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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 216
SEC. 3.1
NO MEMORY ABSTRACTION
185
can reference a private set of addresses local to it.
We will show how this can be
acomplished shortly.
What the IBM 360 did as a stop-gap solution was modify the
second program on the fly as it loaded it into memory using a technique known as
static relocation
.
It worked like this. When a program was loaded at address
16,384, the constant 16,384 was added to every program address during the load
process (so ‘‘JMP 28’’became ‘‘JMP 16,412’’, etc.).While this mechanism works
if done right, it is not a very general solution and slows down loading.
Fur-
thermore, it requires extra information in all executable programs to indicate which
words contain (relocatable) addresses and which do not. After all, the ‘‘28’’ in
Fig. 3-2(b) has to be relocated but an instruction like
MOV REGISTER1,28
which moves the number 28 to
REGISTER1
must not be relocated. The loader
needs some way to tell what is an address and what is a constant.
Finally, as we pointed out in Chap. 1, history tends to repeat itself in the com-
puter world. While direct addressing of physical memory is but a distant memory
on mainframes, minicomputers, desktop computers, notebooks, and smartphones,
the lack of a memory abstraction is still common in embedded and smart card sys-
tems. Devices such as radios, washing machines, and microwave ovens are all full
of software (in ROM) these days, and in most cases the software addresses abso-
lute memory. This works because all the programs are known in advance and users
are not free to run their own software on their toaster.
While high-end embedded systems (such as smartphones) have elaborate oper-
ating systems, simpler ones do not.
In some cases, there is an operating system,
but it is just a library that is linked with the application program and provides sys-
tem calls for performing I/O and other common tasks. The
e-Cos
operating system
is a common example of an operating system as library.
3.2 A MEMORY ABSTRACTION: ADDRESS SPACES
All in all, exposing physical memory to processes has several major draw-
backs. First, if user programs can address every byte of memory, they can easily
trash the operating system, intentionally or by accident, bringing the system to a
grinding halt (unless there is special hardware like the IBM 360’s lock-and-key
scheme). This problem exists even if only one user program (application) is run-
ning. Second, with this model, it is difficult to have multiple programs running at
once (taking turns, if there is only one CPU).
On personal computers, it is com-
mon to have several programs open at once (a word processor, an email program, a
Web browser), one of them having the current focus, but the others being reacti-
vated at the click of a mouse. Since this situation is difficult to achieve when there
is no abstraction from physical memory, something had to be done.
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