Process Processes are executing or executable instances of programs. Processes in a modern OS haveOS –management information –resources –unique process identifier or PID Management information covers –allocated resources –event handling –permissions –scheduling information –utilisation of resources Resources include –memory allocated - address space –open files, I/O channels, devices –secondary storage allocations - swap space, memory mapping Address space is protected so only process may read/write to it. Memory protection stops process –writing to address space of other processes or kernel –crashing OS or other processes if it goes wrongOS

Unix Process (I) Address space of Unix process is divided into 3 memory segmentsUnix – text read only program code –Data dynamic values of program data structures –Stack execution stack of program Unix processes operate either in application or kernel mode.Unix Unix processes in kernel mode –execute system calls synchronously –run in separate address space –use copy of calling arguments as input –copy return value into process address space Unix kernel keeps process information in fixed size process table.

Unix Process (II) Unix processes are in one of 5 states State Status Description nascent created but without resources runnableRrunning or in a run queue sleepingSawaiting an event stoppedTstopped by a signal or by parent process signal zombieZdeceased but termination not notified to parent

Process Creation (I)

Process Creation (II)