Processes

• In C, the fork() command is used to create a process. The fork() returns a 0 in the child process and some number greater than 0 in the parent process. This is number is a PID and is used to identify the different processes.
• In C, the exit() function ends the process immediately.
• The call of exit(0) indicates that the process terminated normally. A nonzero value indicates an error.
• In addition to the typical executable object file, there is some additional virtual memory allocated for the process control block (PCB).
• When new processes are created, a pointer to its process control block exists in the process table.
• The wait() function indicates a state change in a child process.
• All processes are assigned a process identifier or PID.
• The operating system has two modes: user mode and kernel mode.
• User mode provides restricted access to system resources. In order for a program perform kernel operations such as memory management, process scheduling and file management, it must use API's or system calls.
• Kernel mode allows for direct access to all system resources. The operating system kernel operates in kernel mode, since it needs to keep track of the CPU context of a process. In kernel mode, the OS can directly access the hardware components, schedule processes, handle interrupts, and manage memory.
• A system call is a command to hardware devices. It is a kernel mode privilege.
• When an exception occurs, the CPU transfers control to the Operating system. The OS has an exception table and a designated handler for each exception. A handler is a set of instructions.
• There are 3 types of synchronous exceptions: Traps, Faults, and Aborts. Asynchronous exceptions occur due to external circumstances, but synchronous exceptions are due to an event within the CPU. Usually, they are predictable.
• A system call is a special instruction that transfers the CPU to kernel mode. System calls are traps that can be used to interrupt the execution of a program synchronously. File manipulation operations, process creation, and process termination are system calls that require the CPU to be in kernel mode.