Operating Systems

================ Start Lecture #3 ================

1.4.3: I/O Devices

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

Devices are often quite complicated to manage and a separate computer, called a controller, is used to translate simple commands (read sector 123456) into what the device requires (read cylinder 321, head 6, sector 765). Actually the controller does considerably more, e.g. calculates a checksum for error detection.

How does the OS know when the I/O is complete?

  1. It can busy wait constantly asking the controller if the I/O is complete. This is the easiest (by far) but has low performance. It is also called polling or PIO (Programmed I/O).
  2. It can tell the controller to start the I/O and then switch to other tasks. The controller must then interrupt the OS when the I/O is done. Less waiting, but harder (concurrency!). Also on modern processors a single is rather costly. Much more than a single memory reference, but much, much less than a disk I/O.
  3. Some controllers can do DMA (Direct Memory Access) in which case they deal directly with memory after being started by the CPU. This takes work from the CPU and halves the number of bus accesses.

We discuss this more in chapter 5. In particular, we explain the last point about halving bus accesses.

1.4.4: Buses

I don't care very much about the names of the buses, but the diagram given in the book doesn't show a modern design. The one below does. On the right is a figure showing the specifications for a modern chip set (introduced in 2000). The chip set has two different width PCI busses, which is not shown below. Instead of having the chip set supply USB, a PCI USB controller may be used. Finally, the use of ISA is decreasing. Indeed my last desktop didn't have an ISA bus and I had to replace my ISA sound card with a PCI version.

1.5: Operating System Concepts

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

1.5.1: Processes

A program in execution. If you run the same program twice, you have created two processes. For example if you have two editors running in two windows, each instance of the editor is a separate process.

Often one distinguishes the state or context (memory image, open files) 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. This is an example of an active entity being viewed as a data structure (cf. discrete event simulations). An observation made by Finkel in his (out of print) OS textbook.

The Process Tree

The set of processes forms a tree via the fork system call. The forker is the parent of the forkee, which is called a child. If the system blocks the parent until the child finishes, the ``tree'' is quite simple, just a line. But the parent (in many OSes) is free to continue executing and in particular is free to fork again producing another child.

A process can send a signal to another process to cause the latter 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.

Each user is assigned User IDentification (UID) and all processes created by that user have this UID. One UID is special (the superuser or administrator) and has extra privileges. A child has the same UID as its parent. It is sometimes possible to change the UID of a running process. A group of users can be formed and given a Group IDentification, GID.

Access to files and devices can be limited to a given UID or GID.






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.

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

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

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

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

1.5.6: Security

Files and directories have associated permissions.

1.5.7: The Shell or Command Interpreter (DOS Prompt)

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

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 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, 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)
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 (assembly language). 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 call read).

A major complication is that the system call handler may block. Indeed for read it is likely that a block will occur. 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. (The fork() system call returns true in the parent and false in the child.) 

while (true)
    display_prompt()
    read_command(command)

    if (fork() != 0)
        waitpid(...)
    else
        execve(command)
    endif
endwhile

Homework: 18.

1.6A: Addendum on Transfer of Control

The transfer of control between user processes and the operating system kernel can be quite complicated, especially in the case of blocking system calls, hardware interrupts, and page faults. Before tackling these issues later, we begin with a more familiar situation, a procedure call within a user-mode process, i.e., a review of material from 101 and 201.

An important OS objective is that, even in the more complicated cases of page faults and blocking system calls requiring device interrupts, simple procedure call semantics are observed from a user process viewpoint. The complexity is hidden inside the kernel itself, yet another example of the operating system providing a more abstract, i.e., simpler, virtual machine to the user processes.

More details will be added when we study memory management (and know officially about page faults) and more again when we study I/O (and know officially about device interrupts).

A number of the points below are far from standardized. Such items as where are parameters placed, which routine saves the registers, exact semantics of trap, etc, vary as one changes language/compiler/OS. Indeed some of these are referred to as ``calling conventions'', i.e. their implementation is a matter of convention rather than logical requirement. The presentation below is, we hope, reasonable, but must be viewed as a generic description of what could happen instead of an exact description of what does happen with, say, C compiled by the Microsoft compiler running on Windows NT.

1.6A.1: User-mode procedure calls

Procedure f calls g(a,b,c) in process P.

Actions by f prior to the call:

  1. Complete all previous instructions in f.

  2. Save the registers by pushing them onto the stack (in some implementations this is done by g instead of f).

  3. Push arguments c,b,a onto P's stack.
    Note: Stacks usually grow downward from the top of P's segment, so pushing an item onto the stack actually involves decrementing the stack pointer, SP.

Executing the call itself

  1. Execute PUSHJ <start-address of g>.
    This instruction pushes the program counter PC onto the stack, and then jumps to the start address of g. The value pushed is actually the updated program counter, i.e., the location of the next instruction (the instruction to be executed by f when g returns).

Actions by g upon being called:

  1. Allocate space for g's local variables by suitably decrementing SP.

  2. Start execution from the beginning of the program, referencing the parameters as needed. The execution may involve calling other procedures, possibly including recursive calls to f and/or g.

Actions by g when returning to f:

  1. If g is to return a value, store it in the conventional place.

  2. Undo step 5: Deallocate local variables by incrementing SP.

  3. Undo step 4: Execute POPJ, i.e., pop the stack and set PC to the value popped, which is the return address pushed in step 4.

Actions by f upon the return from g:

  1. We are now at the step in f immediately following the call to g.
    Undo step 3: Remove the arguments from the stack by incrementing SP.

  2. (Sort of) undo step 2: Restore the registers by popping the stack.

  3. Continue the execution of f, referencing the returned value of g, if any.

Properties of (user-mode) procedure calls:

1.6A.2: Kernel-mode procedure calls

We mean one procedure running in kernel mode calling another procedure, which will also be run in kernel mode. Later, we will discuss switching from user to kernel mode and back.

There is not much difference between the actions taken during a kernel-mode procedure call and during a user-mode procedure call. The procedures executing in kernel-mode are permitted to issue privileged instructions, but the instructions used for transferring control are all unprivileged so there is no change in that respect.

One difference is that a different stack is used in kernel mode, but that simply means that the stack pointer must be set to the kernel stack when switching from user to kernel mode. But we are not switching modes in this section; the stack pointer already points to the kernel stack.

1.6A.3: The Trap instruction

The trap instruction, like a procedure call, is a synchronous transfer of control: We can see where, and hence when, it is executed; there are no surprises. Although not surprising, the trap instruction does have an unusual effect, processor execution is switched from user-mode to kernel-mode. That is, the trap instruction itself is executed in user-mode (it is naturally an UNprivileged instruction) but the next instruction executed (which is NOT the instruction written after the trap) is executed in kernel-mode.

Process P, running in unprivileged (user) mode, executes a trap. The code being executed was written in assembler since there are no high level languages that generate a trap instruction. There is no need to name the function that is executing. Compare the following example to the explanation of ``f calls g'' given above.

Actions by P prior to the trap

  1. Complete all previous instructions in P.

  2. Save the registers by pushing them onto the stack.

  3. Store any arguments that are to be passed. The stack is not normally used to store these arguments since the kernel has a different stack. Often registers are used.

Executing the trap itself

  1. Execute TRAP <trap-number>.
    Switch the processor to kernel (privileged) mode, jumps to a location in the OS determined by trap-number, and saves the return address. For example, the processor may be designed so that the next instruction executed after a trap is at physical address 8 times the trap-number. The trap-number should be thought of as the ``name'' of the code-sequence to which the processor will jump rather than as an argument to trap. Indeed arguments to trap, are established before the trap is executed.

Actions by the OS upon being TRAPped into

  1. Jump to the real code.
    Recall that trap instructions with different trap numbers jump to locations very close to each other. There is not enough room between them for the real trap handler. Indeed one can think of the trap as having an extra level of indirection; it jumps to a location that then jumps to the real start address. If you learned about writing jump tables in assembler, this is very similar.

  2. Check all arguments passed. The kernel must be paranoid and assume that the user mode program is evil and written by a bad guy.

  3. Allocate space by decrementing the kernel stack pointer.
    The kernel and user stacks are separate.

  4. Start execution from the jumped-to location, referencing the parameters as needed.

Actions by the OS when returning to user mode

  1. Undo step 7: Deallocate space by incrementing the kernel stack pointer.

  2. Undo step 4: Execute (in assembler) another special instruction, RTI or ReTurn from Interrupt, which returns the processor to user mode and transfers control to the return location saved by the trap.

Actions by P upon the return from the OS

  1. We are now in at the instruction right after the trap
    Undo step 2: Restore the registers by popping the stack.

  2. Continue the execution of P, referencing the returned value(s) of the trap, if any.

Properties of TRAP/RTI: