1. The Abstraction: The Process
COMP 755
The Abstraction: The Process

2. Process
The most fundamental abstractions that the OS provides to users: the process.
A process is a running program.
To view all running processes two Linux commands are commonly used: ps –aux and top (real time, use ctrl C to stop)

3. HOW TO PROVIDE THE ILLUSION OF MANY CPUS?
Answer: The OS creates this illusion by virtualizing the CPU. By running one process, then stopping it and running another, and so forth, the OS can promote the illusion that many virtual CPUs exist when in fact there is only one physical CPU (or a few).
We call this time sharing… time sharing of the CPU, allows users to run as many concurrent processes as they would like; the potential cost is performance, as each will run more slowly if the CPU(s) must be shared.

4. Terms
machine state: what a program can read or update when it is running.
address space: the memory the process can address
registers: process registers it can access
program counter (PC) tells us which instruction of the program is currently being executed
stack pointer and associated frame pointer are used to manage the stack for function parameters, local variables, and return addresses.

5. Overview of Process API
Create: An operating system must include some method to create
new processes.
Destroy: As there is an interface for process creation, systems also
provide an interface to destroy processes forcefully.
Wait: Sometimes it is useful to wait for a process to stop running
Miscellaneous Control: suspend and resume process
Status: There are usually interfaces to get some status information
about a process as well, such as how long it has run for, or what
state it is in.

6. Process States
Running: In the running state, a process is running on a processor.
This means it is executing instructions.
• Ready: In the ready state, a process is ready to run but for some
reason the OS has chosen not to run it at this given moment.
• Blocked: In the blocked state, a process has performed some kind
of operation that makes it not ready to run until some other event
takes place. A common example: when a process initiates an I/O
request to a disk, it becomes blocked and thus some other process
can use the processor.

7. 3 State Model

8. 5 state model

9. Process States Running: The process is being executed
Ready: The process is prepared to execute when given the opportunity
Blocked: The process can not execute until some event occurs, such as the completion of an I/O operation
New: The OS is still creating the data structures and allocating memory for the process.
Exit: The process is no longer eligible for execution. The OS is in the process of releasing the resources allocated to this process.

10. Process Transitions
New  Ready: When a process becomes ready to be executed.
Ready  Running: When the CPU is available, the dispatcher selects a new process to run
Running  Ready: The running process has run long enough for now; the running process gets interrupted because a higher priority process is in the ready state (preempted)
Running  Blocked: When a process requests something for which it must wait (e.g., initiates I/O, wait for input from another process)
Blocked  Ready: When the event for which it was waiting occurs
Running  Exit: A process is done or has failed.