Operating Systems

Start Lecture #3

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A set of processes is deadlocked if each of the processes is blocked by a process in the set. The automotive equivalent, shown below, is called gridlock. (The photograph below was sent to me by Laurent Laor.)


1.5.2 Address Spaces

Clearly, each process requires memory, but there are other issues as well. For example, your linkers (will) produce a load module that assumes the process is loaded at location 0. The result would be that every load module has the same address space. The operating system must ensure that the address spaces of concurrently executing processes are assigned disjoint real memory.

For another example note that current operating systems permit each process to be given more (virtual) memory than the total amount of (real) memory on the machine.

1.5.3 Files

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

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 Unix, one file system can be mounted on (attached to) another. When this is done, access to an existing directory on the second filesystem is temporarily replaced by the entire first file system. Most often the directory chosen is empty before the mount so no files become temporarily invisible.

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
and then move the mouse. You kill the cat by typing cntl-C. I tried this on my linux box (using a text console) and no damage occurred. Your mileage may vary.

Before a file can be accessed, it must be opened and a file descriptor obtained. Subsequent I/O system calls (e.g., read and write) use the file descriptor rather that the file name. This is an optimization that enables the OS to find the file once and save the information in a file table accessed by the file descriptor. Many systems have standard files that are automatically made available to a process upon startup. These (initial) file descriptors are fixed.

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

    dir | wc
which pipes the output of dir into a character/word/line counter, will give the number of files in the directory (plus other info).

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 (really each controller) as well as device-independent I/O code.

1.5.6 Protection

Files and directories have associated permissions.

Memory assigned to a process, i.e., an address space, must also be protected.


Security has of course sadly become a very serious concern. The topic is quite deep and I do not feel that the necessarily superficial coverage that time would permit is useful so we are not covering the topic at all.

1.5.7 The Shell or Command Interpreter (DOS Prompt)

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

Instead of a shell, one can have a more graphical interface.

Homework: 7.

Ontogeny Recapitulates Phylogeny

Some concepts become obsolete and then reemerge due in both cases to technology changes. Several examples follow. Perhaps the cycle will repeat with smart card OS.

Large Memories (and Assembly Language)

The use of assembly languages greatly decreases when memories get larger. When minicomputers and microcomputers (early PCs) were first introduced, they each had small memories and for a while assembly language again became popular.

Protection Hardware (and Monoprogramming)

Multiprogramming requires protection hardware. Once the hardware becomes available monoprogramming becomes obsolete. Again when minicomputers and microcomputers were introduced, they had no such hardware so monoprogramming revived.

Disks (and Flat File Systems)

When disks are small, they hold few files and a flat (single directory) file system is adequate. Once disks get large a hierarchical file system is necessary. When mini and microcomputer were introduced, they had tiny disks and the corresponding file systems were flat.

Virtual Memory (and Dynamically Linked Libraries)

Virtual memory, among other advantages, permits dynamically linked libraries so as VM hardware appears so does dynamic linking.

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 to the appropriate component of the OS. 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 (at the bottom) asynchronously sends interrupts and the user (at the top) synchronously invokes 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, Java, etc.).
  2. Library routine (probably in the C, or similar, language).
  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)
which reads up to nbytes from the file described by fd into buffer. The actual number of bytes read is returned (it might be less than nbytes if, for example, an eof was encountered).
  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. This enters the operating system proper and shifts the computer to privileged mode. Assembly language is again used.
  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 invocation of read).

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

A Few Important Posix/Unix/Linux and Win32 System Calls
Process Management
ForkCreateProcessClone current process
exec(ve)Replace current process
waid(pid)WaitForSingleObjectWait for a child to terminate.
exitExitProcessTerminate process & return status
File Management
openCreateFileOpen a file & return descriptor
closeCloseHandleClose an open file
readReadFileRead from file to buffer
writeWriteFileWrite from buffer to file
lseekSetFilePointerMove file pointer
statGetFileAttributesExGet status info
Directory and File System Management
mkdirCreateDirectoryCreate new directory
rmdirRemoveDirectoryRemove empty directory
link(none)Create a directory entry
unlinkDeleteFileRemove a directory entry
mount(none)Mount a file system
umount(none)Unmount a file system
chdirSetCurrentDirectoryChange the current working directory
chmod(none)Change permissions on a file
kill(none)Send a signal to a process
timeGetLocalTimeElapsed time since 1 jan 1970

1.6.1 System Calls for Process Management

We describe the unix (Posix) system calls. A short description of the Windows interface is in the book.

To show how the four process management calls enable much of process management, consider the following highly simplified shell. The fork() system call duplicates the process (so parent and child are each executing fork()); fork() returns true in the parent and false in the child.)

    while (true)
      if (fork() != 0)

Simply removing the waitpid(...) gives background jobs.

1.6.2 System Calls for File Management

Most files are accessed sequentially from beginning to end. In this case the operations performed are

open -- possibly creating the file
multiple reads and writes

For non-sequential access, lseek is used to move the File Pointer, which is the location in the file where the next read or write will take place.

1.6.3 System Calls for Directory Management

Directories are created and destroyed by mkdir and rmdir. Directories are changed by the creation and deletion of files. As mentioned, open creates files. Files can have several names link is used to give another name and unlink to remove a name. When the last name is gone (and the file is no longer open by any process), the file data is destroyed. This description is approximate, we give the details later in the course where we explain Unix i-nodes.

Homework: 18.

1.6.4 Miscellaneous System Calls


1.6.5 The Windows Win32 API