Processes in Operating Systems

• A program in execution
• May be stopped and later restarted by the OS
• Process table
  • Records information about each process
  • Program code
  • Data
  • Stack
  • Program counter (PC), Stack pointer (SP), and other registers
• Core image
• Process has a parent [process] and may create child processes
• Communication through messages
• Process ownership determined by the UID and GID
• Process id identifies the process
• Special process id's in Unix
  • 0 Swapper
  • 1 /sbin/init
  • 2 Pagedaemon

Process execution modes in Unix

User mode

• Processes can access their own instructions and data but not kernel instructions and data
• Cannot execute certain privileged machine instructions

Kernel mode

• Processes can access both kernel as well as user instructions and data
• No limit to which instructions can be executed
• Runs on behalf of a user process and is a part of the user process

Threads

• A dispatchable unit of work to provide intraprocess concurrency in newer operating systems
• A process may have multiple threads of execution in parallel, each thread executing sequentially