List five services provided by an operating system that are designed to make it more convenient for users to use the computer system In what cases it would be impossible for user-level programs to pr?

Answer 1

1. 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.

2. I/O operations.

Disks, tapes, serial lines, and other devices must be

communicated with at a very low level. The user need only specify

the device and the operation to perform on it, while the system

converts that request into device- or controller-specific commands.

User-level programs cannot be trusted to only access devices they

should have access to and to only access them when they are

otherwise unused.

3. File-system manipulation.

There are many details in file creation, deletion, allocation, and naming that users should not have to perform. Blocks of disk space are used by files and must be tracked. Deleting a file requires removing the name file information and

freeing the allocated blocks. Protections must also be checked to

assure proper file access. User programs could neither ensure

adherence to protection methods nor be trusted to allocate only free

blocks and deallocate blocks on file deletion.

4. 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. Again, user programs might not

coordinate access to the network device, or they might receive

packets destined for other processes.

5. Error detection.

Error detection occurs at both the hardware and

software levels. At the hardware level, all data transfers must be

inspected to ensure that data have not been corrupted in transit. All

data on media must be checked to be sure they have not changed

since they were written to the media. At the software level, media

must be checked for data consistency; for instance, do the number

of allocated and unallocated blocks of storage match the total

number on the device. There, errors are frequently

process-independent (for instance, the corruption of data on a disk),

so there must be a global program (the operating system) that

handles all types of errors. Also, by having errors processed by the

operating system, processes need not contain code to catch and

correct all the errors possible on a system.