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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 1034
SEC. 12.3
space should each one get?
The trade-offs here are similar to those for the process
table. Making key data structures like these dynamic is possible, but complicated.
Another static-dynamic trade-off is process scheduling.
In some systems, es-
pecially real-time ones, the scheduling can be done statically in advance. For ex-
ample, an airline knows what time its flights will leave weeks before their depar-
ture. Similarly, multimedia systems know when to schedule audio, video, and other
processes in advance. For general-purpose use, these considerations do not hold
and scheduling must be dynamic.
Yet another static-dynamic issue is kernel structure.
It is much simpler if the
kernel is built as a single binary program and loaded into memory to run. The
consequence of this design, however, is that adding a new I/O device requires a
relinking of the kernel with the new device driver. Early versions of UNIX worked
this way, and it was quite satisfactory in a minicomputer environment when adding
new I/O devices was a rare occurrence. Nowadays, most operating systems allow
code to be added to the kernel dynamically, with all the additional complexity that
12.3.7 Top-Down vs. Bottom-Up Implementation
While it is best to design the system top down, in theory it can be implemented
top down or bottom up.
In a top-down implementation, the implementers start
with the system-call handlers and see what mechanisms and data structures are
needed to support them. These procedures are written, and so on, until the hard-
ware is reached.
The problem with this approach is that it is hard to test anything with only the
top-level procedures available. For this reason, many developers find it more prac-
tical to actually build the system bottom up. This approach entails first writing
code that hides the low-level hardware, essentially the HAL in Fig. 11-4. Interrupt
handling and the clock driver are also needed early on.
Then multiprogramming can be tackled, along with a simple scheduler (e.g.,
round-robin scheduling).
At this point it should be possible to test the system to
see if it can run multiple processes correctly.
If that works, it is now time to begin
the careful definition of the various tables and data structures needed throughout
the system, especially those for process and thread management and later memory
management. I/O and the file system can wait initially, except for a primitive way
to read the keyboard and write to the screen for testing and debugging. In some
cases, the key low-level data structures should be protected by allowing access
only through specific access procedures—in effect, object-oriented programming,
no matter what the programming language is.
As lower layers are completed, they
can be tested thoroughly.
In this way, the system advances from the bottom up,
much the way contractors build tall office buildings.
If a large team of programmers is available, an alternative approach is to first
make a detailed design of the whole system, and then assign different groups to

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