Lecture 3: Processes

• What is a process? (chapter 4, section 4.1, 4.2 and 4.3)
• a basic way to describe the activities in the computer
• similar terms: "job", "task"
• formal definition: a process is an execution of a program
• example: if you type "vi", and then type "ps", you will see:
                            PID       TT    State    TIME    COMMAND
                            8413    pts/0    S          0:00      vi course-notes
• one user can have many different processes:

example: you can have multiple shells running;
                PID        TT        State        TIME        COMMAND
               596        pts/1       S             0:01            tcsh
               3928      pts/3       S             0:04            tcsh
               7336      pts/4       S             0:02            tcsh
• multiple users can run the same program:

example:
             USER    PID        TT        State        TIME       COMMAND
               cao        596        pts/1       S             0:01            tcsh
               lfan        6717      pts/6       S             0:04            tcsh
• what happens when the same program is run in different processes:

- the program is the same, that is, the "text" sections are  the same;
- the data sections are different;
- the program counters are different;
• analogy with real-life situation: playing a cassette in the walkman, or a CD in the CD player;
• also analogous to real-life situation: processes are created, can be stopped, can be resumed, and terminate (well, most of the time);
• Process states:
• ready, running, blocked;
• new, terminated;
• transition between states:

- during creation, in the "new" state;
- once created, in the "ready" state;
- if chosen to be run on the CPU, in the "running" state;
- if stopped for some reason, in the "blocked" state;
- once finishes, in the "terminated" state;
 
• CPU scheduling:

- the "ready" queue;
- the "blocked" queue;
- simple round-robin: once a clock interrupt occurs, move the current running process back to the ready queue, and move the process at the head of the ready queue to run on the CPU;
 
• Three types of interrupts:

- clock interrupt: occurs every 10ms, moves processes between "running" and "ready" states;
- system call interrupts: can potentially move the running process from "running" to "blocked" state;
- device interrupts: if an I/O request is finished, the interrupt handler moves the process waiting for the I/O request from the "blocked" state to the "ready" state;
- clock interrupts and device interrupts are also called "hardware interrupts";
- system call interrrupts are also called "software interrupts";
 
• What constitute a process:
• program counter, and CPU registers;
• user address space: text, data, heap, stack;
• 32-bit address space;
• infinite recursion will exhaust process address space; (review on procedure calling conventions)
• file descriptors and file pointers: for reading and writing files, and for network communications; standard input, standard output, and standard error;
• accounting information;
• I/O request status;