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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 447
416
INPUT/OUTPUT
CHAP. 5
Manipulating a touch screen with just a single finger is still fairly WIMPy—
you just replace the mouse pointer with your stylus or index finger. Multitouch is a
bit more complicated. Touching the screen with five fingers is like pushing five
mouse pointers across the screen at the same time and clearly changes things for
the window manager. Multitouch screens have become ubiquitous and increasingly
sensitive and accurate. Nevertheless, it is unclear whether the Five Point Palm
Exploding Heart Technique has any effect on the CPU.
5.7 THIN CLIENTS
Over the years, the main computing paradigm has oscillated between cent-
ralized and decentralized computing. The first computers, such as the ENIAC,
were, in fact, personal computers, albeit large ones, because only one person could
use one at once. Then came timesharing systems, in which many remote users at
simple terminals shared a big central computer. Next came the PC era, in which the
users had their own personal computers again.
While the decentralized PC model has advantages, it also has some severe
disadvantages that are only beginning to be taken seriously. Probably the biggest
problem is that each PC has a large hard disk and complex software that must be
maintained. For example, when a new release of the operating system comes out, a
great deal of work has to be done to perform the upgrade on each machine sepa-
rately.
At most corporations, the labor costs of doing this kind of software mainte-
nance dwarf the actual hardware and software costs. For home users, the labor is
technically free, but few people are capable of doing it correctly and fewer still
enjoy doing it. With a centralized system, only one or a few machines have to be
updated and those machines have a staff of experts to do the work.
A related issue is that users should make regular backups of their gigabyte file
systems, but few of them do. When disaster strikes, a great deal of moaning and
wringing of hands tends to follow. With a centralized system, backups can be made
every night by automated tape robots.
Another advantage is that resource sharing is easier with centralized systems.
A system with 256 remote users, each with 256 MB of RAM, will have most of
that RAM idle most of the time. With a centralized system with 64 GB of RAM, it
never happens that some user temporarily needs a lot of RAM but cannot get it be-
cause it is on someone else’s PC. The same argument holds for disk space and
other resources.
Finally, we are starting to see a shift from PC-centric computing to Web-cen-
tric computing. One area where this shift is very far along is email. People used to
get their email delivered to their home machine and read it there. Nowadays, many
people log into Gmail, Hotmail, or Yahoo and read their mail there. The next step
is for people to log into other Websites to do word processing, build spreadsheets,
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