CPU Abstraction: the Process

• Mechanisms: low-level machinery

  • E.x. Time-sharing mechanism

  • "How"

• Policies: algorithms for making some kind of decision within the OS

  • E.x. Scheduling policy

  • "Which"

• Separating the two allows one easily to change policies without having to rethink the mechanism and thus is a form of modularity.

4.1 The Abstraction: A Process

• The abstraction provided by the OS of a running program is something we will call a process.

• What constitutes a process (its machine state)

  • Memory (address space): instructions, data (read, write)

  • Registers: program counter (PC), stack pointer, frame pointer

  • Persistent storage devices

4.2 Process API

• Create, destroy, wait, miscellaneous control, status

4.3 Process Creation: A Little More Detail

• Load the code and static data from disk into memory

• Creating and initializing a stack

• Doing other work as related to I/O setup

• Start the program running at the entry point

4.4 Process States

• Running: on a processor, executing instructions

• Ready

• Blocked

Process: State Transitions

4.5 Data Structures

• Process list

For each process

• Registers (save and restore)

• State of the process, ID

• Parent process

• Open files

• Current directory

• Process memory (start, size)

• Bottom of the kernel stack

• Trap frame for the current interrupt