Process Concept

• Process is a program in execution.
  • A program is a passive entity, while a process is an active entity.
• Process and job are used interchangeably.

The Process

Memory Layout

Memory layout of a process is divided into sections:

Layout of a process in memory.

• Text → executable code
• Data → global variables
• Heap → dynamically allocated memory during program run time
• Stack → temporary data storage when invoking functions
  • pushes activation record for each function call, which contains function parameters, local variables, return address
  • pops the activation record when control is returned from the function

Multiple Processes

• Multiple processes may be associated with the same program (eg. two web browser windows open at the same time)
• Each is a separate process:
  • text area (executable code) is equivalent; represents the passive information
  • data, heap, stack sections are unique; represents the active information

Process State

• A process may be in one of the following states:
  • New → process is being created
  • Running → instructions are executing
  • Waiting → process is waiting for some event to occur (eg. I/O completion, signal reception)
  • Ready → process is waiting to be assigned to a processor
  • Terminated → process has finished execution
• Some additional notes:
  • one process can be running on a single processor core at any instant
  • many processes may be ready and waiting

Process Control Block (PCB)

PCB serves as the repository for all data needed to start or restart a process, along with some accounting data.

Process control block (PCB).

Saved when an interrupt occurs. | | CPU registers | Accumulators, index registers, stack pointers, general-purpose registers, condition-code information.

Saved when an interrupt occurs. | | CPU-scheduling information | Process priority, pointers to scheduling queues, any other scheduling parameters. | | Memory-management information | Value of the base and limit registers, page tables, segment tables. | | Accounting information | Amount of CPU and real time used, time limits, account numbers, process numbers, etc. | | I/O status information | List of allocated I/O devices, open files, etc. |

Threads

• Assumption so far: process is single-threaded.