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Operating System
Notes the most ‘trusted’ in the system - and all requests to do anything significant must go via the
kernel. It provides the most key facilities and functions of the system.
3. Outer OS: Surrounding the kernel are other parts of the operating system. These perform
less critical functions - for example, the graphics system which is ultimately responsible for
what you see on the screen.
4. Interface: The interface provides a mechanism for you to interact with the computer.
5. Applications: There are what do the actual work - they can be complex (for example Offi ce)
or simple (for example the is command commonly found on unix and Linux systems that
lists files in a directory (or folder).
3.4.1 Monolithic Systems
This approach is well known as “The Big Mess”. The operating system is written as a collection of
procedures, each of which can call any of the other ones whenever it needs to. When this technique
is used, each procedure in the system has a well-defined interface in terms of parameters and
results, and each one is free to call any other one, if the latter provides some useful computation
that the former needs.
For constructing the actual object program of the operating system when this approach is used,
one compiles all the individual procedures, or files containing the procedures, and then binds
them all together into a single object file with the linker. In terms of information hiding, there
is essentially none- every procedure is visible to every other one i.e. opposed to a structure
containing modules or packages, in which much of the information is local to module, and only
officially designated entry points can be called from outside the module.
However, even in Monolithic systems, it is possible to have at least a little structure. The services
like system calls provide by the operating system are requested by putting the parameters in
well-defined places, such as in registers or on the stack, and then executing a special trap
instruction known as a kernel call or supervisor call.
Figure 3.3: Monolithic System
Operating Operating Operating
System System System
Process Process Process
Operating Operating Operating
System System System
Process Process Process
Operating System
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