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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 214
SEC. 3.1
NO MEMORY ABSTRACTION
183
(a)
(b)
(c)
0xFFF …
0
0
0
User
program
User
program
User
program
Operating
system in
RAM
Operating
system in
RAM
Operating
system in
ROM
Device
drivers in ROM
Figure 3-1.
Three simple ways of organizing memory with an operating system
and one user process. Other possibilities also exist.
One way to get some parallelism in a system with no memory abstraction is to
program with multiple threads. Since all threads in a process are supposed to see
the same memory image, the fact that they are forced to is not a problem. While
this idea works, it is of limited use since what people often want is
unrelated
pro-
grams to be running at the same time, something the threads abstraction does not
provide. Furthermore, any system that is so primitive as to provide no memory
abstraction is unlikely to provide a threads abstraction.
Running Multiple Programs Without a Memory Abstraction
However, even with no memory abstraction, it is possible to run multiple pro-
grams at the same time. What the operating system has to do is save the entire con-
tents of memory to a disk file, then bring in and run the next program.
As long as
there is only one program at a time in memory, there are no conflicts.
This concept
(swapping) will be discussed below.
With the addition of some special hardware, it is possible to run multiple pro-
grams concurrently, even without swapping. The early models of the IBM 360
solved the problem as follows. Memory was divided into 2-KB blocks and each
was assigned a 4-bit protection key held in special registers inside the CPU.
A ma-
chine with a 1-MB memory needed only 512 of these 4-bit registers for a total of
256 bytes of key storage. The PSW (Program Status Word) also contained a 4-bit
key. The 360 hardware trapped any attempt by a running process to access memo-
ry with a protection code different from the PSW key. Since only the operating sys-
tem could change the protection keys, user processes were prevented from interfer-
ing with one another and with the operating system itself.
Nevertheless, this solution had a major drawback, depicted in Fig. 3-2. Here
we have two programs, each 16 KB in size, as shown in Fig. 3-2(a) and (b).
The
former is shaded to indicate that it has a different memory key than the latter. The
Page 215
184
MEMORY MANAGEMENT
CHAP. 3
first program starts out by jumping to address 24, which contains a
MOV
instruc-
tion. The second program starts out by jumping to address 28, which contains a
CMP
instruction. The instructions that are not relevant to this discussion are not
shown. When the two programs are loaded consecutively in memory starting at
address 0, we have the situation of Fig. 3-2(c). For this example, we assume the
operating system is in high memory and thus not shown.
0
4
8
12
16
20
24
28
0
4
8
12
16
20
24
28
(a)
(b)
0
4
8
12
16
20
24
28
ADD
JMP 24
MOV
(c)
16384
16388
16392
16396
16400
16404
16408
16412
ADD
JMP 24
MOV
0
16380
JMP 28
CMP
0
16380
.
.
.
.
.
.
.
.
.
16380
.
.
.
JMP 28
CMP
0
0
32764
Figure 3-2.
Illustration of the relocation problem.
(a) A 16-KB program.
(b) Another 16-KB program. (c) The two programs loaded consecutively
into memory.
After the programs are loaded, they can be run. Since they have different mem-
ory keys, neither one can damage the other.
But the problem is of a different
nature. When the first program starts, it executes the
JMP 24
instruction, which
jumps to the instruction, as expected. This program functions normally.
However, after the first program has run long enough, the operating system
may decide to run the second program, which has been loaded above the first one,
at address 16,384.
The first instruction executed is
JMP 28
, which jumps to the
ADD
instruction in the first program, instead of the
CMP
instruction it is supposed
to jump to. The program will most likely crash in well under 1 sec.
The core problem here is that the two programs both reference absolute physi-
cal memory. That is not what we want at all.
What we want is that each program
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