Operating System Kernel and User Mode

Kernel and User Mode

• The operating system, or kernel, manages hardware.
• It supports a dedicated computer model.
• It protects against misbehaving programs.
• The kernel is the core of the operating system.
• In essence, the operating system and kernel are just programs.

User Mode Programs

• Programs run in user mode.
• User mode programs act as if they have their own dedicated computer.
• User mode programs need the kernel to access shared resources.

Exceptions

• Exceptions are a type of control flow different from if-then-else statements.
• Exceptions allow a program to transfer control to another function or program.
• They can be triggered by external events (asynchronous).
• Exceptions are caused by hardware or software.
• Exception handling requires both hardware and software components.

System Calls

• A system call is a specific type of exception.
• It lets a program leave its dedicated computer and connect with the kernel.
• System calls are synchronous but non-local, meaning they change control without being dependent on any other external state.

The Kernel and Supervisor Mode

• The kernel doesn't have a dedicated computer.
• The kernel controls the "real computer".
• It operates in a special mode, often called supervisor mode.
• The kernel has direct access to hardware.
• It manages virtual memory mappings and process memory.

Protection Domains

• Protection domains represent the allowed access level for a process to the computer's hardware resources.
• Supervisor mode is a higher-privileged protection domain.
• User mode is a lower-privileged protection domain.
• Protection domains are a hardware feature.
• User programs run in user mode; the kernel runs in supervisor mode.
• Hardware enforces access restrictions on user mode.
• Some hardware has more than two protection domains.

Changing Protection Domains

• Changing protection domains is a supervisor mode operation.
• This action safeguards against programs leaving user mode.
• Switching domains requires a well-defined method, like exceptions.
• Changing protection domains can be computationally expensive.

Exception Flow

• When an exception happens, the control shifts to the kernel.
• If control is already with the kernel, its location changes.
• If the control is within a user mode program, contexts switch and the kernel takes over.

Types of Exceptions

• Interrupts are asynchronous signals from hardware.
• Traps are synchronous exceptions caused by software (on purpose).
• Faults are synchronous exceptions from software due to potentially recoverable errors.
• Aborts are synchronous exceptions from irreparable errors outside of software control.

Interrupts

• Hardware uses interrupts to signal the operating system.
• Examples include network packets arriving, clocks ticking, or disks finishing reading.
• Interrupts are handled by the kernel.

Faults

• Faults are recoverable program errors.
• When a fault occurs, the kernel can try to fix it or notify the program.
• The kernel might choose to terminate the program or shut down.
• Examples of faults include segmentation faults and divide-by-zero errors.

Fault Recovery

• Some faults are not errors; for example, page faults deal with bringing in new pages.
• Other faults might be recoverable by the program.
• Such faults include divide by zero, segmentation faults, and bus errors.
• An error doesn't always have to be fatal (e.g., a potentially usefully value can be returned instead).

Aborts

• Aborts represent unrecoverable errors, frequently resulting in computer rebooting or shutting down.
• The kernel manages abort handling.

Traps

• Traps are software-generated exceptions.
• They are created by special instructions within a program.
• A key feature of traps is that they are handled by the kernel in supervisor mode.
• This enables user-mode programs to call into the kernel safely, facilitating protection domain changes.

System Calls

• System calls are traps used by user-mode programs to interact with kernel functions.
• Many operating systems have specific hardware instructions for system calls (e.g., ARM's SVC, x86-64’s syscall).

System Call Handling

• The hardware switches to supervisor mode when a system call instruction executes.
• It then invokes a specific kernel routine.
• Kernel actions include identifying program requirements, verifying inputs, authenticating requests, and performing operations or indicating failures.
• The kernel makes decisions for accessing resources outside the program's dedicated environment.

Implications of Traps

• User mode cannot control what the kernel does.
• The hardware dictates and configures kernel operations.
• Modern operating systems use this to guard themselves from buggy or malicious programs.

Invoking a System Call

• System calls, like open(), sbrk(), mmap(), are necessary for interactions outside the program's dedicated environment.
• The C library makes system calls appear as regular C functions.

Overhead

• System calls are slow, taking tens to hundreds of thousands of clock cycles.
• This slowness is primarily due to changes in protection domains, validation of input data, memory adjustments, and cache effects.

Summary

• Exceptions are special control flows.
• Protection domains control access to hardware resources.
• Exception handlers run in supervisor mode within the kernel.
• Special trap exceptions facilitate system calls.
• System calls allow user-mode programs to get access to the kernel.

References

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