the highest (layer N) is the user interface.
Advantage
Simplicity of construction and debugging
Each layer uses the functions and services of only lower-level layers
Each hides the existence of certain data structures, operation and hardware
from higher-level layers. It does not need to know how these operations are
implemented
Disadvantage
definition of the various layers is difficult
a layer can use only lower-level layers, it makes planning is very difficult
sometime
Main difficulty with the layer approach
less efficient
每一层都要修改参数，传递数据，频繁的调用参数导致浪费时间较长，效率低
Microkernel Approach
Removing all nonessential components from the kernel and implementing them as
system and user-level programs
provides Minimal process and memory management and Basic Communication
Benefits
Easier to extend a microkernel
Easier to port the operating system to new architectures
More security and reliability
less code is running in kernel mode
Detriments(危害)
Performance decreases due to
increased system function overhead
increased communication between user space and kernel space
Modules Approach
Uses object-oriented approach to create a module kernel
The kernel has a set of core components and dynamical links in additional services
Uses dynamical loadable modules
When needed, any component can be added to the kernel
similar to layers but with more flexible
Can call any other module
similar to microkernel but more efficient
Communication don’t need to invoke message passing
3.Processes
Process Concept
Process include:
Program code
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