- In the layered system structure, operating system is organized as hierarchy of layers; each of the layers is constructed upon one another. The lowest layer (layer 0) contains hardware and the top most layer (layer N) implements the user interface.
- The first system was constructed in this way at the Technische Hogeschool Eindhoven (THE) by D.W. Dijstra and with the help of THE’s students.
- The main advantage of the layered system structure is modularity. The layers are organized such that each layer uses functions and services of only lower-level layers. This approach simplifies debugging and system verification.
- The system has six layers, layer 0 deals with allocation of the processes switching between processes when interrupts occurs or timer expired. It also deals with basic multi programming of the CPU.
Layer 1 :
It deals with the memory management. It allocates space for process in main memory. In layer 1, processes did not have to worry about whether they were in main memory or on disk. The layer 1 software took care of making sure processes were brought into main memory whenever they needed.
Layer 2 :
It handles communication between each process and the operator console.
Layer 3 :
It took care of managing input/output devices and buffering the information to end from them. About layer 3, each process deals with abstract input/output devices and provides high named interface took the user programs.
Layer 4 :
Layer 4 was ,where the actual user programs found. They did not worry about process, memory, control on input/output management.
Layer 5 :
The system operator processes was located in layer 5.
- The major difficulty with layered system structure is the organization of the various layers in a proper manner because a layer can use only those layers which are at a lower level, careful panning is necessary.
- For example the device driver for the disk store must be at a level lower than the memory management routines because the memory management uses the backing store.