Handle Tables

There is a local handle table for each process, and a global handle table within the kernel.

Process Control Block (PCB)

• Data structure to keep track of process state. Each element identified by PID
• OS keeps all of a process' state in proc within the PCB while it isn't running.

proc has many many fields. (pid, pointer to paprent proc, execution state, etc.)

In Linux, defined within task_struct(include/linux/sched.h). Has over 95 fields!

proc and CPU state

context switch: take the currently running processes state from the CPU and save it in the PCB, then grab the next scheduled proccess' proc off of the PCB and load it into the CPU to run.

Choosing the next process to run is called scheduling.

The OS kernel is NOT a process, just a block of code. Remember: the CPI is always executing code in the context of a process. That code may be in either kernel or user mode.

State Queues

Typically there is a queue of procs that correspond to each of the states a process can take (ie WAITING, READY). There might even be many wait queues, one for each type of wait (particular device, timer, message, etc.).

• procs are just data structures, dynamically allocated in the OS memory.
• procs are created by existing processes. creator is parent, creation is child.

Creation

• fork basically clones process
• exec stops the current process, loads a new program into the address space (ie overwrites), initializes hardware and args for program, and places process on ready queue.

Note that exec doesn't create a new process.

To create a new process (ie from a shell), fork the shell process, and then exec the program

Making Creation Faster

method 1: vfork:

the older (now uncommon) way to do it. Instead of making a new child address space being a copy, just point to the parent address space from the child. This was an unenforced "promise" that the child wouldn't modify the address space. The child has the same page table and everything.

method 2: copy on write.