1. 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

2. Process States and Transitions1 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

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

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

5. 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

6. 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

7. 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.

8. 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

9. 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

10. 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

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

12. <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>

13. Pages and Page Tables Logical Page Number Physical Page Number 0 177

14. 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>