Real Time Sistemi operativi

OS.

Unfortunately, microkernels can suffer from performance decreases due to in-

2

creased system function overhead. Windows NT delivered low performance com-

pared with that of Windows 95, so the following OS developped was Windows

XP and its architecture was more monolithic than microkernel.

1.4 Modules

Perhaps the best current methodology for operating-system design involves

using object-oriented programming techniques to create a modular kernel. He-

re, the kernel has a set of core components and dynamically links in additional

services either during boot time or during run time. Such a strategy uses dyna-

mically loadable modules and is common in modern implementations of UNIX.

Such a design allows the kernel to provide core services yet also allows certain

features to be implemented dynamically. For example, device and bus drivers

for specific hardware can be added to the kernel, and support for different file

systems can be added as loadable modules. The overall result resembles a laye-

red system in that each kernel subsection has defined, protected interfaces; but

it is more flexible than a layered system in that any module can call any other

module.

1.5 Virtual Machines

The layered approach described is taken to its logical conclusion in the con-

cept of a virtual machine. The fundamental idea behind a virtual machine is

to abstract the hardware of a single computer (the CPU, memory, disk drives,

network interface cards, and so forth) into several different execution environ-

ments, thereby creating the illusion that each separate execution environment

is running its own private computer. By using CPU scheduling and virtual-

memory techniques, an OS can create the illusion that a process has its own

processor with its own (virtual) memory. Normally, a process has additional

features, such as system calls and a file system, that are not provided by the

bare hardware. The virtual-machine approach does not provide any such ad-

ditional functionality but rather provides an interface that is identical to the

underlying bare hardware. Each process is provided with a (virtual) copy of the

underlying computer.

There are several reasons for creating a virtual machine, all of which are fun-

damentally related to being able to share the same hardware yet run several

different execution environments (that is, different OSs) concurrently. Despite

the advantages of virtual machines, they received little attention for a number

of years after they were first developed. Today, however, virtual machines are

coming back into fashion as a means of solving system compatibility problems.

3 client chiede al server di eseguire un

processo per lui

stub = pacchetto contenente tutto il

necessario per eseguire il processo

per evitare il problema big endian-little

endian c'è un adattatore di linguaggi XDR

The software solution is called Peterson Solotution to critical section

problem, but it has numerous variants