Operating Systems

Start Lecture #1

Chapter -1: Administrivia

I start at -1 so that when we get to chapter 1, the numbering will agree with the text.

(-1).1: Contact Information

(-1).2: Course Web Page

There is a web site for the course. You can find it from my home page, which is http://cs.nyu.edu/~gottlieb

(-1).3: Textbook

The course text is Tanenbaum, "Modern Operating Systems", Third Edition (3e).

(-1).4: Computer Accounts and Mailman Mailing List

(-1).5: Grades

Grades are based on the labs and the final exam, with each very important. The weighting will be approximately
40%*LabAverage + 60%*FinalExam (but see homeworks below).

(-1).6: The Upper Left Board

I use the upper left board for lab/homework assignments and announcements. I should never erase that board. Viewed as a file it is group readable (the group is those in the room), appendable by just me, and (re-)writable by no one. If you see me start to erase an announcement, let me know.

I try very hard to remember to write all announcements on the upper left board and I am normally successful. If, during class, you see that I have forgotten to record something, please let me know. HOWEVER, if I forgot and no one reminds me, the assignment has still been given.

(-1).7: Homeworks and Labs

I make a distinction between homeworks and labs.

Labs are

Homeworks are

(-1).7.1: Homework Numbering

Homeworks are numbered by the class in which they are assigned. So any homework given today is homework #1. Even if I do not give homework today, the homework assigned next class will be homework #2. Unless I explicitly state otherwise, all homeworks assignments can be found in the class notes. So the homework present in the notes for lecture #n is homework #n (even if I inadvertently forgot to write it to the upper left board).

(-1).7.2: Doing Labs on non-NYU Systems

You may solve lab assignments on any system you wish, but ...

(-1).7.3: Obtaining Help with the Labs

Good methods for obtaining help include

  1. Asking me during office hours (see web page for my hours).
  2. Asking the mailing list.
  3. Asking another student, but ...
    Your lab must be your own.
    That is, each student must submit a unique lab. Naturally, simply changing comments, variable names, etc. does not produce a unique lab.

(-1).7.4: Computer Language Used for Labs

You may write your lab in Java, C, or C++.

(-1).8: A Grade of Incomplete

The new rules set by GSAS state:

  3.6.  Incomplete Grades: An unresolved grade, I, reverts to F one
        year after the beginning of the semester in which the course
        was taken unless an extension of the incomplete grade has been
        approved by the Vice Dean.
     3.6.1.  At the request of the departmental DGS and with the
             approval of the course instructor, the Vice Dean will
             review requests for an extension of an incomplete grade.
     3.6.2.  A request for an extension of incomplete must be
             submitted before the end of one year from the beginning
             of the semester in which the course was taken.
     3.6.3.  An extension of an incomplete grade may be requested for
             a period of up to, but not exceeding, one year
     3.6.4.  Only one one-year extension of an incomplete may be granted.
     3.6.5.  If a student is approved for a leave of absence (See 4.4)
             any time the student spends on that leave of absence will
             not count toward the time allowed for completion of the

(-1).9 Academic Integrity Policy

This email from the assistant director, describes the policy.

    Dear faculty,

    The vast majority of our students comply with the
    department's academic integrity policies; see


    Unfortunately, every semester we discover incidents in
    which students copy programming assignments from those of
    other students, making minor modifications so that the
    submitted programs are extremely similar but not identical.

    To help in identifying inappropriate similarities, we
    suggest that you and your TAs consider using Moss, a
    system that automatically determines similarities between
    programs in several languages, including C, C++, and Java.
    For more information about Moss, see:


    Feel free to tell your students in advance that you will be
    using this software or any other system.  And please emphasize,
    preferably in class, the importance of academic integrity.

    Rosemary Amico
    Assistant Director, Computer Science
    Courant Institute of Mathematical Sciences

(-1).10: An Introductory OS Course with a Programming Prerequisite

(-1).10.1: A Introductory Course ...

I do not assume you have had an OS course as an undergraduate, and I do not assume you have had extensive experience working with an operating system.

If you have already had an operating systems course, this course is probably not appropriate. For example, if you can explain the following concepts/terms, the course is probably too elementary for you.

(-1).10.2: ... with a Programming Prerequisite

I do assume you are an experienced programmer, at least to the extent that you are comfortable writing modest size (several hundred line) programs.

Chapter 0: Interlude on Linkers

Originally called a linkage editor by IBM.

A linker is an example of a utility program included with an operating system distribution. Like a compiler, the linker is not part of the operating system per se, i.e. it does not run in supervisor mode. Unlike a compiler it is OS dependent (what object/load file format is used) and is not (normally) language dependent.

0.1: What does a Linker Do?

Link of course.

When the compiler and assembler have finished processing a module, they produce an object module that is almost runnable. There are two remaining tasks to be accomplished before object modules can be run. Both are involved with linking (that word, again) together multiple object modules. The tasks are relocating relative addresses and resolving external references.

The output of a linker is called a load module because, with relative addresses relocated and the external addresses resolved, the module is ready to be loaded and run.


0.1.1: Relocating Relative Addresses

The compiler and assembler (mistakenly) treat each module as if it will be loaded at location zero.

To convert this relative address to an absolute address, the linker adds the base address of the module to the relative address. The base address is the address at which this module will be loaded.

How does the linker know that Module A is to be loaded starting at location 2300?

0.1.2: Resolving External Reverences

If a C (or Java, or Pascal, or ada, etc) program contains a function call
to a function f() that is compiled separately, the resulting object module must contain some kind of jump to the beginning of f.

0.1.3: An Example from the Lab

To see how a linker works lets consider the following example, which is the first dataset from lab #1. The description in lab1 is more detailed.

The target machine is word addressable and each word consists of 4 decimal digits. The first (leftmost) digit is the opcode and the remaining three digits form an address.

Each object module contains three parts, a definition list, a use list, and the program text itself. Each definition is a pair (sym, loc). Each use is a symbol that will be pointed to by the program text.

The program text consists of a count N followed by N pairs (type, word), where word is a 4-digit instruction described above and type is a single character indicating if the address in the word is Immediate, Absolute, Relative, or External.

The actions taken by the linker depend on the type of the instruction, as we now illustrate. Consider the first input set from the lab.

    Input set #1

    1 xy 2
    2 z 2 -1 xy 4 -1
    5 R 1004  I 5678  E 2000  R 8002  E 7001
    1 z 1 2 3 -1
    6 R 8001  E 1000  E 1000  E 3000  R 1002  A 1010
    1 z 1 -1
    2 R 5001  E 4000
    1 z 2
    2 xy 2 -1 z 1 -1
    3 A 8000  E 1001  E 2000

The first pass simply finds the base address of each module and produces the symbol table giving the values for xy and z (2 and 15 respectively). The second pass does the real work using the symbol table and base addresses produced in pass one.

The resulting output (shown below) is more detailed than I expect you to produce. The detail is there to help me explain what the linker is doing. All I would expect from you is the symbol table and the rightmost column of the memory map.

    Symbol Table

    0:       R 1004      1004+0 = 1004
    1:       I 5678               5678
    2: xy:   E 2000 ->z           2015
    3:       R 8002      8002+0 = 8002
    4:       E 7001 ->xy          7002
    0        R 8001      8001+5 = 8006
    1        E 1000 ->z           1015
    2        E 1000 ->z           1015
    3        E 3000 ->z           3015
    4        R 1002      1002+5 = 1007
    5        A 1010               1010
    0        R 5001      5001+11= 5012
    1        E 4000 ->z           4015
    0        A 8000               8000
    1        E 1001 ->z           1015
    2 z:     E 2000 ->xy          2002

You must process each module separately, i.e. except for the symbol table and memory map your space requirements should be proportional to the largest module not to the sum of the modules. This does NOT make the lab harder.

Remark: It is faster (less I/O) to do a one pass approach, but is harder since you need fix-up code whenever a use occurs in a module that precedes the module with the definition.

The linker on unix was mistakenly called ld (for loader), which is unfortunate since it links but does not load.

Historical remark: Unix was originally developed at Bell Labs; the seventh edition of unix was made publicly available (perhaps earlier ones were somewhat available). The 7th ed man page for ld begins (see http://cm.bell-labs.com/7thEdMan).
      .TH LD 1
      .SH NAME
      ld \- loader
      .B ld
      [ option ] file ...
      .I Ld
      combines several
      object programs into one, resolves external
      references, and searches libraries.

By the mid 80s the Berkeley version (4.3BSD) man page referred to ld as link editor and this more accurate name is now standard in unix/linux distributions.

During the 2004-05 fall semester a student wrote to me

BTW - I have meant to tell you that I know the lady who wrote ld. She told me that they called it loader, because they just really didn't have a good idea of what it was going to be at the time.

The wikipedia reference

Lab #1: Implement a two-pass linker. The specific assignment is detailed on the class home page.

Chapter 1: Introduction

Homework: Read Chapter 1 (Introduction)

Levels of abstraction (virtual machines)

Software (and hardware, but that is not this course) is often implemented in layers. The higher layers use the facilities provided by lower layers.

Alternatively said, the upper layers are written using a more powerful and more abstract virtual machine than the lower layers.

In other words, each layer is written as though it runs on the virtual machine supplied by the lower layers and in turn provides a more abstract (pleasant) virtual machine for the higher layers to run on.

Using a broad brush, the layers are.

  1. Applications (e.g., web browser) and utilities (e.g., compiler, linker).
  2. User interface (UI). It may be text oriented (Unix/Linux shell, DOS prompt) or graphical (GUI, e.g., MS Windows, Gnome/KDE, MAC).
  3. Libraries (e.g., libc).
  4. The OS proper (the kernel).
  5. Hardware.

An important distinction is that the kernel runs in privileged/kernel/supervisor mode); whereas compilers, editors, shell, linkers. browsers etc run in user mode.

The kernel itself is itself normally layered, e.g.

  1. Filesystems
  2. Machine independent I/O
  3. Machine dependent device drivers

The machine independent I/O part is written assuming virtual (i.e. idealized) hardware. For example, the machine independent I/O portion simply reads a block from a disk. But in reality one must deal with the specific disk controller.

Often the machine independent part is more than one layer.

The term OS is not well defined. Is it just the kernel? How about the libraries? The utilities? All these are certainly system software but it is not clear how much is part of the OS.

1.1: What is an operating system?

As mentioned above, the OS raises the abstraction level by providing a higher level virtual machine. A second related key objective for the OS is to manage the resources provided by this virtual machine.

1.1.1: The Operating System as an Extended Machine

The kernel itself raises the level of abstraction and hides details. For example a user (of the kernel) can write to a file (a concept not present in hardware) and ignore whether the file resides on a floppy, a CD-ROM, or a hard disk. The user can also ignore issues such as whether the file is stored contiguously or is broken into blocks.

Well designed abstractions are a key to managing complexity.

1.1.2: The Operating System as a Resource Manager

The kernel must manage the resources to resolve conflicts between users. Note that when we say users, we are not referring directly to humans, but instead to processes (typically) running on behalf of humans.

Typically the resource is shared or multiplexed between the users. This can take the form of time-multiplexing, where the users take turns (e.g., the processor resource) or space-multiplexing, where each user gets a part of the resource (e.g., a disk drive).

With sharing comes various issues such as protection, privacy, fairness, etc.

Homework: What are the two main functions of an operating system?

How is an OS fundamentally different from a compiler (say)?

Answer: Concurrency! Per Brinch Hansen in Operating Systems Principles (Prentice Hall, 1973) writes.

The main difficulty of multiprogramming is that concurrent activities can interact in a time-dependent manner, which makes it practically impossibly to locate programming errors by systematic testing. Perhaps, more than anything else, this explains the difficulty of making operating systems reliable.

Homework: 1. (unless otherwise stated, problems numbers are from the end of the current chapter in Tanenbaum.)

1.2: History of Operating Systems

The subsection heading describe the hardware as well as the OS; we are naturally more interested in the latter. These two development paths are related as the improving hardware enabled the more advanced OS features.

1.2.1: The first Generation (1945-55) Vacuum Tubes (and No OS)

One user (program; perhaps several humans) at a time. Although this time frame predates my own usage, computers without serious operating systems existed during the second generation and were now available to a wider (but still very select) audience.

I have fond memories of the Bendix G-15 (paper tape) and the IBM 1620 (cards; typewriter; decimal). During the short time you had the machine, it was truly a personal computer.

1.2.2: The Second Generation (1955-65) Transitors and Batch Systems

Many jobs were batched together, but the systems were still uniprogrammed, a job once started was run to completion without interruption and then flushed from the system.

A change from the previous generation is that the OS was not reloaded for each job and hence needed to be protected from the user's execution. Previously, the beginning of your job contained the trivial OS-like support features.

The batches of user jobs were prepared offline (cards to magnetic tape) using a separate computer (an IBM 1401 with a 1402 card reader/punch). The tape was brought to the main computer (an IBM 7090/7094) where the output to be printed was written on another tape. This tape went back to the 1401 and was printed on a 1403.

1.2.3: The Third Generation (1965-1980) and Multiprogramming

In my opinion this is the biggest change from the OS point of view. It is with multiprogramming (many processes executing concurrently) that we have the operating system fielding requests whose arrival order is non-deterministic (at the pragmatic level). Now operating systems become notoriously hard to get right due to the inability to test a significant percentage of the possible interactions and the inability to reproduce bugs on request.

The Purpose of Multiprogramming

The purpose of multiprogramming is to overlap CPU and I/O activity and thus greatly improve CPU utilization. Recall that these computers, in particular the processors, were very expensive.

Multiple Batch Streams

Time Sharing

This is multiprogramming with rapid switching between jobs (processes) and with individual users spooling their own jobs on a remote terminal. Deciding when to switch and which process to switch to is called scheduling.

We will study scheduling when we do processor management.

1.2.4: The Fourth Generation (1980-Present) Personal Computers

Serious PC Operating systems such as Unix/Linux, Windows NT/2000/XP/Vista/7 and (the newer) MacOS are multiprogrammed OSes.

GUIs have become important. What is not clear is whether the GUI should be part of the kernel.

Early PC operating systems were uniprogrammed and their direct descendants lasted for quite some time (e.g., Windows ME), but now all (non-embedded) OS are multiprogrammed.

Homework: Why was timesharing not widespread on second generation computers?

Homework: 2.

Remark: I very much recommend reading all of 1.2, not for this course especially, but for general interest. Tanenbaum writes well and is my age so lived through much of the history himself.

1.3: Computer Hardware Review

The picture above is very simplified. (For one thing, today separate buses are used to Memory and Video.)

A bus is a set of wires that connect two or more devices. Only one message can be on the bus at a time. All the devices receive the message: There are no switches in between to steer the message to the desired destination, but often some of the wires form an address that indicates which devices should actually process the message.

1.3.1: Processors

Only at a few points will we get into sufficient detail to need to understand the various processor registers such as program counter (a.k.a, instruction pointer), stack pointers, and Program Status Words (PSWs). We will ignore computer design issues such as pipelining and superscalar.

We do, however, need the notion of a trap, that is an instruction that atomically switches the processor into privileged mode and jumps to a pre-defined physical address. We will have much more to say about traps later in the course.

Multithreaded and Multicore Chips

Many of the OS issues introduced by multi-processors of any flavor are also found in a uni-processor, multi-programmed system. In particular, successfully handling the concurrency offered by the second class of systems, goes a long way toward preparing for the first class. The remaining multi-processor issues are not covered in this course.

1.3.2: Memory

We will ignore caches, but will (later) discuss demand paging, which is very similar (although demand paging and caches use largely disjoint terminology). In both cases, the goal is to combine large, slow memory with small, fast memory to achieve the effect of large, fast memory. We cover caches in our computer design (aka architecture) courses (you can access my class notes off my home page).

The central memory in a system is called RAM (Random Access Memory). A key point is that it is volatile, i.e. the memory loses its data if power is turned off.


ROM (Read Only Memory) is used for (low-level control) software that often comes with devices on general purpose computers, and for the entire software system on non-user-programmable devices such as microwaves and wristwatches. It is also used for non-changing data. A modern, familiar ROM is CD-ROM (or the denser DVD, or the even denser Blu-ray). ROM is non-volatile.

But often this unchangable data needs to be changed (e.g., to fix bugs). This gives rise first to PROM (Programmable ROM), which, like a CD-R, can be written once (as opposed to being mass produced already written like a CD-ROM), and then to EPROM (Erasable PROM), which is like a CD-RW. Early EPROMs needed UV light for erasure; EEPROM, Electrically EPROM or Flash RAM) can be erased by normal circuitry, which is much more convenient.

Memory Protection and Context Switching

As mentioned above when discussing OS/MFT and OS/MVT, multiprogramming requires that we protect one process from another. That is we need to translate the virtual addresses of each program into physical addresses such that, at any point in time, the physical address of each process are disjoint. The hardware that performs this translation is called the MMU or Memory Management Unit.

When context switching from one process to another, the translation must change, which can be an expensive operation.

1.3.3: Disks

When we do I/O for real, I will show a real disk opened up and illustrate the components

Devices are often quite difficult to manage and a separate computer, called a controller, is used to translate OS commands into what the device requires.

1.3.4: Tapes

The bottom of the memory hierarchy, tapes have large capacities, tiny cost per byte, and very long access times.