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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 218
SEC. 3.2
A MEMORY ABSTRACTION: ADDRESS SPACES
187
programs are loaded into consecutive memory locations wherever there is room
and without relocation during loading, as shown in Fig. 3-2(c). When a process is
run, the base register is loaded with the physical address where its program begins
in memory and the limit register is loaded with the length of the program.
In
Fig. 3-2(c), the base and limit values that would be loaded into these hardware reg-
isters when the first program is run are 0 and 16,384, respectively. The values used
when the second program is run are 16,384 and 32,768, respectively.
If a third
16-KB program were loaded directly above the second one and run, the base and
limit registers would be 32,768 and 16,384.
Every time a process references memory, either to fetch an instruction or read
or write a data word, the CPU hardware automatically adds the base value to the
address generated by the process before sending the address out on the memory
bus. Simultaneously, it checks whether the address offered is equal to or greater
than the value in the limit register, in which case a fault is generated and the access
is aborted.
Thus, in the case of the first instruction of the second program in
Fig. 3-2(c), the process executes a
JMP 28
instruction, but the hardware treats it as though it were
JMP 16412
so it lands on the
CMP
instruction as expected. The settings of the base and limit
registers during the execution of the second program of Fig. 3-2(c) are shown in
Fig. 3-3.
Using base and limit registers is an easy way to give each process its own pri-
vate address space because every memory address generated automatically has the
base-register contents added to it before being sent to memory.
In many imple-
mentations, the base and limit registers are protected in such a way that only the
operating system can modify them. This was the case on the CDC 6600, but not on
the Intel 8088, which did not even have the limit register.
It did have multiple base
registers, allowing program text and data, for example, to be independently relocat-
ed, but offered no protection from out-of-range memory references.
A disadvantage of relocation using base and limit registers is the need to per-
form an addition and a comparison on every memory reference. Comparisons can
be done fast, but additions are slow due to carry-propagation time unless special
addition circuits are used.
3.2.2 Swapping
If the physical memory of the computer is large enough to hold all the proc-
esses, the schemes described so far will more or less do. But in practice, the total
amount of RAM needed by all the processes is often much more than can fit in
memory.
On a typical Windows, OS X, or Linux system, something like 50–100
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