List five services provided by an operating system. Explain how each provides convenience to the users. Explain also in which cases it would be impossible for user-level programs to provide these services.
Program execution. The operating system loads the contents (or sections) of a file into memory and begins its execution. A user-level program could not be trusted to properly allocate CPU time. I/O operations.
File-system manipulation. There are many details in file creation, allocation, deletion, and naming that user should not have to perform. Blocks of disk space are used by files and must be tracked. Deleting a file needs removing the name file information and freeing the allocated blocks.
Communications. Message passing between systems requires messages be turned into packets of information, sent to the network controller, transmitted across a communications medium, and reassembled by the destination system. Packet ordering and data correction must take place.
Error detection. Error detection happens at both the hardware and software levels. At the hardware level, all data transfers must be inspected to make sure that data have not been corrupted in transit. All data on media must be checked to be sure they have not altered since they were written to the media.