Modern Operating Systems by Herbert Bos ...
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Modern Operating Systems by Herbert Bos and Andrew...
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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 69
occur at a certain moment (or within a certain range), we have a
hard real-time
Many of these are found in industrial process control, avionics, military,
and similar application areas. These systems must provide absolute guarantees that
a certain action will occur by a certain time.
soft real-time system
, is one where missing an occasional deadline, while
not desirable, is acceptable and does not cause any permanent damage. Digital
audio or multimedia systems fall in this category.
Smartphones are also soft real-
time systems.
Since meeting deadlines is crucial in (hard) real-time systems, sometimes the
operating system is simply a library linked in with the application programs, with
everything tightly coupled and no protection between parts of the system.
An ex-
ample of this type of real-time system is eCos.
The categories of handhelds, embedded systems, and real-time systems overlap
considerably. Nearly all of them have at least some soft real-time aspects.
The em-
bedded and real-time systems run only software put in by the system designers;
users cannot add their own software, which makes protection easier. The handhelds
and embedded systems are intended for consumers, whereas real-time systems are
more for industrial usage. Nevertheless, they have a certain amount in common.
1.4.9 Smart Card Operating Systems
The smallest operating systems run on smart cards, which are credit-card-sized
devices containing a CPU chip. They have very severe processing power and mem-
ory constraints.
Some are powered by contacts in the reader into which they are
inserted, but contactless smart cards are inductively powered, which greatly limits
what they can do. Some of them can handle only a single function, such as elec-
tronic payments, but others can handle multiple functions. Often these are propri-
etary systems.
Some smart cards are Java oriented. This means that the ROM on the smart
card holds an interpreter for the Java Virtual Machine (JVM).
Java applets (small
programs) are downloaded to the card and are interpreted by the JVM interpreter.
Some of these cards can handle multiple Java applets at the same time, leading to
multiprogramming and the need to schedule them. Resource management and pro-
tection also become an issue when two or more applets are present at the same
time. These issues must be handled by the (usually extremely primitive) operating
system present on the card.
Most operating systems provide certain basic concepts and abstractions such as
processes, address spaces, and files that are central to understanding them.
In the
following sections, we will look at some of these basic concepts ever so briefly, as

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