Java Reference
In-Depth Information
usually by typing the name of the program or by clicking on an icon. The
operating system locates the program code, loads it into memory, and starts
it.
ȗ Managing files. A storage device, such as a hard disk is, electronically,
simply a device capable of storing a huge sequence of zeroes and ones. It is
up to the operating system to bring some structure to the storage layout and
organize it into files, folders, and so on. The operating system also needs to
impose some amount of security and redundancy into the file system so that
a power outage does not jeopardize the contents of an entire hard disk. Some
operating systems do a better job in this regard than others.
ȗ Virtual memory. RAM is expensive, and few computers have enough RAM
to hold all programs and their data that a user would like to run
simultaneously. Most operating systems extend the available memory by
storing some data on the hard disk. The application programs do not realize
whether a particular data item is in memory or in the virtual memory disk
storage. When a program accesses a data item that is currently not in RAM,
the processor senses this and notifies the operating system. The operating
system swaps the needed data from the hard disk into RAM, simultaneously
swapping out a memory block of equal size that had not been accessed for
some time.
ȗ Handling multiple users. The operating systems of large and powerful
computers allow simultaneous access by multiple users. Each user is
connected to the computer through a separate terminal. The operating
system authenticates users by checking that each one has a valid account and
password. It gives each user a small slice of processor time, then serves the
next user.
ȗ Multitasking. Even if you are the sole user of a computer, you may want to
run multiple applicationsȌfor example, to read your e-mail in one window
and run the Java compiler in another. The operating system is responsible
for dividing processor time between the applications you are running, so that
each can make progress.
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