8088 (the
heart of the original IBM PC), is to equip each CPU with two special hardware
registers, usually called the
base
and
limit
registers. When these registers are used,
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