Structure of Processes Chapter 6 Structure of Processes Process State and Transition Data Structure for Process Layout of System Memory THE DESIGN OF THE UNIX OPERATING SYSTEM Maurice J. bach Prentice Hall

Process States and Transitions 1 2 9 7 3 4 6 5 8 User Running Preempted Zombie Asleep in Memory Sleep, Swapped Ready to Run, Swapped fork Created Ready to Run in Memory Kernel Running not enough mem (swapping system only) swap out wakeup enough mem sleep reschedule process preempt return to user system call, interrupt interrupt, interrupt return

Data Structures for Process Text Stack Data File Descriptor Table Per Process Region Table Kernel Process Table Kernel Region Table A Process U Area

Data Structure for Process (contd.) per process region table Kernel region table u area main memory Kernel process table

State of a Process Process table entry U area Contains general fields of processes that must be always be accessible to the kernel U area further characteristics of the process only need to be accessible to the running process itself

Process table entry State field: user running, kernel running etc. Fields that allow the kernel to locate the process and u area. Requires while context switch Process size : kernel know how much space to allocate for the process. User ID Process ID

Process table entry (contd.) Event descriptor. Used when the process is in the "sleep" state. Scheduling parameters. Allow the kernel to determine the order in which processes move to the states "kernel running" and "user running” A signal field. keeps the signals sent to a process but not yet handled. Various timers: process execution time, resource utilization etc.

U Area A pointer to the process table entry User IDs various Timer: Execution time in user mode Execution Time in kernel mode An error field: keeps error during system call Return value field: result of system call

U Area (contd.) I/O parameters The current directory and current root Amount of data transfer Address of source and target etc. The current directory and current root User file descriptor table Limit fields Restrict process size Restrict size of the file it can write The control terminal field: login terminal associated with the process, if one exists An array indicates how the process wishes to react to signal

Per Process Region Table (Pregion) Each pregion entry points to the kernel region table Starting virtual (absolute) address of the region Permission filed: read-only, read-write, read-execute

Kernel Region table Kernel region table contains the pointer to the page table which keeps the physical memory address

<Processes and Regions> 8K 16K 32K b a d c e 4K Text Data Stack Process A B Per Proc Region Tables (Virtual Addresses) Region <Processes and Regions>

Pages and Page Tables Logical Page Number Physical Page Number 0 177 0 177 1 54 2 209 3 17

Pages and Page Tables (contd.) 8K 32K 64K Text Data Stack 541K 783K 986K 897K 87K 552K 727K 941K empty 137K 852K 764K . 1096K 2001K 433K 333K Per Proc Region Table Page Tables(Physical Addresses) Virtual Addresses <Mapping Virtual Addresses to Physical Address>