OPERATING SYSTEM

The (sw) LAYER of a computer system between the HW and user programs which--
i) CONTROLS ALL OF THE COMPUTER'S RESOURCES, and
ii) PROVIDES AN INTERFACE to give user processes access to the functionality of the computer.

Transforming a physical resource w/ a complex interface into a "virtual resource" which has full functionality but has a simpler interface.

Creating the illusion of several resources from 1 resource

Using a resource for different virtual computers at different times (ex using time slices by CPU)

Dividing a resource into smaller versions of itself and each virtual computer is given a part of the resource (ex many processes sharing memory at the same time).

Deciding which processes should get the resources they need and when they should get them

Calls provided to user processes (by the OS) to provide an interface btw the OS and applications and to protect system resources fro user processes.

Having a single cpu appear as many cpu's by using time-slices and sharing the cpu with multiple processes.

Dividing physical memory into pieces so that entire programs need not reside in memory at the same time and giving the illusion that the computer has more memory than it really does--thus, allowing many processes to share memory at the same time.

Allowing the user to see a hierarchical file system with files of any size appearing to be kept "together" in one place when, in fact, the physical computer stores files block by block.  This hides the details of where the file is stored (track, sector, ...) from the user.

Hiding the gory details of I/O from the user by providing device independence and allowing the user to use such commands as "inFile >> num1;"

Storing more than one program in memory at a time, thus allowing many processes to share memory.

Those running on different clocks at different speeds and, perhaps but not necessarily, needing occasional synchronization and
cooperation.

            Continually checking to see if an event has occurred.  Wasteful if done by the cpu.

MULTITASKING

Allowing more than one process to run "at a time", thereby allowing the user to switch back and forth between applications.

One in which processes are parceled out into sub-processes or sub-computations t be executed on other processors, requiring a much more complicated OS.

    A "program in execution", which includes the following info:  the executable program, its data and stack, its program counter, the stack pointer and the content of other registers (saved when the process is interrupted), and all other info needed to run the program.

The command interpreter of the OS whose job it is to read and interpreet user commands from the terminal.

In UNIX, the way a process is created.  "fork()" creates a copy (except for the text segment) of the original process for the child process.  The parent and child processes then go their separate ways, although the parent has a "wait" system call available to it to wait for the child process to terminate if required.

SIGNAL

A means for a process to communicate with another process in special unexpected circumstances.  (ex:  to "kill" a process in UNIX).

Attempt by the OS to hide the details of file handling from the user by providing commands that hide these details and, in UNIX, for example, treating devices just like files (ex:  "/dev/tty4" is the file name for terminal #4.)

Not really files!  Provided to make I/O devices look like files.  Allows use of the same system calls to read/write from/to devices as to read/write from/to regular files.  Ex:  BLOCK special files (model disks and other randomly accessible devices) and CHARACTER special files (model printers, terminals, modems, and other devices that accept or output character streams).

A pseudo-file that connects two processes together, allowing one process to send its output as input to another process as input.
Ex:    ls -al | more

Moving as much code as possible (all but that which really needs direct access to the hardware) up into higher layers from the kernel, thus, yielding a minimal kernel and a group of processes ("servers", running as user processes) ) to handle requests.  (Advantage:  Eachpart of the OS is smaller and more manageable (modular!) ; if one server goes down the others aren't affected; adaptable for distributed systems.)