Operating System

1.5.2: Deadlocks

A set of processes each of which is blocked by a process in the set. The automotive equivalent, shown at right, is gridlock.

1.5.3: Memory Management

Each process requires memory. The loader produces a load module that assumes the process is loaded at location 0. The operating system ensures that the processes are actually given disjoint memory. Current operating systems permit each process to be given more (virtual) memory than the total amount of (real) memory on the machine.

1.5.4: Input/Output

There are a wide variety of I/O devices that the OS must manage. For example, if two processes are printing at the same time, the OS must not interleave the output. The OS contains device specific code (drivers) for each device as well as device-independent I/O code.

1.5.5: Files

Modern systems have a hierarchy of files. A file system tree.

• In MSDOS the hierarchy is a forest not a tree. There is no file, or directory that is an ancestor of both a:\ and c:\.
• In unix the existence of (hard) links weakens the tree to a DAG.
• Unix also has symbolic links, which when used indiscriminately, permit directed cycles (i.e., the result is not a DAG).

You can name a file via an absolute path starting at the root directory or via a relative path starting at the current working directory.

In addition to regular files and directories, Unix also uses the file system namespace for devices (called special files, which are typically found in the /dev directory. Often utilities that are normally applied to (ordinary) files can be applied as well to some special files. For example, when you are accessing a unix system using a mouse and do not have anything serious going on (e.g., right after you log in), type the following command

    cat /dev/mouse

Before a file can be accessed, it must be opened and a file descriptor obtained. Many systems have standard files that are automatically made available to a process upon startup. These (initial) file descriptors are fixed

• standard input: fd=0
• standard output: fd=1
• standard error: fd=2

A convenience offered by some command interpretors is a pipe or pipeline. The pipeline

  dir | wc

1.5.6: Security

Files and directories normally have permissions

• Normally have at least rwx.
• User, group, world
• A more general mechanism is an access control lists.
• Often files have ``attributes'' as well. For example the linux ext2 file system supports a ``d'' attribute that is a hint to the dump program not to backup this file.
• When a file is opened, permissions are checked and, if the open is permitted, a file descriptor is returned that is used for subsequent operations

1.5.7: The Shell or Command Interpreter (DOS Prompt)

The command line interface to the operating system. The shell permits the user to

• Invoke commands
• Pass arguments to the commands
• Redirect the output of a command to a file or device
• Pipe one command to another (as illustrated above via ls | wc)

Homework: 8

1.6: System Calls

System calls are the way a user (i.e., a program) directly interfaces with the OS. Some textbooks use the term envelope for the component of the OS responsible for fielding system calls and dispatching them. On the right is a picture showing some of the OS components and the external events for which they are the interface.

Note that the OS serves two masters. The hardware (below) asynchronously sends interrupts and the user makes system calls and generates page faults.

Homework: 14
What happens when a user executes a system call such as read()? We show a more detailed picture below, but at a high level what happens is

1. Normal function call (in C, Ada, Pascal, etc.).
2. Library routine (in C).
3. Small assembler routine.
   1. Move arguments to predefined place (perhaps registers).
   2. Poof (a trap instruction) and then the OS proper runs in supervisor mode.
   3. Fix up result (move to correct place).

The following actions occur when the user executes the (Unix) system call

count = read(fd,buffer,nbytes)

1. Push third parameter on to the stack.
2. Push second parameter on to the stack.
3. Push first parameter on to the stack.
4. Call the library routine, which involves pushing the return address on to the stack and jumping to the routine.
5. Machine/OS dependent actions. One is to put the system call number for read in a well defined place, e.g., a specific register. This requires assembly language.
6. Trap to the kernel (assembly language). This enters the operating system proper and shifts the computer to privileged mode.
7. The envelope uses the system call number to access a table of pointers to find the handler for this system call.
8. The read system call handler processes the request (see below).
9. Some magic instruction returns to user mode and jumps to the location right after the trap.
10. The library routine returns (there is more; e.g., the count must be returned).
11. The stack is popped (ending the function call read).

A major complication is that the system call handler may block. Indeed for read it is likely. In that case a switch occurs to another process. This is far from trivial and is discussed later in the course.

Process Management
Posix	Win32	Description
Fork	CreateProcess	Clone current process
exec(ve)		Replace current process
waid(pid)	WaitForSingleObject	Wait for a child to terminate.
exit	ExitProcess	Terminate current process & return status
File Management
Posix	Win32	Description
open	CreateFile	Open a file & return descriptor
close	CloseHandle	Close an open file
read	ReadFile	Read from file to buffer
write	WriteFile	Write from buffer to file
lseek	SetFilePointer	Move file pointer
stat	GetFileAttributesEx	Get status info
Directory and File System Management
Posix	Win32	Description
mkdir	CreateDirectory	Create new directory
rmdir	RemoveDirectory	Remove empty directory
link	(none)	Create a directory entry
unlink	DeleteFile	Remove a directory entry
mount	(none)	Mount a file system
umount	(none)	Unmount a file system
Miscellaneous
Posix	Win32	Description
chdir	SetCurrentDirectory	Change the current working directory
chmod	(none)	Change permissions on a file
kill	(none)	Send a signal to a process
time	GetLocalTime	Elapsed time since 1 jan 1970

A Few Important Posix/Unix/Linux and Win32 System Calls

The table on the right shows some systems calls; the descriptions are accurate for Unix and close for win32. To show how the four process management calls enable much of process management, consider the following highly simplified shell.

while (true)
    display_prompt()
    read_command(command)

    if (fork() != 0)  // true in parent false in child
        waitpid(...)
    else
        execve(command) // the command itself executes exit()
    endif
endwhile

Homework: 18.

1.7: OS Structure

I must note that Tanenbaum is a big advocate of the so called microkernel approach in which as much as possible is moved out of the (supervisor mode) kernel into separate processes. The (hopefully small) portion left in supervisor mode is called a microkernel.

In the early 90s this was popular. Digital Unix (now called True64) and Windows NT are examples. Digital Unix is based on Mach, a research OS from Carnegie Mellon university. Lately, the growing popularity of Linux has called into question the belief that ``all new operating systems will be microkernel based''.

1.7.1: Monolithic approach

The previous picture: one big program

The system switches from user mode to kernel mode during the poof and then back when the OS does a ``return''.

But of course we can structure the system better, which brings us to.

1.7.2: Layered Systems

Some systems have more layers and are more strictly structured.

An early layered system was ``THE'' operating system by Dijkstra. The layers were.

1. The operator
2. User programs
3. I/O mgt
4. Operator-process communication
5. Memory and drum management

The layering was done by convention, i.e. there was no enforcement by hardware and the entire OS is linked together as one program. This is true of many modern OS systems as well (e.g., linux).

The multics system was layered in a more formal manner. The hardware provided several protection layers and the OS used them. That is, arbitrary code could not jump to or access data in a more protected layer.

1.7.3: Virtual Machines

Use a ``hypervisor'' (beyond supervisor, i.e. beyond a normal OS) to switch between multiple Operating Systems. Made popular by IBM's VM/CMS

• Each App/CMS runs on a virtual 370.
• CMS is a single user OS.
• A system call in an App traps to the corresponding CMS.
• CMS believes it is running on the machine so issues I/O. instructions but ...
• ... I/O instructions in CMS trap to VM/370.
• This idea is still used. A modern version (used to ``produce'' a multiprocessor from many uniprocessors) is ``Cellular Disco'', ACM TOCS, Aug. 2000.
• Another modern usage is JVM the ``Java Virtual Machine''.

1.7.4: Exokernels

Similar to VM/CMS but the virtual machines have disjoint resources (e.g., distinct disk blocks) so less remapping is needed.

1.7.5: Client Server

When implemented on one computer, a client server OS is using the microkernel approach in which the microkernel just supplies interprocess communication and the main OS functions are provided by a number of separate processes.

This does have advantages. For example an error in the file server cannot corrupt memory in the process server. This makes errors easier to track down.

But it does mean that when a (real) user process makes a system call there are more processes switches. These are not free.

A distributed system can be thought of as an extension of the client server concept where the servers are remote.

Homework: 23

Chapter 2: Process and Thread Management

Tanenbaum's chapter title is ``Processes and Threads''. I prefer to add the word management. The subject matter is processes, threads, scheduling, interrupt handling, and IPC (InterProcess Communication--and Coordination).

2.1: Processes

Definition: A process is a program in execution.

• We are assuming a multiprogramming OS that can switch from one process to another.
• Sometimes this is called pseudoparallelism since one has the illusion of a parallel processor.
• The other possibility is real parallelism in which two or more processes are actually running at once because the computer system is a parallel processor, i.e., has more than one processor.
• We do not study real parallelism (parallel processing, distributed systems, multiprocessors, etc) in this course.

2.1.1: The Process Model

Even though in actuality there are many processes running at once, the OS gives each process the illusion that it is running alone.

• Virtual time: The time used by just this processes. Virtual time progresses at a rate independent of other processes. Actually, this is false, the virtual time is typically incremented a little during systems calls used for process switching; so if there are more other processors more ``overhead'' virtual time occurs.
  
  
• Virtual memory: The memory as viewed by the process. Each process typically believes it has a contiguous chunk of memory starting at location zero. Of course this can't be true of all processes (or they would be using the same memory) and in modern systems it is actually true of no processes (the memory assigned is not contiguous and does not include location zero).

  Think of the individual modules that are input to the linker. Each numbers its addresses from zero; the linker eventually translates these relative addresses into absolute addresses. That is the linker provides to the assembler a virtual memory in which addresses start at zero.

Virtual time and virtual memory are examples of abstractions provided by the operating system to the user processes so that the latter ``sees'' a more pleasant virtual machine than actually exists.

2.1.2: Process Creation

From the users or external viewpoint there are several mechanisms for creating a process.

1. System initialization, including daemon processes.
2. Execution of a process creation system call by a running process.
3. A user request to create a new process.
4. Initiation of a batch job.

But looked at internally, from the system's viewpoint, the second method dominates. Indeed in unix only one process is created at system initialization (the process is called init); all the others are children of this first process.

Why have init? That is why not have all processes created via method 2?
Ans: Because without init there would be no running process to create any others.

2.1.3: Process Termination

Again from the outside there appear to be several termination mechanism.

1. Normal exit (voluntary).
2. Error exit (voluntary).
3. Fatal error (involuntary).
4. Killed by another process (involuntary).

And again, internally the situation is simpler. In Unix terminology, there are two system calls kill and exit that are used. Kill (poorly named in my view) sends a signal to another process. If this signal is not caught (via the signal system call) the process is terminated. There is also an ``uncatchable'' signal. Exit is used for self termination and can indicate success or failure.

2.1.4: Process Hierarchies

Modern general purpose operating systems permit a user to create and destroy processes.

• In unix this is done by the fork system call, which creates a child process, and the exit system call, which terminates the current process.
• After a fork both parent and child keep running (indeed they have the same program text) and each can fork off other processes.
• A process tree results. The root of the tree is a special process created by the OS during startup.
• A process can choose to wait for children to terminate. For example, if C issued a wait() system call it would block until G finished.

Old or primitive operating system like MS-DOS are not multiprogrammed, so when one process starts another, the first process is automatically blocked and waits until the second is finished.

2.1.5: Process States and Transitions

The diagram on the right contains much information.

• Consider a running process P that issues an I/O request
  • The process blocks
  • At some later point, a disk interrupt occurs and the driver detects that P's request is satisfied.
  • P is unblocked, i.e. is moved from blocked to ready
  • At some later time the operating system looks for a ready job to run and picks P.
  
  
• A preemptive scheduler has the dotted line preempt;
  A non-preemptive scheduler doesn't.
  
  
• The number of processes changes only for two arcs: create and terminate.
  
  
• Suspend and resume are medium term scheduling
  • Done on a longer time scale.
  • Involves memory management as well.
  • Sometimes called two level scheduling.