1 Kernel Mode vs. User Mode

All multi-purposed chips today have (at least) two modes in which they can operate: user mode and kernel mode¹. Thus far you have probably only written user-mode software, and most of you will never write kernel-mode software during your entire software development careers.

1.1 Definitions

When running in kernel mode, the hardware allows the software to do all operations the hardware supports. In user mode, instead, some operations are not allowed.

The code that runs in kernel mode is called the kernel.

If the code running in user mode wants to perform one of the prohibited operations, it must instead ask the kernel to do so on its behalf.

1.2 Motivation

Running software in user mode only restricts what that software can do; it does not provide access to any additional functionality. But this restriction provides several advantages.

1.2.1 Limiting Bugs’ Potential for Evil

One reason to not run in kernel mode is to limit the impact a software bug can have. Inside the computer is all of the code and data that handles drawing the screen and tracking the mouse and reacting to keys and so on; I’d rather not be able to make a mistake when writing my Hello World program and accidentally mess up those core functionalities. By running in user mode, if my code tries to touch any of that it will be blocked from doing so: my program will crash, but the computer will keep running.

1.2.2 Each Program Thinks it is in Control

Physical computers have limited physical resources. One of the benefits of running in user mode is that the particular limitations are hidden from you by the kernel and hardware working together. For example, even if you only have 8GB of memory you can still use addresses larger than that limit: the kernel and hardware will work together to provide the illusion that there was enough memory even if there was not.

This illusion is even more important when you are running more than one process simultaneously. Perhaps you have a terminal, an editor, and your developed code all running at once. The kernel and hardware will work together to ensure that your code cannot even see, let alone modify, the memory being used by the terminal or editor. Each program is given the illusion of being the only one running on an unlimited machine.

1.2.3 Wrapper around Hardware

Each piece of hardware attached to a chip has its own protocols and nuances of operation. Fortunately, only the kernel needs to know about those details. If the mouse tries to talk to your program directly, the kernel intercepts the communication and handles it for you so you don’t notice it happened unless you asked the kernel to inform you about it. If you try to talk to the disk, the kernel likewise moderates that communication, keeping you from needing to know the specific protocol the currently-attached disk expects.

1.2.4 Multiple Users

Because the kernel is an unavoidable intermediary for talking to hardware, it can choose to forbid some interactions. One common use of this is to allow multiple user accounts. Since the kernel is the one that controls access to the disk, for example, it can choose to prohibit my processeses from accessing the part of the disk it has assigned to you and not me.

1.3 Implementation

1.3.1 Protected Instructions and Memory

When in user mode, there are a set of instructions that cannot be executed and segments of memory that cannot be accessed. Examples of things you cannot directly access in user mode include

• the part of memory that stores information about which user account you are logged into
• instructions that send signals out of the chip to communicate with disks, networks, etc.
• instructions that let user-mode code execute in kernel mode

1.3.2 Mode Bit

The simplest way to create two modes is to have a single-bit register somewhere on the chip that indicates if the chip is currently executing in user mode or in kernel mode. Each protected instruction then checks the contents of that register and, if it indicates user mode, causes an exception instead of executing the instruction. A special protected instruction is added to change the contents of this mode register, meaning kernel-mode code can become user-mode, but not vice versa.

Modern processors often have more than one operating mode with more than one level of privilege, which (among other benefits) can help make virtualization efficient; the details of these additional modes are beyond the scope of this course.

1.3.3 Mode-switch via Exceptions

The core mechanic for switching to the kernel is a hardware exception. An exception results in several functionally-simultaneous changes to processor state, notably including both (a) changing to kernel mode and (b) jumping to code in kernel-only memory. Thus, the only mechanisms that exist for entering kernel mode will run kernel code for the next instruction, preventing user code from running itself in kernel mode.

The nature of these hardware exceptions and the jump to kernel code that is associated with them is a large enough topic to deserve its own section.

2 Exceptions

Hardware exceptions are examples of exceptional control flow: that is, the sequence of instructions being operated around an exception is an exception to the usual rules of sequentiality and jumps. They also tend to appear in exceptional (that is, unusual) situations.

2.1 Kinds

There are several kinds of hardware exceptions.

2.1.1 Interrupts

An interrupt occurs independently from the code being executed when it occurs. It might result from the mouse moving, a network packet arriving, or the like.

Input devices commonly triggering interrupts allows the operating system to process and act on input even if that input occurs while an unrelated program is running.

Interrupts are typically handled by the kernel in a way that is invisible to the user code. The user code is frozen, the interrupt is handled, and then the user code is resumed as if nothing had happened.

2.1.2 Faults

A fault is the result of an instruction failing to succeed in its execution. Examples include dividing by zero, dereferencing a null pointer, or attempting to execute a protected instruction while in user mode.

There are two basic families of responses to faults: either the kernel fixes the problem and reruns the user code that generated the fault, or it cannot fix the problem and kills the process that caused the fault. In practice, fixable faults happen quite often, most frequently the page fault that occurs when using virtual memory. (As described in more detail in the virtual memory reading, these occur when a program accesses memory that is not yet assigned to the program (as far as the hardware knows). The OS can choose to respond to the fault by assigning the memory to the program. Then, it can allow the memory acess to be retried and this time, the hardware will be able to complete the memory access.)

Even faults the kernel cannot fix on its own may be handled by having the kernel’s fault handler ask the user code if it knows how to fix them. On Unix-like systems like Linux, this is done through a mechanism called signals. However, many programs do not handle the signals, so the kernel’s handler will often end up just terminating the program regardless.

2.1.3 Traps

A trap is an exception caused by a special instruction whose sole job is to generate exceptions. Traps are the main mechanism for intentionally switching from user mode to kernel mode and are the core of all system calls. System calls are typically function-call like interfaces provided by kernels to allow programs to access protected functionality. System calls are used to interact with the file system, spawn and wait on threads, listen for key presses, and anything else that you cannot do with simple code alone.

2.2 Handling

When an exception occurs, the processor switches to kernel mode and jumps to a special function in kernel code called an exception handler. Because interrupts exist, this can happen without any particular instruction causing the jump, and thus might happen at any point during code execution. In order for the handler to later resume user code, the exception-handling process must also save the processor state before executing the handler.

2.2.1 Save-Handle-Restore

The basic mechanism for any exception to be handled is

1. Save (some of) the current state of the processor (program counter, kernel mode bit, etc.)
2. Enter kernel mode
3. Jump to code designed to react to the exception in question, called an exception handler
4. When the handler finishes, enter user mode and restore processor state (program counter, kernel mode bit, etc.)

These steps (except for the actual execution of the exception handler) are all done atomically by the hardware.

Processors vary in how much of the processor state they save and restore in hardware. The PC has to be saved and restored since jumping to the exception handler overwrites the old PC value. The processor may also save and restore the values of all or some of the other registers. If the processor itself does not, then the operating system’s exception handler code can do this instead.

2.2.2 Which Handler?

In general, there can be as many different exception handlers as there are exception causes. To select which one to run, the hardware consults something called an exception table. The exception table is just an array of code addresses; the index into that array is determined by the kind of exception generated (e.g., divide-by-zero uses index 0, invalid instructions use index 6, etc.). The index is called an exception number or vector number and the array of code addresses is called the exception table.

switch(x) {
    case 0: f0();
    case 1: f1();
    case 3: f3();
    case 2: f2();
        break;
    case 4:
    case 5: f5();
    case 6: f6();
        break;
    case 8: f6();
}

    cmpl    $8, %eax
    ja      AfterSwitch
    jmpq    * Table(,%rax,8)
Case0:
    callq   f0
Case1:
    callq   f1
Case3:
    callq   f3
Case2:
    callq   f2
    jmp     AfterSwitch
Case5:
    callq   f5
Case6:
    callq   f6
    jmp     AfterSwitch
Case8:
    callq   f8
AfterSwitch:

    .section    .rodata
Table:
    .quad   Case0
    .quad   Case1
    .quad   Case2
    .quad   Case3
    .quad   Case5
    .quad   Case5
    .quad   Case6
    .quad   AfterSwitch
    .quad   Case8

2.2.3 After the Handler

Handlers may either abort the user code or resume its operation. Aborting is primarily used when there is no obvious way to recover from the cause of the exception.

When resuming, the handler can either re-run the instruction that was running when it was generated or continue with the next instruction instead. A trap, for example, is similar to a callq in its behavior and thus resumes with the subsequent instruction. A fault handler, on the other hand, is supposed to remove the barrier to success that caused the fault and thus generally re-runs the faulting instruction.

2.2.4 Example: Linux system calls

In Linux, almost all communications from user- to kernel-code are handled by a trap with exception number 128. Instead of using the exception number to select the particular action the user is requesting of the kernel, that action is put in a number in %rax; this decision allows Linux to have space for billions of different system calls instead of just the 200 or so that would fit in an exception number and are not reserved by the hardware.

To see a list of Linux system calls, see man 2 syscall. Most of these are wrapped in their own library function, listed in man 2 syscalls.

socket:
    endbr64 
    mov    $0x29,%eax
    syscall 

2.3 Exception-Like Constructs

2.3.1 Signals

One view of exceptions is that they enable communication from user code to the kernel. Signals permit the opposite direction of communication.

Signals are roughly the kernel-to-user equivalent of an interrupt. At any time, while executing any line of code, a signal may appear. It will suspend the currently executing code and see if you’ve told the kernel about a signal handler, a special function you believe will handle that signal. After running the handler, the interrupted code will resume.

Linux defines many different signals, listed in man 7 signal. Each of them has a default action if no handler is registered, most commonly terminating the process.

#include <stdio.h>
#include <signal.h>
#include <unistd.h>

static void handler(int signum) {
    // send string to stdout (1) without <stdio.h>
    // not using printf() here in case the signal handler runs
    // while printf() is running
    write(1, "Provide end-of-file to end program.\n",
          strlen("Provide end-of-file to end program.\n"));
}

int main() {
    struct sigaction sa;       // how we tell the OS what we want to handle
    sa.sa_handler = handler;   // the function pointer to invoke
    sigemptyset(&sa.sa_mask);  // do not "block" additional signals while signal handler is running
    sa.sa_flags = SA_RESTART;  // restart functions that can recognize they were interrupted

    if (sigaction(SIGINT, &sa, NULL) == -1) {
        fprintf(stderr, "unable to override ^C signal");
        return 1;
    }

    char buf[1024];
    while(scanf("%1023s", buf) && !feof(stdin)) {
        printf("token: %s\n", buf);
    }
}

#include <stdio.h>
#include <signal.h>

static void handler(int signum) {
    printf("Ignoring segfault.\n");
}

int main() {
    struct sigaction sa;       // how we tell the OS what we want to handle
    sa.sa_handler = handler;   // the function pointer to invoke
    sigemptyset(&sa.sa_mask);  // do not "block" additional signals while signal handler is running
    sa.sa_flags = SA_RESTART;  // restart functions that can recognize they were interrupted

    if (sigaction(SIGSEGV, &sa, NULL) == -1) {
        fprintf(stderr, "unable to override segfault signal");
        return 1;
    }

    // let's make a segfault -- enters infinite handler loop
    char * buf;
    buf[1234567] = 89;
}

There is a system call named kill that asks the kernel to send a signal to a different process. While this can be used for inter-process communication, better systems (like sockets and pipes) exist for that if it is to be a major part of an application’s design.

2.3.1.1 Signal safety

When a signal handlers runs in the middle of another function, this might severely limit what operations the signal handler can use. For example, if a signal handler runs in the middle of a call to malloc() (because of when you pressed Ctrl+C), then you probably cannot call malloc() from that signal handler. If you did so, probably malloc’s internal data structures would be corrupted.

To let programmers avoid this issue, The Unix-like operating system standard POSIX documents a list of functions that are async-signal-safe. Linux reproduces this list in man 7 signal-safety. These functions are gaurenteed to behave correctly even if they signal handler calls them while they are already running. write is on this list, which is why I used it in the signal handler in the example above. Another way to avoid this problem is to temporarily block signal handlers from running in the middle of unsafe operations or explicitly tell the OS when to handle them, as described in the next section.

2.3.1.2 Blocking signals and handling signals without signal handlers

Sometimes we may not want a signal to be handled right away. For example, a program with a handler for Ctrl+C that handles saving before exiting may not want the handler to run while it’s loading a new file. To do this, you can functions like sigprocmask to block signals, temporarily disabling signals handlers from running:

sigset_t sigint_as_set;
sigemptyset(&sigint_as_set);
sigaddset(&sigint_as_set, SIGINT);
sigprocmask(SIG_BLOCK, &sigint_as_set, NULL);
... /* do stuff without signal handler running yet */
sigprocmask(SIG_UNBLOCK, &sigint_as_set, NULL);

If a signal is sent to a process while it is blocked, then the OS will track that is pending. When the pending signal is unblocked, its signal handler will be run. Processors typically provide an analagous mechanism for operating systems to disable interrupts.

Unix-like systems also provide functions that work with blocked signals to allow processing signals without having a signal handler interrupt normale execution. For example, sigsuspend will temporarily unblock a blocked signal just long enough to run its signal handler. Alternately, sigwait will wait until one of a specific set of signals are sent and handle them by returning instead of running any signal handler.

2.3.2 setjmp, longjmp, and software exceptions

Portions of the save- and restore-state functionality used by exception handlers is exposed by the C library functions setjmp and longjmp.

setjmp, given a jmp_buf as an argument, fills that structure with information needed to resume execution at that location and then returns 0. Thereafter, longjmp can be called with that same jmp_buf as an argument; longjmp never returns, instead returning from setjmp for a second time. longjmp also provides an alternative return value for setjmp.

#include <setjmp.h>
#include <stdio.h>

jmp_buf env;
int n = 0;

void b() {
    printf("b1: n = %d\n", n);
    n += 1;
    printf("b2: n = %d\n", n);
    longjmp(env, 1);
    printf("b3: n = %d\n", n);
    n += 1;
    printf("b4: n = %d\n", n);
}

void a() {
    printf("a1: n = %d\n", n);
    n += 1;
    printf("a2: n = %d\n", n);
    b();
    printf("a3: n = %d\n", n);
    n += 1;
    printf("a4: n = %d\n", n);
}

int main() {
    printf("main1: n = %d\n", n);
    n += 1;
    printf("main2: n = %d\n", n);
    int got = setjmp(env);
    if (got) {
        printf("main3: n = %d\n", n);
        n += 1;
        printf("main4: n = %d\n", n);
    } else {
        printf("main5: n = %d\n", n);
        n += 1;
        printf("main6: n = %d\n", n);
        a();
    }
    printf("end of main\n");
}

main1: n = 0
main2: n = 1
main5: n = 1
main6: n = 2
a1: n = 2
a2: n = 3
b1: n = 3
b2: n = 4
main3: n = 4
main4: n = 5
end of main

There was a time when setjmp/longjmp were seen as effective ways of achieving try/catch-like error recovery, which cannot be handled with simple goto because it may skip multiple function call/returns.

try {
    f();
} catch {
    g();
}

if (!setjmp(env)) {
    f();
} else {
    g();
}

More efficient approaches to this have since been developed. setjmp records much of the state of the program in advance, which gives it a significant cost in time; because try is generally assumed to be executed far more often than throw, we’d rather make throw the expensive one, not try. Most of the information stored by setjmp into the jmp_buf can be found somewhere in the setjmp-invoking function’s stack frame, which many languages maintain with sufficient discipline that it can be unwound to restore a previous state upon a throw. C, however, lets you do anything, including violating assumptions about the stack organization, so C has an expensive setjmp and less expensive longjmp instead of an expensive throw and less expensive try.

3 Processes and Threads

2023-10-09: Most of the content of this section was moved to the processes threads reading to reduce duplication. You can find the old contents here.

A process is a software-only operating system construct that roughly parallels the end-user idea of a running program. The hardware knows the difference between user and kernel mode, but not the difference between a web browser and a text editor.

We discuss processes in more detail in the processes and threads reading.

4 Typical I/O patterns

4.1 Input

When a program retrieves input from a device (like a keyboard or the network) often multiple exceptions and context switches are involved. A typical input operation might work like this:

1. A program needs to request input from the OS. It will do this with a system call.

2. In some cases, the OS may already have the input available in its memory and be able to return that immediately to the program in the system call handler.

3. Otherwise, most likely the program that made the system call will not be able to continue running until that input is available. So, rather than wasting processor time, the OS will, if possible, context switch to something else until input is available.

4. Eventually, the device will indicate it has more input by triggering an interrupt. The OS can handle interrupt by copying the data and marking the waiting program as runnable again. Possibly, the could context switch to the waiting program immediately, or it might decide to let other programs run more first.

4.2 Output

The typical output scenario is very similar to the input scenario:

1. A program will request that the OS send output to a device on its behalf.

2. In many cases, the OS will be able to send the output immediately to the device.

3. Otherwise, the OS might need to wait until the device is ready to accept its output. If the current program can run while that’s happening, the OS can return to the current program. Otherwise, it would context switch to other programs until the device is ready for more output.

4. Eventually, the device will indicate it can accept more output by triggering an interrupt. The OS will handle this interrupt by completing the output operation.

5 Further resources

5.1 General

• Arpaci-Dusseau and Arpaci-Dusseau, Operating Systems: Three Easy Pieces chapters 4, 6, 36. More detailed textbook on all these topics, intended in the context of a more dedicated operating systems course.

  In chapter 6, this textbook talks about exceptions referring to hardware exceptions triggered by attempting ot run an instruction (what we call system calls or faults) as traps, and exceptions triggered by external events as interrupts. It does not use the term exception explicitly.

• Dive Into Systems, Chapter 13

  Like Arpaci-Dusseau, this textbook uses the term traps to refer to exceptions triggered by attempting to run an instruction and interrupts to refer exceptions triggered by external events.

5.2 For signal handling

• The POSIX documentation for sigaction; the POSIX documentation for signal handling generally

5.3 Operating system organization examples

• Sections 1.3–1.6 of Bovet and Cesati, Understanding the Linux Kernel, Third Edition, available through O’Reilly (click institution not listed and enter your UVa email)

• Chapter 2 of McKusick and Neville-Neil, The Design and Implementation of the FreeBSD Operating System (also available through O’Reilly)

• Chapter 2 of Yosifovich, Ionescu, Russinovich, and Solomon, Windows Internals Seventh Edition Part 1 (Also available through O’Reilly)