1.If we delivery less information to computers,we prefer to using message passing than shared-memory model.
Message passing: information through the operating system provide links between trips to the Exchange.
Pros: message delivery method is useful only in the minority when the information needs to be exchanged, but also for communication between computers, or easier to make than the shared memory.
Cons: Messaging mode each message requires the operating system, so it is not efficient, slow access speed.
Shared-memory model: travel share memory create shared memory attach System call the future and the other collector has access to the memory region. Therefore prevention of shared problems, such as critical section address in the future, and ensure you do not write to the same area.
Pros: maximum communication speed and convenience, because it is the speed of conversion operation with memory to operate.
Cons: travel protection and synchronization of shared memory still exist many problems.
2.functions:
System calls are made from an application program to request the OS to perform certain services that use the resources controlled by the OS.
purposes:
process control，file manipulation，device manipulation，information maintenance，communication，protection
3.At one extreme , a system administrator can use it to modify a copy of the source code of the operating system.The operating system then is completely compiled.
At a slightly less tailored level,the system description can lead to the creation of tables and the selection of modules from a precompiled library.these modules are linked together to form the generated operating system.
At the other extreme,it is possible to contrust a system that is completely table driven. All the code is always part of the system,and selection occurs at execution time,rather than at compile or link time.System generation involves simply creating the appropriate tables to describe the system.
The major differences among these approaches are the size and generality of the generated system and the ease of modifying it as the hardware configuration changes.