Unix foundations

When to Switch a Process

• Interrupts
• Clock
• process has executed for the maximum allowable time slice
• I/O
• Memory fault
• memory address is in virtual memory so it must be brought into main memory
• Trap
• error occurred
• may cause process to be moved to Exit state
• Supervisor call
• such as file open

Change of Process State

• Save context of processor including program counter and other registers
• Update the process control block with the new state and any accounting information
• Move process control block to appropriate queue - ready, blocked
• Select another process for execution
• Update the process control block of the process selected
• Update memory-management data structures
• Restore context of the selected process

UNIX Process State

• Initial (idle)
• Ready to run
• Kernel/User running
• Zombie
• Asleep
• for 4BSD: stopped/suspend

Process states and state transitions

Process Context

• User address space:
• code, data, stack, shared memory regions,
• Control information:
• u area, proc, kernel stack, ATM
• Credentials: UID & GID
• Environment variables:
• inherited from the parent
• Hardware context(in PCB of u area):
• PC, SP, PSW, MMR, FPU

User Credentials

• Superuser: UID=0, GID=1
• Real IDs: log in, send signals
• Effective IDs: file creation and access
• exec:
• suid mode: that of the owner;
• sgid mode: that of the calling process
• setuid / setgid:
• SV & BSD are different with these
• saved UID, saved GID in SV
• setgroup in BSD

The u Area

• part of the process space, needed only when running.
• PCB
• proc pointer
• UID s
• Arguments, results, error status from system calls
• Signal handlers and related information
• Info from the program header, text, data, stack size, MM info
• Open file descriptor
• Vnodes & controlling terminal pointers
• CPU usage statistics, profiling info, disk quotas, & resource limits
• Per-process kernel stack