Processes

Processes are operating system (OS) managed and in case of a modern OS they are truly concurrent in the presence of suitable hardware support (multiprocessor and multicore systems).

Processes are scheduled by the operating system's [scheduler](http://en.wikipedia.org/wiki/Scheduling_(computing\)). They may be made up of multiple [threads of execution](http://en.wikipedia.org/wiki/Thread_(computer_science\)) that execute instructions concurrently.

Processes exist within their own address space. No other process can read or write to another one's memory, because the OS secures this with process isolation as the OS is the one who manages the processes.

Processes are heavy and spawning many of them (in contrast to other concurrency models) is not recommended. Creating a process requires creating an entirely new virtual address space.

Context switching between processes requires a lot of work. This means saving of the entire CPU state (all processor registers that were in use, program counter, etc.) into PCB (usually). Context switching also involves switching the MMU context and keeping OS related data.

Processes can't talk to each other directly (in modern OS). Instead the OS provides facilities for inter-process communication.

Typical ways of inter-process communication involve files, [signals](http://en.wikipedia.org/wiki/Signal_(computing\)), sockets, message queues, (named) [pipes](http://en.wikipedia.org/wiki/Pipeline_(Unix\)), [semaphores](http://en.wikipedia.org/wiki/Semaphore_(programming\)), shared memory and even memory-mapped files.

Processes are all about preemptive multitasking (today) meaning that the OS decides when a process is preempted ("goes to sleep") and which process goes "alive" next.