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NOTE:  These class notes are now on the web
http://allan.ultra.nyu.edu/gottlieb/os/class-notes

HOMEWORK 3.8

System structure

    As we said before layering used to raise level of abstraction.

    Book give examples.

        MS-DOS and (orig) unix are simple

Virtual Machines

    Give each process the illusion it has an entire machine

    Supply devices not present (assuming you program at a low level)
        Ask for a specific device get a virtual one

    Useful for debugging operating systems

        New OS runs in virtual supervisor mode

    Emulate one system on top of another

        intel on alpha

        win 3.1 on win 95

System design goals

    At highest level the goals are vague and like motherhood

        fast, robust, simple, cheap, correct, convenient, ...

    At more detailed level get hard tradeoffs

Mechanisms and policy

    how vs what

    policy: users cannot delete the OS

    mechanism 1: file permissions

    mechanism 2: ROM

    micro-kernel provides (low-level) mechanism

    higher level processes implement policy

HOMEWORK 3.11

Nowadays mostly written in "high-level" languages

System Generation

    Configuration

    Bootstrap loading

---------- Part II, i.e. the "real course" ------

Chapter 4 Process management

Process is an active entity.  A program in execution.

    Terminology not standard (sadly).

        Will try to use  process = task + thread(s)

Standard picture with running/ready/waiting (blocked)

    start / terminate

    suspended (medium term sched)

Process control block PCB

    Active entity now view as passive data

        When the process moves from blocked on a device to ready
        the PCB moves from the corresp device Q to the ready Q.

    accting/sched/mem info    

    why the process is blocked

    process state (registers, condition codes, etc)

Process switch

    also called context switch

    Big time state change.

    user --> kernel --> user

    in kernel 

        save state of old process in PCB

        decide on new process to run

            this is called process scheduling.

                really short term scheduling (if on ready Q)

                Also have med term sched (degree of multiprogramming)

                Also have long term job scheduling.

            we will study scheduling policies soon

        restore state from pcb

HOMEWORK 4.1, 4.2

Process Creation

    We did the unix fork/exec/wait/exit

        Parent can wait or keep executing (& in shell)

    VMS has "spawn" (fork+exec): new process is not copy of old

    Win/NT has both

    Get a tree of processes

    Termination

        What to do if parent terminates before child?

        Book is wrong about unix.  Init inherits the children

Process cooperation (coordination)

    A big topic that is one of my very favorites.

    Trivial example.  Bounded buffer with ONE producer and ONE consumer

        This is shared memory communication

		    shared in, out, buffer[n]

                    loop -- producer
			 produce an item
			 while (in+1 mod n) = out do nothing;
			 buffer[in] = item;
			 in = in+1 mod n;

		    loop -- consumer
			 while in = out do nothing;
			 item = buffer[out]
			 out = out+1 mod n;
			 consume the item 

	Fails for multiple consumers or producers (in=in+1)

    Only permits n-1 items in buffer[n].  So useless for n=1

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