1.5 OPERATING SYSTEM CONCEPTS

1.5.1 Processes

1. A process is fundamentally a container that holds all the information
   needed to run a program.
2. For example, the process may have sev eral files open for reading at once. Associated with each of these files is a pointer giving the current position (i.e., the number of the byte or record to be read next). When a process is temporarily suspended, all these pointers must be saved so that a read call executed after the process is restarted will read the proper data.
3. All the information about each process, other than the contents of its own address space, is stored in an operating system table called the process table, which is an array of structures, one for each process currently in existence. Thus, a (suspended) process consists of its address space, usually called the core image , and its process table entry, which contains the contents of its registers and many other items needed to restart the process later.
4. Related processes that are cooperating to get some job done often need to communicate with one another and synchronize their activities. This communication is called interprocess communication.

1.5.2 Address Spaces

1. More sophisticated operating systems allow multiple programs to be in memory at the same time. To keep them from interfering with one another (and with the operating system), some kind of protection mechanism is needed. While this mechanism has to be in the hardware, it is controlled by the operating system.

1.5.3 Files

1. System calls are obviously needed to create files, remove files, read files, and write files. Before a file can be read, it must be located on the disk and opened, and after being read it should be closed, so calls are provided to do these things.

1.5.6 The Shell

1. The operating system is the code that carries out the system calls. Editors, compilers, assemblers, linkers, utility programs, and command interpreters definitely are not part of the operating system, even though they are important and useful.

1.6 SYSTEM CALLS

Any single-CPU computer can execute only one instruction at a time. If a process is running a user program in user mode and needs a system service, such as reading data from a file, it has to execute a trap instruction to transfer control to the operating system. The operating system then figures out what the calling process wants by inspecting the parameters. Then it carries out the system call and returns control to the instruction following the system call. In a sense, making a system call is like making a special kind of procedure call, only system calls enter the kernel and procedure calls do not.