Page 33 - DCAP507_SYSTEM_SOFTWARE

Unit 2: Evolution of Operating System

For constructing the actual object program of the operating system when this approach is used, Notes
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.

2.2.2 Layered Systems

A generalization of the approach as shown below in the figure for organizing the operating
system as a hierarchy of layers, each one constructed upon the one below it.
Figure 2.5: An Operating System Layer

Layer M

New
operations 

Layer M - 1
hidden
operations 

Existing
operations 

The system has 6 layers. Layer 0 dealt with allocation of the processor, switching between
processes when interrupts occurred or timers expired. Above layer 0, the system consisted of
sequential processes, each of which could be programmed without having to worry about the
fact that multiple processes were running on a single processor. In other words, layer 0 provided
the basic multiprogramming of the CPU.

Figure 2.6: Diagram of Layered System

T

LOVELY PROFESSIONAL UNIVERSITY 27