Administrivia

Web Pages

There is a web page for the course. You can find it from my home page.

• Can find these notes there. Let me know if you can't find it.
• They will be updated as bugs are found.
• Will also have each lecture available as a separate page. I will produce the page after the lecture is given. These individual pages might not get updated.

Textbook

Text is Tanenbaum, "Modern Operating Systems".

• Available in bookstore.
• We will do part 1, starting with chapter 1.

Computer Accounts and majordomo mailing list

• You are entitled to a computer account, get it.
• Sign up for majordomo mailing list for the course. But I use the web more. If you want to send mail to me, use gottlieb@nyu.edu not the mailing list.
• You may do assignments on any system you wish, but ...
  • You are responsible for the machine. I extend deadlines if the nyu machines are down, not if yours are.
  • Be sure to upload your assignments to the nyu systems.
    • If somehow your assignment is misplaced by me or a grader, we need a to have a copy ON AN NYU SYSTEM that can be used to verify the date the lab was completed.
    • When you complete a lab (and have it on an nyu system), do not edit those files. Indeed, put the lab in a separate directory and keep out of the directory. You do not want to alter the dates.

Homeworks and Labs

I make a distinction between homework and labs.

Labs are

• Required
• Due several lectures later (date given on assignment)
• Graded and form part of your final grade
• Penalized for lateness

• Optional
• Due the beginning of Next lecture
• Not accepted late
• Mostly from the book
• Collected and returned
• Can help, but not hurt, your grade

Upper left board for assignments and announcements.

Homework: Read Chapter 1 (Introduction)

Chapter 1. Introduction

Levels of abstraction (virtual machines)

• Software (and hardware, but that is not this course) is done 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.
• Alternatively said, each layer is written as though it runs a the virtual machine supplied by the lower layer and provides a more abstract (pleasent) virtual machine for the higher layer to run on.
• Using a broad brush, the layers are.
  1. Scripts (e.g. shell scripts)
  2. Applications and utilities
  3. Libraries
  4. The OS proper (the kernel)
  5. Hardware
• The kernel itself is itself normally layered, e.g.
  1. ...
  2. Filesystems
  3. Machine independent I/O
  4. Machine dependent device drivers
• The machine independent part is written assuming "virtual (i.e. idealized) hardware". Simply read 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 not clear how much is part of the OS.

1.1: What is an operating system?

The kernel itself raises the level of abstraction and hides details. Can write to a file (a concept not present in hardware) and ignore whether it is a floppy or hard disk.

The kernel is a resource manager (so users don't conflict).

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.

1.2 History of Operating Systems

1. Single user (no OS)
2. Batch, uniprogrammed, run to completion
3. Multiprogrammed
   • Overlap CPU and I/O
   • Multiple batches
     • IBM OS/MFT
     • IBM OS/MVT (then other names)
     • MVT is a more ``efficient'' user of resources but is more difficult.
     • When we study memory management, we will see that with varying size partitions questions like compaction and ``holes'' arise.
   • Time sharing
     • This is multiprogramming with rapid switching.
     • We will study scheduling when we do processor management
4. Multiple computers
   • Multiprocessors: Almost from the beginning of the computer age but now are not exotic.
   • Network OS: Make use of the multiple PCs/workstations on a LAN.
   • Distributed OS: A ``seamless'' version of above
5. Real time systems
   • Often in embedded systems
   • Soft vs hard real time. In the latter missing a deadline is a fatal error--sometimes literally.

1.3: Operating System Concepts

This will be brief. Much of the rest of the course will consist in ``filling in the details''.

1.3.1: Processes

A program in execution.

Often one distinguishes the state or context (memory image, open file) from the thread of control. Then if one has many threads running in the same task, the result is a ``multithreaded processes''.

The OS keeps information about all processes in the process table. Indeed, the OS views the process as the entry. An example of an active entity being viewed as a data structure (cf. discrete event simulations).

The set of processes forms a tree via the fork system call. The forker is the parent of the forkee.

A signal can be sent to a process to cause it to execute a predefined function (the signal handler). This can be tricky to program since the programmer does not know when in his ``main'' program the signal handler will be invoked.