Modern Operating Systems by Herbert Bos ...
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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 42
SEC. 1.2
was an immediate success, and the idea of a family of compatible computers was
soon adopted by all the other major manufacturers. The descendants of these ma-
chines are still in use at computer centers today.
Nowadays they are often used for
managing huge databases (e.g., for airline reservation systems) or as servers for
World Wide Web sites that must process thousands of requests per second.
The greatest strength of the ‘‘single-family’’ idea was simultaneously its great-
est weakness. The original intention was that all software, including the operating
, had to work on all models.
It had to run on small systems, which
often just replaced 1401s for copying cards to tape, and on very large systems,
which often replaced 7094s for doing weather forecasting and other heavy comput-
ing. It had to be good on systems with few peripherals and on systems with many
peripherals. It had to work in commercial environments and in scientific environ-
ments. Above all, it had to be efficient for all of these different uses.
There was no way that IBM (or anybody else for that matter) could write a
piece of software to meet all those conflicting requirements. The result was an
enormous and extraordinarily complex operating system, probably two to three
orders of magnitude larger than FMS.
It consisted of millions of lines of assembly
language written by thousands of programmers, and contained thousands upon
thousands of bugs, which necessitated a continuous stream of new releases in an
attempt to correct them. Each new release fixed some bugs and introduced new
ones, so the number of bugs probably remained constant over time.
One of the designers of OS/360, Fred Brooks, subsequently wrote a witty and
incisive book (Brooks, 1995) describing his experiences with OS/360.
While it
would be impossible to summarize the book here, suffice it to say that the cover
shows a herd of prehistoric beasts stuck in a tar pit. The cover of Silberschatz et al.
(2012) makes a similar point about operating systems being dinosaurs.
Despite its enormous size and problems, OS/360 and the similar third-genera-
tion operating systems produced by other computer manufacturers actually satis-
fied most of their customers reasonably well. They also popularized several key
techniques absent in second-generation operating systems. Probably the most im-
portant of these was
On the 7094, when the current job
paused to wait for a tape or other I/O operation to complete, the CPU simply sat
idle until the I/O finished. With heavily CPU-bound scientific calculations, I/O is
infrequent, so this wasted time is not significant. With commercial data processing,
the I/O wait time can often be 80 or 90% of the total time, so something had to be
done to avoid having the (expensive) CPU be idle so much.
The solution that evolved was to partition memory into several pieces, with a
different job in each partition, as shown in Fig. 1-5. While one job was waiting for
I/O to complete, another job could be using the CPU.
If enough jobs could be held
in main memory at once, the CPU could be kept busy nearly 100% of the time.
Having multiple jobs safely in memory at once requires special hardware to protect
each job against snooping and mischief by the other ones, but the 360 and other
third-generation systems were equipped with this hardware.

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