Process - Operating Systems

Process

• program in the execution
• Attributes held by process include hardware state, memory, CPU
• Process memory is divided into four sections:
• The text section is made up of the compiled program code, read in from non-volatile storage when the program is launched.
• The data section is made up the global and static variables, allocated and initialized prior to executing the main.
• The heap is used for the dynamic memory allocation, and is managed via calls to new, delete, malloc, free, etc.
• The stack is used for local variables. Space on the stack is reserved for local variables when they are declared.

Processes can be any of the following states :

• New - The process is in the stage of being created.
• Ready - The process has all the resources available that it needs to run, but the CPU is not currently working on this process's instructions.
• Running - The CPU is working on this process's instructions.
• Waiting - The process cannot run at the moment, because it is waiting for some resource to become available or for some event to occur.
• Terminated - The process has completed.

PROCESS CONTROL BLOCK
There is a Process Control Block for each process, enclosing all the information about the process. It is a data structure, which contains the following :

• Process State - It can be running, waiting etc.
• Process ID and parent process ID.
• CPU registers and Program Counter. Program Counter holds the address of the next instruction to be executed for that process.
• CPU Scheduling information - Such as priority information and pointers to scheduling queues.
• Memory Management information - Eg. page tables or segment tables.
• Accounting information - user and kernel CPU time consumed, account numbers, limits, etc.
• I/O Status information - Devices allocated, open file tables, etc.