Processes

What is a process?

The OS's abstraction of a running program. A process is a program in execution.

Simplest case:

• An address space
• A sinle thread of execution

What's "in" a process?

Consists of (at least)

• An address space, containing the code (instructions) and data of the running program
• (At least one) CPU state, consisting of the instruction pointer (EIP), stack pointer (ESP), and other general purpose registers.
• A set of OS resources, including open files, open pipes, open network connections, etc.

Address Space

Remember stack grows down (ie push decrements ESP, pop increments ESP). Stack contains all runtime frames.

A processes address space (idealized):

+----------------------+ <- 0x7FFFFFFF
|        Stack         |
|   (dynamic memory)   |
+----------------------+ <- ESP
|          |           |
|          v           |
|                      |
|          ^           |
|          |           |
+----------------------+
|        Heap          |
|   (dynamic memory)   |
+----------------------+
|        Data          |
|   (data segment)     |
+----------------------+
|        Code          | <- EIP
|   (text segment)     |
+----------------------+ 0x00000000

OS Process Namespace

• Each process has a unique identifier (PID)
• The PID namespace is global to the OS
• Operations that create processes (e.g. fork) returns the pid
• Operations on processes take pid as an argument (e.g. kill)