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Layered Architecture
The operating system is to be organized as a hierarchy of layers, each one constructed upon the one below it. The first system constructed in this way was the THE system built at the Technische Hogeschool Eindhoven in the Netherlands by E. W. Dijkstra (1968) and his students. The THE system was a simple batch system for a Dutch computer. The system had 6 layers, as shown in Fig. “Structure of the THE operating system”. Layer 0 dealt with allocation of the processor, switching between processes when interrupts occurred or timers expired. It also provided the basic multiprogramming of the CPU. Layer 1 did the memory management. It allocated space for processes in main memory and on a 512K word drum used for holding parts of processes (pages) for which there was no room in main memory. Above layer 1, processes did not have to worry about whether they were in memory or on the drum; the layer 1 software took care of making sure pages were brought into memory whenever they were needed. Layer 2 handled communication between each process and the operator console. Above this layer each process effectively hat its own operator console. Layer 3 took care of managing the I/O devices and buffering the information streams to and from them. Above layer 3 each process could deal with abstract I/O devices with nice properties, instead of real devices with many peculiarities. Layer 4 was where the user programs were found. They did not have to worry about process, memory, console or I/O management. The system operator process was located in layer5. The concept of layered architecture is broadly used in several areas, in particular in the area of computer networks. The well-known ISO-OSI (Open Systems Architecture) Reference Model is the theoretical basis for building hierarchies of network services and protocols. A good link: Dijkstra and Parnas: Structure matters [Source: Tanenbaum,Operating Systems: Design and Implementation; 1987]
 

 

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