How a user program asks for service from the kernel through a system call?

Please discuss in detail the procedure user program of asking for service program through a system call?
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•The kernel passes it up to Venus, by allowing Venus to read the request from the character device/dev/cfso. Venus tries to answer the request, by looking in it’s cache, asking servers, or possibly by declaring disconnection $ servicing it in disconnected mode.•Disconnected mode kicks in when there is no n/w connection to any server which has the files; Also if server fails, disconnected operation can come into action.•When the kernel passes the open request to venues for the first time, Venus letches the entire file from the servers, using RPC (Remote Procedure Calls) to reach the servers. It then stores the file as a container file in the cache area. •The file is now an ordinary file on the local disk and read­write operations to the file do not reach Venus, but are (almost) entirely handled by the local file system. coda read­ write operation take place at the same speed as those to local files.•If the file is opened a second time, it will not be fetched from the servers again,but the local copy will be available for use immediately. Source:
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Something call is usually an user interface between a consumer-living space diligence along with a program the center provides. Because the assistance will be furnished within the center, a primary get in touch with can not be executed; as a substitute, you should make use of a procedure for crossing anyone-living space/centre boundary. The way you do that varies using the specific structures. For this reason, We'll go through most popular structure, i386. "
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